Exokernel: An Operating System Architecture forApplication-Level Resource Management
Dawson R. Engler, M. Frans Kaashoek, and James O’Toole Jr.
M.I.T. Laboratory for Computer Science
Cambridge, MA 02139, U.S.A
engler, kaashoek, james  @lcs.mit.edu 
Traditional operating systems limit the performance, flexibility, andfunctionality of applications by fixing the interface and implementation of operating system abstractions such as interprocess communication and virtual memory. The exokernel operating systemarchitecture addresses this problem by providing application-level
management of physical resources. In the exokernel architecture, asmall kernel securely exports all hardware resources through a lowlevel interface to untrusted library operating systems. Library operating systems use this interface to implement system objects andpolicies. This separation of resource protection from management
allows application-specific customization of traditional operating
system abstractions by extending, specializing, or even replacing
libraries.
We have implemented a prototype exokernel operating system.
Measurements show that most primitive kernel operations (suchas exception handling and protected control transfer) are ten to 100
times faster than in Ultrix, a mature monolithic UNIX operating system. In addition, we demonstrate that an exokernel allows applications to control machine resources in ways not possible in traditional
operating systems. For instance, virtual memory and interprocess
communication abstractions are implemented entirely within an
application-level library. Measurements show that application-levelvirtual memory and interprocess communication primitives are five
to 40 times faster than Ultrix’s kernel primitives. Compared tostate-of-the-art implementations from the literature, the prototype
exokernel system is at least five times faster on operations such as
exception dispatching and interprocess communication.

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Operating systems define the interface between applications andphysical resources. Unfortunately, this interface can significantly
limit the performance and implementation freedom of applications.
Traditionally, operating systems hide information about machine
resources behind high-level abstractions such as processes, files,
address spaces and interprocess communication. These abstractions define a virtual machine on which applications execute; theirimplementation cannot be replaced or modified by untrusted applications. Hardcoding the implementations of these abstractions is
This research was supported in part by the Advanced Research Projects Agency undercontract N00014-94-1-0985 and by a NSF National Young Investigator Award.



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inappropriate for three main reasons: it denies applications the advantages of domain-specific optimizations, it discourages changes
to the implementations of existing abstractions, and it restricts theflexibility of application builders, since new abstractions can only
be added by awkward emulation on top of existing ones (if they canbe added at all).
We believe these problems can be solved through applicationlevel (i.e., untrusted) resource management. To this end, we havedesigned a new operating system architecture, exokernel, in which
traditional operating system abstractions, such as virtual memory(VM) and interprocess communication (IPC), are implemented entirely at application level by untrusted software. In this architecture,a minimal kernel—which we call an exokernel—securely multiplexes available hardware resources. Library operating systems,
working above the exokernel interface, implement higher-level abstractions. Application writers select libraries or implement theirown. New implementations of library operating systems are incorporated by simply relinking application executables.
Substantial evidence exists that applications can benefit greatly
from having more control over how machine resources are used
to implement higher-level abstractions. Appel and Li [5] reported
that the high cost of general-purpose virtual memory primitives
reduces the performance of persistent stores, garbage collectors, anddistributed shared memory systems. Cao et al. [10] reported thatapplication-level control over file caching can reduce application
running time by 45%. Harty and Cheriton [26] and Krueger et
al. [30] showed how application-specific virtual memory policies
can increase application performance. Stonebraker [47] argued
that inappropriate file-system implementation decisions can have adramatic impact on the performance of databases. Thekkath andLevy [50] demonstrated that exceptions can be made an order ofmagnitude faster by deferring signal handling to applications.
To provide applications control over machine resources, an exokernel defines a low-level interface. The exokernel architecture isfounded on and motivated by a single, simple, and old observation:
the lower the level of a primitive, the more efficiently it can be
implemented, and the more latitude it grants to implementors ofhigher-level abstractions.
To provide an interface that is as low-level as possible (ideally,
just the hardware interface), an exokernel designer has a single
overriding goal: to separate protection from management. For
instance, an exokernel should protect framebuffers without understanding windowing systems and disks without understanding filesystems. One approach is to give each application its own virtual
machine [17]. As we discuss in Section 8, virtual machines canhave severe performance penalties. Therefore, an exokernel uses a
different approach: it exports hardware resources rather than emulating them, which allows an efficient and simple implementation.
An exokernel employs three techniques to export resources securely.
First, by using secure bindings, applications can securely bind tomachine resources and handle events. Second, by using visible re-stractions and show how the exokernel architecture addresses these
problems.

source revocation, applications participate in a resource revocation
protocol. Third, by using an abort protocol, an exokernel can breaksecure bindings of uncooperative applications by force.

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We have implemented a prototype exokernel system based on
secure bindings, visible revocation, and abort protocols. It includes
an exokernel (Aegis) and an untrusted library operating system(ExOS). We use this system to demonstrate several important properties of the exokernel architecture: (1) exokernels can be madeefficient due to the limited number of simple primitives they must
provide; (2) low-level secure multiplexing of hardware resources
can be provided with low overhead; (3) traditional abstractions,such as VM and IPC, can be implemented efficiently at applicationlevel, where they can be easily extended, specialized, or replaced;and (4) applications can create special-purpose implementations ofabstractions, tailored to their functionality and performance needs.

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Fixed high-level abstractions hide information from applications. For instance, most current systems do not make low-level
exceptions, timer interrupts, or raw device I/O directly available toapplication-level software. Unfortunately, hiding this information
makes it difficult or impossible for applications to implement theirown resource management abstractions. For example, database implementors must struggle to emulate random-access record storage
on top of file systems [47]. As another example, implementing
lightweight threads on top of heavyweight processes usually requires compromises in correctness and performance, because theoperating system hides page faults and timer interrupts [4]. In suchcases, application complexity increases because of the difficulty ofgetting good performance from high-level abstractions.

This paper focuses on the exokernel architecture design and how
it can be implemented securely and efficiently. Section 2 provides
a more detailed case for exokernels. Section 3 discusses the issues
that arise in their design. Section 4 overviews the status of our prototype and explains our experimental methodology. Sections 5 and 6
present the implementation and summarize performance measurements of Aegis and ExOS. Section 7 reports on experiments that
demonstrate the flexibility of the exokernel architecture. Section 8summarizes related work and Section 9 concludes.
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Fixed high-level abstractions hurt application performance because there is no single way to abstract physical resources or to
implement an abstraction that is best for all applications. In implementing an abstraction, an operating system is forced to maketrade-offs between support for sparse or dense address spaces,
read-intensive or write-intensive workloads, etc. Any such tradeoff penalizes some class of applications. For example, relational
databases and garbage collectors sometimes have very predictabledata access patterns, and their performance suffers when a generalpurpose page replacement strategy such as LRU is imposed by
the operating system. The performance improvements of suchapplication-specific policies can be substantial; Cao et al. [10] measured that application-controlled file caching can reduce application
running time by as much as 45%.

Aegis also gives ExOS (and other application-level software)
flexibility that is not available in microkernel-based systems. For
instance, virtual memory is implemented at application level, where
it can be tightly integrated with distributed shared memory systems
and garbage collectors. Aegis’s efficient protected control transfer allows applications to construct a wide array of efficient IPC
primitives by trading performance for additional functionality. In
contrast, microkernel systems such as Amoeba [48], Chorus [43],Mach [2], and V [15] do not allow untrusted application software todefine specialized IPC primitives because virtual memory and message passing services are implemented by the kernel and trusted
servers. Similarly, many other abstractions, such as page-tablestructures and process abstractions, cannot be modified in microkernels. Finally, many of the hardware resources in microkernel systems, such as the network, screen, and disk, are encapsulated in heavyweight servers that cannot be bypassed or tailored
to application-specific needs. These heavyweight servers can beviewed as fixed kernel subsystems that run in user-space.

   The essential observation about abstractions in traditional operating
systems is that they are overly general. Traditional operating systems attempt to provide all the features needed by all applications.
As previously noted by Lampson and Sproul [32], Anderson etal. [4] and Massalin and Pu [36], general-purpose implementationsof abstractions force applications that do not need a given feature to
pay substantial overhead costs. This longstanding problem has become more important with explosive improvements in raw hardware
performance and enormous growth in diversity of the application
software base. We argue that preventing the modification of theimplementation of these high-level abstractions can reduce the performance, increase the complexity, and limit the functionality of
application programs.

In practice, our prototype exokernel system provides applications with greater flexibility and better performance than monolithic and microkernel systems. Aegis’s low-level interface allows
application-level software such as ExOS to manipulate resourcesvery efficiently. Aegis’s protected control transfer is almost seventimes faster than the best reported implementation [33]. Aegis’s
exception dispatch is five times faster than the best reported implementation [50]. On identical hardware, Aegis’s exception dispatch
and control transfer are roughly two orders of magnitude faster thanin Ultrix 4.2, a mature monolithic system.

  Fixed high-level abstractions limit the functionality of applications, because they are the only available interface between applications and hardware resources. Because all applications must
share one set of abstractions, changes to these abstractions occur
rarely, if ever. This may explain why few good ideas from thelast decade of operating systems research have been adopted into
widespread use: how many production operating systems support
scheduler activations [4], multiple protection domains within a single address space [11], efficient IPC [33], or efficient and flexible
virtual memory primitives [5, 26, 30]?

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Traditionally, operating systems have centralized resource management via a set of abstractions that cannot be specialized, extended,or replaced. Whether provided by the kernel or by trusted userlevel servers (as in microkernel-based systems), these abstractions
are implemented by privileged software that must be used by all
applications, and therefore cannot be changed by untrusted software. Typically, the abstractions include processes, files, address
spaces, and interprocess communication. In this section we discuss
the problems with general-purpose implementations of these ab-

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The familiar “end-to-end” argument applies as well to low-level
operating system software as it does to low-level communication
protocols [44]. Applications know better than operating systems
what the goal of their resource management decisions should be
and therefore, they should be given as much control as possible over2

system can be smaller, since most of the operating system runs in
the address space of the application.
MosaicApplications

DSM

WWW
POSIX

Library operating systems can provide as much portability andcompatibility as is desirable. Applications that use an exokernelinterface directly will not be portable, because the interface will include hardware-specific information. Applications that use library
operating systems that implement standard interfaces (e.g., POSIX)
will be portable across any system that provides the same interface.
An application that runs on an exokernel can freely replace theselibrary operating systems without any special privileges, which simplifies the addition and development of new standards and features.
We expect that most applications will use a handful of available
library operating systems that implement the popular interfaces;
only designers of more ambitious applications will develop new
library operating systems that fit their needs. Library operating
systems themselves can be made portable by designing them to use
a low-level machine-independent layer to hide hardware details.

Barnes−Hut

VM

IPC

TCP
Library operating systems

Exokernel

Traps

Secure bindings

Hardware Frame buffer

TLB

Network

MemoryDisk

Figure 1: An example exokernel-based system consisting of a thinexokernel veneer that exports resources to library operating systems
through secure bindings. Each library operating system implements
its own system objects and policies. Applications link against standard libraries (e.g., WWW, POSIX, and TCP libraries for Web applications) or against specialized libraries (e.g., a distributed sharedmemory library for parallel applications).

Extending or specializing a library operating system might beconsiderably simplified by modular design. It is possible that objectoriented programming methods, overloading, and inheritance canprovide useful operating system service implementations that canbe easily specialized and extended, as in the VM++ library [30].
To reduce the space required by these libraries, support for sharedlibraries and dynamic linking will be an essential part of a complete
exokernel-based system.

those decisions. Our solution is to allow traditional abstractions to
be implemented entirely at application level.

As in microkernel systems, an exokernel can provide backward
compatibility in three ways: one, binary emulation of the operating
system and its programs; two, by implementing its hardware abstraction layer on top of an exokernel; and three, re-implementing
the operating system’s abstractions on top of an exokernel.

To provide the maximum opportunity for application-level resource management, the exokernel architecture consists of a thinexokernel veneer that multiplexes and exports physical resources
securely through a set of low-level primitives. Library operating
systems, which use the low-level exokernel interface, implement
higher-level abstractions and can define special-purpose implementations that best meet the performance and functionality goals of
applications (see Figure 1). (For brevity, we sometimes refer to“library operating system” as “application.”) This structure allows
the extension, specialization and even replacement of abstractions.
For instance, page-table structures can vary among library operating systems: an application can select a library with a particular
implementation of a page table that is most suitable to its needs.
To the best of our knowledge, no other secure operating systemarchitecture allows applications so much useful freedom.

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The challenge for an exokernel is to give library operating systems
maximum freedom in managing physical resources while protecting
them from each other; a programming error in one library operating system should not affect another library operating system. To
achieve this goal, an exokernel separates protection from management through a low-level interface.
In separating protection from management, an exokernel performs three important tasks: (1) tracking ownership of resources,(2) ensuring protection by guarding all resource usage or binding
points, and (3) revoking access to resources. To achieve thesetasks, an exokernel employs three techniques. First, using securebindings, library operating systems can securely bind to machine
resources. Second, visible revocation allows library operating systems to participate in a resource revocation protocol. Third, anabort protocol is used by an exokernel to break secure bindings of
uncooperative library operating systems by force.

This paper demonstrates that the exokernel architecture is an effective way to address the problems listed in Section 2.1. Many ofthese problems are solved by simply moving the implementation of
abstractions to application level, since conflicts between applicationneeds and available abstractions can then be resolved without theintervention of kernel architects. Furthermore, secure multiplexing
does not require complex algorithms; it mostly requires tables totrack ownership. Therefore, the implementation of an exokernel canbe simple. A simple kernel improves reliability and ease of maintenance, consumes few resources, and enables quick adaptation to
new requirements (e.g., gigabit networking). Additionally, as istrue with RISC instructions, the simplicity of exokernel operations
allows them to be implemented efficiently.
In this section, we enumerate the central design principles ofthe exokernel architecture. Then, we discuss in detail the threetechniques that we use to separate protection from management.

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An exokernel specifies the details of the interface that library operating systems use to claim, release, and use machine resources.
This section articulates some of the principles that have guided
our efforts to design an exokernel interface that provides library
operating systems the maximum degree of control.

The implementations of abstractions in library operating systems
can be simpler and more specialized than in-kernel implementations, because library operating systems need not multiplex a resource among competing applications with widely different demands. In addition, since libraries are not trusted by an exokernel,
they are free to trust the application. For example, if an application
passes the wrong arguments to a library, only that application will
be affected. Finally, the number of kernel crossings in an exokernel

Securely expose hardware. The central tenet of the exokernel architecture is that the kernel should provide secure low-level3

primitives that allow all hardware resources to be accessed as directly as possible. An exokernel designer therefore strives to safely
export all privileged instructions, hardware DMA capabilities, andmachine resources. The resources exported are those provided by
the underlying hardware: physical memory, the CPU, disk memory,translation look-aside buffer (TLB), and addressing context identifiers. This principle extends to less tangible machine resources
such as interrupts, exceptions, and cross-domain calls. An exokernel should not impose higher-level abstractions on these events(e.g., Unix signal or RPC semantics). For improved flexibility,
most physical resources should be finely subdivided. The number, format, and current set of TLB mappings should be visible toand replaceable by library operating systems, as should any “privileged” co-processor state. An exokernel must export privileged
instructions to library operating systems to enable them to implement traditional operating system abstractions such as processes
and address spaces. Each exported operation can be encapsulated
within a system call that checks the ownership of any resources
involved.

allocation requests to grant and from which applications to revoke
resources, an exokernel can enforce traditional partitioning strategies, such as quotas or reservation schemes. Since policy conflicts
boil down to resource allocation decisions (e.g., allocation of seektime, physical memory, or disk blocks), an exokernel handles themin a similar manner.
    One of the primary tasks of an exokernel is to multiplex resources
securely, providing protection for mutually distrustful applications.
To implement protection an exokernel must guard each resource. To
perform this task efficiently an exokernel allows library operating
systems to bind to resources using secure bindings.
A secure binding is a protection mechanism that decouples authorization from the actual use of a resource. Secure bindings
improve performance in two ways. First, the protection checks
involved in enforcing a secure binding are expressed in terms of
simple operations that the kernel (or hardware) can implement
quickly. Second, a secure binding performs authorization only atbind time, which allows management to be decoupled from protection. Application-level software is responsible for many resources
with complex semantics (e.g., network connections). By isolating
the need to understand these semantics to bind time, the kernel canefficiently implement access checks at access time without understanding them. Simply put, a secure binding allows the kernel toprotect resources without understanding them.

Phrased negatively, this principle states that an exokernel should
avoid resource management. It should only manage resources tothe extent required by protection (i.e., management of allocation,
revocation, and ownership). The motivation for this principle is ourbelief that distributed, application-specific, resource management
is the best way to build efficient flexible systems. Subsequent
principles deal with the details of achieving this goal.
Expose allocation. An exokernel should allow library operating systems to request specific physical resources. For instance, if alibrary operating system can request specific physical pages, it can
reduce cache conflicts among the pages in its working set [29]. Furthermore, resources should not be implicitly allocated; the library
operating system should participate in every allocation decision.
The next principle aids the effectiveness of this participation.

Operationally, the one requirement needed to support securebindings is a set of primitives that application-level software canuse to express protection checks. The primitives can be implemented either in hardware or software. A simple hardware securebinding is a TLB entry: when a TLB fault occurs the complex mapping of virtual to physical addresses in a library operating system’s
page table is performed and then loaded into the kernel (bind time)
and then used multiple times (access time). Another example is thepacket filter [37], which allows predicates to be downloaded into
the kernel (bind time) and then run on every incoming packet to determine which application the packet is for (access time). Without apacket filter, the kernel would need to query every application or network server on every packet reception to determine who the packet
was for. By separating protection (determining who the packet isfor) from authorization and management (setting up connections,sessions, managing retransmissions, etc.) very fast network multiplexing is possible while still supporting complete application-levelflexibility.

Expose Names. An exokernel should export physical names.
Physical names are efficient, since they remove a level of indirection
otherwise required to translate between virtual and physical names.
Physical names also encode useful resource attributes. For example,
in a system with physically-indexed direct-mapped caches, the nameof a physical page (i.e., its page number) determines which pages
it conflicts with. Additionally, an exokernel should export bookkeeping data structures such as freelists, disk arm positions, andcached TLB entries so that applications can tailor their allocation
requests to available resources.
Expose Revocation. An exokernel should utilize a visible resource revocation protocol so that well-behaved library operating
systems can perform effective application-level resource management. Visible revocation allows physical names to be used easily
and permits library operating systems to choose which instance ofa specific resource to relinquish.

We use three basic techniques to implement secure bindings:
hardware mechanisms, software caching, and downloading application code.
Appropriate hardware support allows secure bindings to becouched as low-level protection operations such that later operations can be efficiently checked without recourse to high-levelauthorization information. For example, a file server can buffer
data in memory pages and grant access to authorized applications
by providing them with capabilities for the physical pages. An
exokernel would enforce capability checking without needing any
information about the file system’s authorization mechanisms. As
another example, some Silicon Graphics frame buffer hardware associates an ownership tag with each pixel. This mechanism can beused by the window manager to set up a binding between a library
operating system and a portion of the frame buffer. The application
can access the frame buffer hardware directly, because the hardware
checks the ownership tag when I/O takes place.

  An exokernel hands over resource policy decisions to library operating systems. Using this control over resources, an applicationor collection of cooperating applications can make decisions about
how best to use these resources. However, as in all systems, an exokernel must include policy to arbitrate between competing library
operating systems: it must determine the absolute importance ofdifferent applications, their share of resources, etc. This situation
is no different than in traditional kernels. Appropriate mechanisms
are determined more by the environment than by the operating
system architecture. For instance, while an exokernel cedes management of resources over to library operating systems, it controls
the allocation and revocation of these resources. By deciding which

Secure bindings can be cached in an exokernel. For instance,
an exokernel can use a large software TLB [7, 28] to cache address

4

translations that do not fit in the hardware TLB. The software TLB
can be viewed as a cache of frequently-used secure bindings.

Software support for message demultiplexing can be provided
by packet filters [37]. Packet filters can be viewed as an implementation of secure bindings in which application code—the filters—
are downloaded into the kernel. Protocol knowledge is limited tothe application, while the protection checks required to determine
packet ownership are couched in a language understood by the kernel. Fault isolation is ensured by careful language design (to boundruntime) and runtime checks (to protect against wild memory references and unsafe operations).

Secure bindings can be implemented by downloading code into
the kernel. This code is invoked on every resource access or event todetermine ownership and the actions that the kernel should perform.
Downloading code into the kernel allows an application thread ofcontrol to be immediately executed on kernel events. The advantages of downloading code are that potentially expensive crossingscan be avoided and that this code can run without requiring theapplication itself to be scheduled. Type-safe languages [9, 42], interpretation, and sandboxing [52] can be used to execute untrusted
application code safely [21].

Our prototype exokernel uses packet filters, because our current network does not provide hardware mechanisms for message
demultiplexing. One challenge with a language-based approach is
to make running filters fast. Traditionally, packet filters have beeninterpreted, making them less efficient than in-kernel demultiplexing routines. One of the distinguishing features of the packet filter
engine used by our prototype exokernel is that it compiles packet
filters to machine code at runtime, increasing demultiplexing performance by more than an order of magnitude [22].

We provide examples of each of these three techniques below
and discuss how secure bindings apply to the secure multiplexingof physical memory and network devices.

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The one problem with the use of a packet filter is ensuring thatthat a filter does not “lie” and accept packets destined to anotherprocess. Simple security precautions such as only allowing a trusted
server to install filters can be used to address this problem. On asystem that assumes no malicious processes, our language is simple
enough that in many cases even the use of a trusted server can be
avoided by statically checking a new filter to ensure that it cannot
accept packets belonging to another; by avoiding the use of any
central authority, extensibility is increased.

Secure bindings to physical memory are implemented in our prototype exokernel using self-authenticating capabilities [12] and address translation hardware. When a library operating system allocates a physical memory page, the exokernel creates a securebinding for that page by recording the owner and the read and write
capabilities specified by the library operating system. The ownerof a page has the power to change the capabilities associated with
it and to deallocate it.
To ensure protection, the exokernel guards every access to aphysical memory page by requiring that the capability be presented
by the library operating system requesting access. If the capability is
insufficient, the request is denied. Typically, the processor contains
a TLB, and the exokernel must check memory capabilities when alibrary operating system attempts to enter a new virtual-to-physicalmapping. To improve library operating system performance byreducing the number times secure bindings must be established,
an exokernel may cache virtual-to-physical mappings in a large
software TLB.

Sharing the network interface for outgoing messages is easy.
Messages are simply copied from application space into a transmit
buffer. In fact, with appropriate hardware support, transmission
buffers can be mapped into application space just as easily as physical memory pages [19].

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Using capabilities to protect resources enables applications togrant access rights to other applications without kernel intervention. Applications can also use “well-known” capabilities to shareresources easily.

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Application-specific Safe Handlers (ASHs) are a more interesting example of downloading code into our prototype exokernel.
These application handlers can be downloaded into the kernel toparticipate in message processing. An ASH is associated with apacket filter and runs on packet reception. One of the key featuresof an ASH is that it can initiate a message. Using this feature,roundtrip latency can be greatly reduced, since replies can be transmitted on the spot instead of being deferred until the applicationis scheduled. ASHs have a number of other useful features (seeSection 6).

To break a secure binding, an exokernel must change the associated capabilities and mark the resource as free. In the case ofphysical memory, an exokernel would flush all TLB mappings andany queued DMA requests.

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%In addition to implementing secure bindings, downloading code canbe used to improve performance. Downloading code into the kernel
has two main performance advantages. The first is obvious: elimination of kernel crossings. The second is more subtle: the execution
time of downloaded code can be readily bounded [18]. The crucial
importance of “tamed” code is that it can be executed when theapplication is not scheduled. This decoupling allows downloaded
code to be executed in situations where context switching to theapplication itself is infeasible (e.g., when only a few microsecondsof free processing time is available). Packet filters are an exampleof this feature: since the packet-filter runtime is bounded, the kernel
can use it to demultiplex messages irrespective of what applicationis scheduled; without a packet filter the operating system would
have to schedule each potential consumer of the packet [37].

If the underlying hardware defines a page-table interface, thenan exokernel must guard the page table instead of the TLB. Although
the details of how to implement secure memory bindings will varydepending on the details of the address translation hardware, thebasic principle is straightforward: privileged machine operations
such as TLB loads and DMA must be guarded by an exokernel. As
dictated by the exokernel principle of exposing kernel book-keeping
structures, the page table should be visible (read only) at applicationlevel.

    Multiplexing the network efficiently is challenging, since protocolspecific knowledge is required to interpret the contents of incoming
messages and identify the intended recipient.

A salient issue in downloading code is the level at which thecode is specified. High-level languages have more semantic information, which provides more information for optimizations. For
example, our packet-filter language is a high-level declarative language. As a result packet filters can be merged [56] in situations

Support for network demultiplexing can be provided either insoftware or hardware. An example of a hardware-based mechanismis the use of the virtual circuit in ATM cells to securely bind streams
to applications [19].

5One possible abort protocol is to simply kill any library operating system and its associated application that fails to respondquickly to revocation requests. We rejected this method because
we believe that most programmers have great difficulty reasoning
about hard real-time bounds. Instead, if a library operating systemfails to comply with the revocation protocol, an exokernel simply
breaks all existing secure bindings to the resource and informs thelibrary operating system.

where merging a lower-level, imperative language would be infeasible. However, in cases where such optimizations are not done,(e.g., in an exception handler) a low-level language is more in keeping with the exokernel philosophy: it allows the broadest range
of application-level languages to be targeted to it and the simplest
implementation. ASHs are another example of this tradeoff: mostASHs are imported into the kernel in the form of the object codeof the underlying machine; however, in the few key places where
higher level semantics are useful we have extended the instruction
set of the machine.

To record the forced loss of a resource, we use a repossession
vector. When an exokernel takes a resource from a library operating system, this fact is registered in the vector and the library
operating system receives a “repossession” exception so that it can
update any mappings that use the resource. For resources withstate, an exokernel can write the state into another memory or disk
resource. In preparation, the library operating system can pre-load
the repossession vector with a list of resources that can be used forthis purpose. For example, it could provide names and capabilities
for disk blocks that should be used as backing store for physicalmemory pages.

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

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Once resources have been bound to applications, there must be a way
to reclaim them and break their secure bindings. Revocation caneither be visible or invisible to applications. Traditionally, operating
systems have performed revocation invisibly, deallocating resources
without application involvement. For example, with the exceptionof some external pagers [2, 43], most operating systems deallocate(and allocate) physical memory without informing applications.
This form of revocation has lower latency than visible revocation
since it requires no application involvement. Its disadvantages arethat library operating systems cannot guide deallocation and haveno knowledge that resources are scarce.

Another complication is that an exokernel should not arbitrarilychoose the resource to repossess. A library operating system may
use some physical memory to store vital bootstrap information suchas exception handlers and page tables. The simplest way to deal withthis is to guarantee each library operating system a small numberof resources that will not be repossessed (e.g., five to ten physicalmemory pages). If even those resources must be repossessed, someemergency exception that tells a library operating system to submit
itself to a “swap server” is required.

An exokernel uses visible revocation for most resources. Even
the processor is explicitly revoked at the end of a time slice; a library
operating system can react by saving only the required processor
state. For example, a library operating system could avoid saving
the floating point state or other registers that are not live. However,since visible revocation requires interaction with a library operating
system, invisible revocation can perform better when revocations
occur very frequently. Processor addressing-context identifiers area stateless resource that may be revoked very frequently and are
best handled by invisible revocation.

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The use of physical resource names requires that an exokernel reveal each revocation to the relevant library operating system so thatit can relocate its physical names. For instance, a library operating
system that relinquishes physical page “5” should update any of its
page-table entries that refer to this page. This is easy for a library
operating system to do when it deallocates a resource in reaction toan exokernel revocation request. An abort protocol (discussed below) allows relocation to be performed when an exokernel forcibly
reclaims a resource.

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Aegis and ExOS are implemented on MIPS-based DECstations.
Aegis exports the processor, physical memory, TLB, exceptions,and interrupts. In addition, it securely exports the network interface using a packet filter system that employs dynamic code generation. ExOS implements processes, virtual memory, user-level
exceptions, various interprocess abstractions, and several network
protocols (ARP/RARP, IP, UDP, and NFS). A native extensible filesystem that implements global buffer management is under development. Currently, our prototype system has no real users, but isused extensively for development and experimentation.

We view the revocation process as a dialogue between an exokernel and a library operating system. Library operating systems
should organize resource lists so that resources can be deallocatedquickly. For example, a library operating system could have a simple vector of physical pages that it owns: when the kernel indicatesthat some page should be deallocated, the library operating systemselects one of its pages, writes it to disk, and frees it.


We have implemented two software systems that follow the exokernel architecture: Aegis, an exokernel, and ExOS, a library operating
system. Another prototype exokernel, Glaze, is being built for an
experimental SPARC-based shared-memory multiprocessor [35],along with PhOS, a parallel operating system library.

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The next three sections describe the implementation of Aegis,
ExOS, and extensions to ExOS. Included in the discussion are experiments that test the efficacy of the exokernel approach. Theseexperiments test four hypotheses:  

Exokernels can be very efficient.



Low-level, secure multiplexing of hardware resources can be
implemented efficiently.



An exokernel must also be able to take resources from library operating systems that fail to respond satisfactorily to revocation requests.
An exokernel can define a second stage of the revocation protocol
in which the revocation request (“please return a memory page”)
becomes an imperative (“return a page within 50 microseconds”).
However, if a library operating system fails to respond quickly, thesecure bindings need to be broken “by force.” The actions taken
when a library operating system is recalcitrant are defined by theabort protocol.

Traditional operating system abstractions can be implemented
efficiently at application level.
Applications can create special-purpose implementations ofthese abstractions.

On identical hardware we compare the performance of Aegis
and ExOS with the performance of Ultrix4.2, a mature monolithic
UNIX operating system. It is important to note that Aegis and6Machine
DEC2100 (12.5 MHz)
DEC3100 (16.67 MHz)
DEC5000/125 (25 MHz)

Processor
R2000
R3000
R3000

SPEC rating
8.7 SPECint89
11.8 SPECint89
16.1 SPECint92

System call
Yield
Scall
Acall
Alloc
Dealloc

MIPS
11
15
25

Description
Yield processor to named process
Synchronous protected control transfer
Asynchronous protected control transfer
Allocation of resources (e.g., physical page)
Deallocation of resources

Table 1: Experimental platforms.
Table 2: A subset of the Aegis system call interface.
ExOS do not offer the same level of functionality as Ultrix. We
do not expect these additions to cause large increases in our timing
measurements.

Primitive operations
TLBwr
FPUmodCIDswitch
TLBvadelete

The comparisons with Ultrix serve two purposes. First, theyshow that there is much overhead in today’s systems, which canbe easily removed by specialized implementations. Second, they
provide a well-known, easily-accessible point of reference for understanding Aegis’s and ExOS’s performance. Ultrix, despite its
poor performance relative to Aegis, is not a poorly tuned system; itis a mature monolithic system that performs quite well in comparison to other operating systems [39]. For example, it performs twoto three times better than Mach 3.0 in a set of I/O benchmarks [38].
Also, its virtual memory performance is approximately twice thatof Mach 2.5 and three times that of Mach 3.0 [5].

Description
Insert mapping into TLB
Enable/disable FPU
Install context identifier
Delete virtual address from TLB

Table 3: A sample of Aegis’s primitive operations.




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This section describes the implementation and performance ofAegis. The performance numbers demonstrate that Aegis and lowlevel multiplexing can be implemented efficiently. We describe indetail how Aegis multiplexes the processor, dispatches exceptions,translates addresses, transfers control between address spaces, andmultiplexes the network.

In addition, we attempt to assess Aegis’s and ExOS’s performance in the light of recent advances in operating systems research.
These advances have typically been evaluated on different hardwareand frequently use experimental software, making head-to-headcomparisons impossible. In these cases we base our comparisonson relative SPECint ratings and instruction counts.

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Table 2 lists a subset of the Aegis interface. We discuss theimplementation of most of the system calls in this section. Aegis
also supports a set of primitive operations that encapsulate privileged instructions and are guaranteed not to alter application-visible
registers (see Table 3 for some typical examples). These primitive
operations can be viewed as pseudo-instructions (similar to the Alpha’s use of PALcode [45]). In this subsection we examine how
Aegis protects time slices and processor environments; other resources are protected as described in Section 3.

Table 1 shows the specific machine configurations used in the experiments. For brevity, we refer to the DEC5000/125 as DEC5000.
The three machine configurations are used to get a tentative measure of the scalability of Aegis. All times are measured using the“wall-clock.” We used clock on the Unix implementations anda microsecond counter on Aegis. Aegis’s time quantum was set
at 15.625 milliseconds. All benchmarks were compiled using an
identical compiler and flags: gcc version 2.6.0 with optimization
flags “-O2.” None of the benchmarks use floating-point instructions; therefore, we do not save floating-point state. Both systems
were run in “single-user” mode and were isolated from the network.

The per-operation cost was obtained by repeating the operation alarge number of times and averaging. As a result, the measurements
do not consider cold start misses in the cache or TLB, and therefore
represent a “best case.” Because, Ultrix has a much larger cacheand virtual memory footprint than Aegis, this form of measurement
is more favorable to Ultrix. Because Ultrix was sensitive to theinstance of the type of machine it was run on, we took the best timemeasured. The exokernel numbers are the median of three trials.

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Aegis represents the CPU as a linear vector, where each element
corresponds to a time slice. Time slices are partitioned at the clock
granularity and can be allocated in a manner similar to physicalmemory. Scheduling is done “round robin” by cycling throughthe vector of time slices. A crucial property of this representationis position, which encodes an ordering and an approximate upperbound on when the time slice will be run. Position can be used tomeet deadlines and to trade off latency for throughput. For example,
a long-running scientific application could allocate contiguous timeslices in order to minimize the overhead of context switching, while
an interactive application could allocate several equidistant timeslices to maximize responsiveness.

A few of our benchmarks are extremely sensitive to instruction
cache conflicts. In some cases the effects amounted to a factor ofthree performance penalty. Changing the order in which ExOS’s
object files are linked was sufficient to remove most conflicts. A
happy side-effect of using application-level libraries is that object
code rearrangement is extremely straightforward (i.e., a “makefile” edit). Furthermore, with instruction cache tools, conflicts
between application and library operating system code can be removed automatically—an option not available to applications using
traditional operating systems. We believe that the large impact of
instruction cache conflicts is due to the fact that most Aegis operations are performed at near hardware speed; as a result, even minor
conflicts are noticeable.

Timer interrupts denote the beginning and end of time slices, andare delivered in a manner similar to exceptions (discussed below): a
register is saved in the “interrupt save area,” the exception program
counter is loaded, and Aegis jumps to user-specified interrupt handling code with interrupts re-enabled. The application’s handlers
are responsible for general-purpose context switching: saving andrestoring live registers, releasing locks, etc. This framework gives
applications a large degree of control over context switching. For
example, it can be used to implement scheduler activations [4].
Fairness is achieved by bounding the time an application takes tosave its context: each subsequent timer interrupt (which demarcatesa time slice) is recorded in an excess time counter. Applications pay
7for each excess time slice consumed by forfeiting a subsequent timeslice. If the excess time counter exceeds a predetermined threshold,the environment is destroyed. In a more friendly implementation,
Aegis could perform a complete context switch for the application.

Machine
DEC2100
DEC2100
DEC3100
DEC3100
DEC5000
DEC5000

This simple scheduler can support a wide range of higher-level
scheduling policies. As we demonstrate in Section 7, an application
can enforce proportional sharing on a collection of sub-processes.

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OS
Ultrix
Aegis
Ultrix
Aegis
Ultrix
Aegis

Procedure call
0.57
0.56
0.42
0.42
0.28
0.28

Syscall (getpid)
32.2
3.2 / 4.7
33.7
2.9 / 3.5
21.3
1.6 / 2.3

Table 4: Time to perform null procedure and system calls. Twonumbers are listed for Aegis’s system calls: the first for system callsthat do not use a stack, the second for those that do. Times are in
microseconds.

An Aegis processor environment is a structure that stores the information needed to deliver events to applications. All resource
consumption is associated with an environment because Aegis mustdeliver events associated with a resource (such as revocation exceptions) to its designated owner.

Machine
DEC2100
DEC2100
DEC3100
DEC3100
DEC5000
DEC5000

Four kinds of events are delivered by Aegis: exceptions, interrupts, protected control transfers, and address translations. Processor environments contain the four contexts required to support these
events:OS
Ultrix
Aegis
Ultrix
Aegis
Ultrix
Aegis

unalign
n/a
2.8
n/a
2.1
n/a
1.5

overflow
208.0
2.8
151.0
2.1
130.0
1.5

coproc
n/a
2.8
n/a
2.1
n/a
1.5

prot
238.0
3.0
177.0
2.3
154.0
1.5

Exception context: for each exception an exception context
contains a program counter for where to jump to and a pointer tophysical memory for saving registers.

Table 5: Time to dispatch an exception in Aegis and Ultrix; times
are in microseconds.

Interrupt context: for each interrupt an interrupt context includes a program counters and register-save region. In the caseof timer interrupts, the interrupt context specifies separate program
counters for start-time-slice and end-time-slice cases, as well as
status register values that control co-processor and interrupt-enableflags.

through the same fault handler as system calls. Therefore, Ultrix
must take great care not to disturb any registers that will be required
to “patch up” an interrupted TLB miss. Because Aegis does not
map its data structures (and has no page tables) it can avoid such
intricacies. We expect this to be the common case with exokernels.

Protected Entry context: a protected entry context specifies
program counters for synchronousand asynchronousprotected control transfers from other applications. Aegis allows any processor
environment to transfer control into any other; access control ismanaged by the application itself.

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1. It saves three scratch registers into an agreed-upon “savearea.” (To avoid TLB exceptions, Aegis does this operation
using physical addresses.)
2. It loads the exception program counter, the last virtual addressthat failed to have a valid translation, and the cause of the
exception.

These are the event-handling contexts required to define a process. Each context depends on the others for validity: for example,
an addressing context does not make sense without an exception
context, since it does not define any action to take when an exception or interrupt occurs.


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Aegis dispatches all hardware exceptions to applications (save forsystem calls) using techniques similar to those described in Thekkathand Levy [50]. To dispatch an exception, Aegis performs the following actions:

Addressing context: an addressing context consists of a setof guaranteed mappings. A TLB miss on a virtual address that is
mapped by a guaranteed mapping is handled by Aegis. Library
operating systems rely on guaranteed mappings for bootstrapping
page-tables, exception handling code, and exception stacks. The
addressing context also includes an address space identifier, a status
register, and a tag used to hash into the Aegis software TLB (seeSection 5.4). To switch from one environment to another, Aegis
must install these three values.



3. It uses the cause of the exception to perform an indirect jump
to an application-specified program counter value, where execution resumes with the appropriate permissions set (i.e., in
user-mode with interrupts re-enabled).

 

After processing an exception, applications can immediately resume
execution without entering the kernel. Ensuring that applications
can return from their own exceptions (without kernel intervention)requires that all exception state be available for user reconstruction.
This means that all registers that are saved must be in user-accessible
memory locations.

The base cost for null procedure and system calls are shown in
Table 4. The null procedure call shows that Aegis’s scheduling
flexibility does not add overhead to base operations. Aegis has two
system call paths: the first for system calls that do not require astack, the second for those that do. With the exception of protected
control transfers, which are treated as a special case for efficiency,
all Aegis system calls are vectored along one of these two paths.
Ultrix’s getpid is approximately an order of magnitude slower
than Aegis’s slowest system call path—this suggests that the basecost of demultiplexing system calls is significantly higher in Ultrix.
Part of the reason Ultrix is so much less efficient on this basic operation is that it performs a more expensive demultiplexing operation.
For example, on a MIPS processor, kernel TLB faults are vectored

Currently, Aegis dispatches exceptions in 18 instructions. The
low-level nature of Aegis allows an extremely efficient implementation: the time for exception dispatching on a DECstation5000/125
is 1.5 microseconds. This time is over five times faster than themost highly-tuned implementation in the literature (8 microseconds on DECstation5000/200 [50], a machine that is 1.2 faster on
SPECint92 than our DECstation5000/125). Part of the reason forthis improvement is that Aegis does not use mapped data structures,and so does not have to separate kernel TLB misses from the more8

general class of exceptions in its exception demultiplexing routine. Fast exceptions enable a number of intriguing applications:efficient page-protection traps can be used by applications such as
distributed shared memory systems, persistent object stores, andgarbage collectors [5, 50].

OS
Aegis
Aegis
Aegis
L3

Table 5 shows exception dispatch times for unaligned pointeraccesses (unalign), arithmetic overflow (overflow), attempted useof the floating point co-processor when it is disabled (coproc) andaccess to protected pages (prot). The times for unalign are notavailable under Ultrix since the kernel attempts to “fix up” an unaligned access and writes an error message to standard error. Additionally, Ultrix does not allow applications to disable co-processors,and hence cannot utilize the coproc exception. Times are given in
Table 5. In each case, Aegis’s exception dispatch times are approximately two orders of magnitude faster than Ultrix.
    Machine
DEC2100
DEC3100
DEC5000
486

MHz
12.5MHz
16.67MHz
25MHz
50MHz

Transfer cost
2.9
2.2
1.4
9.3 (normalized)

Table 6: Time to perform a (unidirectional) protected control transfer; times are in microseconds.
contrast, performing an upcall to application level on a TLB miss,followed by a system call to install a new mapping is at least three
to six microseconds more expensive.
As dictated by the exokernel principle of exposing kernel bookkeeping structures, the STLB can be mapped using a well-known
capability, which allows applications to efficiently probe for entries.

  
This section looks at two problems in supporting application-levelvirtual memory: bootstrapping and efficiency. An exokernel must
provide support for bootstrapping the virtual naming system (i.e., itmust support translation exceptions on both application page-tables
and exception code). Aegis provides a simple bootstrapping mechanism through the use of a small number of guaranteed mappings.
A miss on a guaranteed mapping will be handled automatically byAegis. This organization frees the application from dealing with
the intricacies of boot-strapping TLB miss and exception handlers,
which can take TLB misses. To implement guaranteed mappings
efficiently, an application’s virtual address space is partitioned into
two segments. The first segment holds normal application data andcode. Virtual addresses in the segment can be “pinned” using guaranteed mappings and typically holds exception handling code andpage-tables.

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Aegis provides a protected control transfer mechanism as a substrate
for efficient implementations of IPC abstractions. Operationally, a
protected control transfer changesthe program counter to an agreedupon value in the callee, donates the current time slice to the callee’s
processor environment, and installs the required elements of thecallee’s processor context (addressing-context identifier, addressspace tag, and processor status word).
Aegis provides two forms of protected control transfers: synchronous and asynchronous. The difference between the two is
what happens to the processor time slice. Asynchronous calls donate only the remainder of the current time slice to the callee. Synchronous calls donate the current time and all future instantiationsof it; the callee can return the time slice via a synchronous control transfer call back to the original caller. Both forms of control
transfer guarantee two important properties. First, to applications,a protected control transfer is atomic: once initiated it will reachthe callee. Second, Aegis will not overwrite any application-visible
register. These two properties allow the large register sets of modern
processors to be used as a temporary message buffer [14].

On a TLB miss, the following actions occur:
1. Aegis checks which segment the virtual address resides in. If
it is in the standard user segment, the exception is dispatcheddirectly to the application. If it is in the second region,
Aegis first checks to see if it is a guaranteed mapping. If so,
Aegis installs the TLB entry and continues; otherwise, Aegis
forwards it to the application.

Currently, our synchronous protected control transfer operation
takes 30 instructions. Roughly ten of these instructions are used to
distinguish the system call “exception” from other hardware exceptions on the MIPS architecture. Setting the status, co-processor, andaddress-tag registers consumes the remaining 20 instructions, andcould benefit from additional optimizations. Because Aegis implements the minimum functionality required for any control transfer
mechanism, applications can efficiently construct their own IPC
abstractions. Sections 6 and 7 provide examples.

2. The application looks up the virtual address in its page-tablestructure and, if the access is not allowed raises the appropriate exception (e.g., “segmentation fault”). If the mapping
is valid, the application constructs the appropriate TLB entry and its associated capability and invokes the appropriate
Aegis system routine.

Table 6 shows the performance in microseconds of a “barebone” protected control transfer. This time is derived by dividing
the time to perform a call and reply in half (i.e., we measure the timeto perform a unidirectional control transfer). Since the experimentis intended to measure the cost of protected control transfer only, no
registers are saved and restored. However, due to our measurement
code, the time includes the overhead of incrementing a counter andperforming a branch.

3. Aegis checks that the given capability corresponds to theaccess rights requested by the application. If it does, themapping is installed in the TLB and control is returned to theapplication. Otherwise an error is returned.
4. The application performs cleanup and resumes execution.
In order to support application-level virtual memory efficiently,
TLB refills must be fast. To this end, Aegis caches TLB entries(a form of secure bindings) in the kernel by overlaying the hardware TLB with a large software TLB (STLB) to absorb capacity
misses [7, 28]. On a TLB miss, Aegis first checks to see whether therequired mapping is in the STLB. If so, Aegis installs it and resumes
execution; otherwise, the miss is forwarded to the application.

We attempt a crude comparison of our protected control transfer
operation to the equivalent operation on L3 [33]. The L3 implementation is the fastest published result, but it runs on an Intel 486
DX-50 (50 MHz). For Table 6, we scaled the published L3 results(5 microseconds) by the SPECint92 rating of Aegis’s DEC5000
and L3’s 486 (16.1 vs. 30.1). Aegis’s trusted control transfer
mechanism is 6.6 times faster than the scaled time for L3’s RPC
mechanism.

The STLB contains 4096 entries of 8 bytes each. It is directmapped and resides in unmapped physical memory. An STLB “hit”
takes 18 instructions (approximately one to two microseconds). In
9Filter
MPF
PATHFINDERDPFClassification Time
35.0
19.0
1.5

Machine
DEC2100
DEC2100
DEC3100
DEC3100
DEC5000
DEC5000

Table 7: Time on a DEC5000/200 to classify TCP/IP headers destined for one of ten TCP/IP filters; times are in microseconds.


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pipe: measures the latency of sending a word-sized message
from one process to another using pipes by “ping-ponging” a counter
between two processes. The Ultrix pipe implementation uses standard UNIX pipes. The ExOS pipe implementation uses a sharedmemory circular buffer. Writes to full buffers and reads from empty
ones cause the current time slice to be yielded by the current process
to the reader or writer of the buffer, respectively. We use two pipe
implementations: the first is a naive implementation (pipe), while
the second (pipe’) exploits the fact that this library exists in application space by simply inlining the read and write calls. ExOS’s
unoptimized pipe implementation is an order of magnitude more
efficient than the equivalent operation under Ultrix.

We measured DPF’s time to classify packets destined for oneof ten TCP/IP filters, and compare its times to times for MPF [56](a widely used packet filter engine) and PATHFINDER [6] (thefastest packet filter engine in the literature). To ensure meaningfulcomparisons between the systems, we ran our DPF experiments on
the same hardware (a DECstation 5000/200) in user space. Table 7
presents the time to perform this message classification; it was derived from the average of one million trials. This experiment and thenumbers for both MPF and PATHFINDER are taken from [6]. On
this experiment DPF is 20 times faster than MPF and 10 times fasterthan PATHFINDER. The bulk of this performance improvement isdue to the use of dynamic code generation,


lrpc
n/a
13.9
n/a
10.4
n/a
6.3

Fast interprocess communication is crucial for building efficient anddecoupled systems [8, 27, 33]. As described in Section 5, the Aegis
protected control transfer mechanism is an efficient substrate for
implementing IPC abstractions. This section describes experiments
used to measure the performance of ExOS’s IPC abstractions on
top of the Aegis primitives. The results of these experiments are
summarized in Table 8. The experiments are:

The key in our approach to making filters run fast is dynamic
code generation. Dynamic code generation is the creation of executable code at runtime. DPF exploits dynamic code generation in
two ways: (1) by using it to eliminate interpretation overhead bycompiling packet filters to executable code when they are installed
into the kernel and (2) by using filter constants to aggressively
optimize this executable code. To gain portability, DPF compiles
filters using VCODE, a portable, very fast, dynamic code generation system [20]. VCODE generates machine code in approximately
10 instructions per generated instruction and runs on a number ofmachines (e.g., MIPS, Alpha and SPARC).



shm
187.0
12.4
139.0
9.3
118.0
5.7

The most unusual aspect of ExOS is that it manages fundamental
operating system abstractions (e.g., virtual memory and process)at application level, completely within the address space of theapplication that is using it. This section demonstrates that basic
system abstractions can be implemented at application level in a
direct and efficient manner. Due to space constraints we focus on
IPC, virtual memory, and remote communication.

Message demultiplexing is the process of determining whichapplication an incoming message should be delivered to. Packet
filters are a well-known technique used to implement extensible
kernel demultiplexing [6, 56]. Traditionally, packet filters are interpreted, which entails a high computational cost. Aegis uses
Dynamic Packet Filter (DPF), a new packet filter system that isover an order of magnitude more efficient than previous systems.



pipe’
n/a
24.8
n/a
18.6
n/a
10.7



Aegis’s network subsystem uses aggressive dynamic code generation techniques to provide efficient message demultiplexing andhandling. We briefly discuss some key features of this system. A
complete discussion can be found in [22].



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pipe
326.0
30.9
243.0
22.6
199.0
14.2

Table 8: Time for IPC using pipes, shared memory, and LRPC
on ExOS and Ultrix; times are in microseconds. Pipe and sharedmemory are unidirectional, while LRPC is bidirectional.

Architectural characteristics of the Intel 486 partially accountfor Aegis’s better performance. L3 pays a heavy penalty to enterand leave the kernel (71 and 36 cycles, respectively) and must flush
the TLB on a context switch.
OS
UltrixExOS
UltrixExOS
UltrixExOS

shm: measures the time for two processes to “ping-pong” usinga shared counter. ExOS uses Aegis’s yield system call to switch
between partners. Ultrix does not provide a yield primitive, so
we synthesized it using signals. ExOS’s shm is 15 to 20 times fasterthan Ultrix’s shm. ExOS’s shm is about twice as fast as its pipe
implementation, which must manipulate circular buffers.
lrpc: this experiment measures the time to perform an LRPC
into another address space, increment a counter and return its value.
ExOS’s LRPC is built on top of Aegis’s protected control transfer
mechanism. lrpc saves all general-purpose callee-saved registers.
The lrpc implementation assumes that only a single function is ofinterest (e.g., it does not use the RPC number to index into a table)
and it does not check permissions. The implementation is also
single-threaded.

The main conclusion we draw from these experiments is that an
exokernel can be implemented efficiently. The reasons for Aegis’s
good performance are the following. One, keeping track of ownership is a simple task and can therefore be implemented efficiently.
Two, since the kernel provides very little functionality beyond lowlevel multiplexing, it is small and lean: for instance, it keeps its
data structures in physical memory. Three, by caching secure bindings in a software TLB, most hardware TLB misses can be handled
efficiently. Four, by downloading packets filters and by employing
dynamic code generation, secure binding to the network can be
implemented efficiently.

Because Ultrix is built around a set of fixed high-level abstractions, new primitives can be added only by emulating them on top ofexisting ones. Specifically, implementations of lrpc must use pipes
or signals to transfer control. The cost of such emulation is high:on Ultrix, lrpc using pipes costs 46 to 60 more than on ExOS andusing signals costs 26 to 37 more than on ExOS. These experiments

10Machine
DEC2100
DEC2100
DEC3100
DEC3100
DEC5000
DEC5000

OS
UltrixExOS
UltrixExOS
UltrixExOS

can be eliminated by implementing abstractions at application level.
Second, application-level software can implement functionality thatis frequently not provided by traditional operating systems.

matrix
7.1
7.0
5.2
5.2
3.8
3.7

If we compare the time for dirty to the time for prot1, we seethat over half the time spent in prot1 is due to the overhead ofparsing the page table. As we show in Section 7.2, this overhead
can be reduced through the use of a data structure more tuned toefficient lookup (e.g., a hash table). Even with this penalty, ExOS
performs prot1 almost twice as fast as Ultrix. The likely reason forthis difference is that, as shown in Table 4, Aegis dispatches system
calls an order of magnitude more efficiently than Ultrix.

Table 9: Time to perform a 150x150 matrix multiplication; time inseconds.

In general, our exokernel-based system performs well on thisset of benchmarks. The exceptions are prot100 and unprot100.
Ultrix is extremely efficient in protecting and unprotecting contiguous ranges of virtual addresses: it performs these operations 1.1
to 1.6 times faster than Aegis. One reason for this difference is
the immaturity of our implementation. Another is that changing
page protections in ExOS requires access to two data structures(Aegis’s STLB and ExOS’s page-table). However, even with poor
performance on these two operations, the benchmark that uses thisoperation (appel2) is close to an order of magnitude more efficienton ExOS than on Ultrix. In fact, we can expect further improvements in performance from more sophisticated page-table structuresand hand-coded assembly language for some operations. The useof a high-level language (C) to handle exceptions adds overhead forsaving and restoring all caller-saved registers when a trap handler
starts and returns.

indicate that an Ultrix user either pays substantially in performancefor new functionality, or is forced to modify the kernel.
    %  #$

 




)

 

ExOS provides a rudimentary virtual memory system (approximately 1000 lines of heavily commented code). Its two main limitations are that it does not handle swapping and that page-tables are
implemented as a linear vector (address translations are looked up
in this structure using binary search). Barring these two limitations,
its interface is richer than other virtual memory systems we know
of. It provides flexible support for aliasing, sharing, disabling andenabling of caching on a per-page basis, specific page-allocation,
and DMA.
The overhead of application-level memory is measured by performing a 150 by 150 integer matrix multiplication. Because thisnaive version of matrix multiply does not use any of the special
abilities of ExOS or Aegis (e.g., page-coloring to reduce cache
conflicts), we expect it to perform equivalently on both operating
systems. The times in Table 9 indicate that application-level virtual
memory does not add noticeable overhead to operations that havereasonable virtual memory footprints. Of course, this is hardly aconclusive proof.

        




%



 



  ASHExOS operates efficiently in spite of executing at application levelin part because the cost of crossing between kernel and user space isextremely low in our prototype (18 instructions). Most applicationspecific optimizations can therefore be implemented in libraries atapplication level. However, in the context of networking, there are
two reasons for ExOS to download code: the first is technology
driven, while the second is more fundamental. First, the network
buffers on our machines cannot be easily mapped into application
space in a secure way. Therefore, by downloading code into thekernel, applications can integrate operations such as checksumming during the copy of the message from these buffers to user
space. Such integration can improve performance on a DECstation5000/200 by almost a factor of two [22]. Second, if the runtimeof downloaded code is bounded, it can be run in situations when
performing a full context switch to an unscheduled application is
impractical. Downloading code thus allows applications to decouple latency-critical operations such as message reply from process
scheduling.

Table 10 compares Aegis and ExOS to Ultrix on seven virtual
memory experiments based on those used by Appel and Li [5].
These experiments are of particular interest, since they measure
the cost of VM operations that are crucial for the construction of
ambitious systems, such as page-based distributed shared memory
systems and garbage collectors. Note that Ultrix’s VM performance is quite good compared to other systems [5]. The operations
measured are the following:
dirty: time to query whether a page is “dirty.” Since it does not
require examination of the TLB, this experiment measures the basecost of looking up a virtual address in ExOS’s page-table structure.
This operation is not provided by Ultrix.
prot1: time to change the protection of a single page.

We examine these issues using application-specific handlers(ASHs). ASHs are untrusted application-level message-handlersthat are downloaded into the kernel, made safe by a combinationof code inspection [18] and sandboxing [52], and executed uponmessage arrival. The issues in other contexts (e.g., disk I/O) aresimilar.

prot100: time to “read-protect” 100 pages.
unprot100: time to remove read-protections on 100 pages.
trap: time to handle a page-protection trap.
appel1: time to access a random protected page and, in thefault handler, protect some other page and unprotect the faulting
page (this benchmark is “prot1+trap+unprot” in Appel and Li [5]).

An ASH can perform general computation. We have augmented
this ability with a set of message primitives that enable the following
four useful abilities:

appel2: time to protect 100 pages, then access each page in arandom sequence and, in the fault-handler, unprotect the faulting
page (this benchmark is “protN+trap+unprot” in Appel and Li [5]).
Note that appel2 requires less time than appel1 since appel1 must
both unprotect and protect different pages in the fault handler.

1. Direct, dynamic message vectoring. An ASH controls where
messages are copied in memory, and can therefore eliminate
all intermediate copies, which are the bane of fast networking
systems.

The dirty benchmark measures the average time to parse thepage-table for a random entry. This operation illustrates two consequences of the exokernel architecture. First, kernel transitions

2. Dynamic integrated layer processing (ILP) [1, 16]. ASHs
can integrate data manipulations such as checksumming and11Machine
DEC2100
DEC2100
DEC3100
DEC3100
DEC5000
DEC5000

OS
UltrixExOS
UltrixExOS
Ultrix
ExOS

dirty
n/a
17.5
n/a
13.1
n/a
9.8

prot1
51.6
32.5
39.0
24.4
32.0
16.9

prot100
175.0
213.0
133.0
156.0
102.0
109.0

unprot100
175.0
275.0
133.0
206.0
102.0
143.0

trap
240.0
13.9
185.0
10.1
161.0
4.8

appel1
383.0
74.4
302.0
55.0
262.0
34.0

appel2
335.0
45.9
267.0
34.0
232.0
22.0

Table 10: Time to perform virtual memory operations on ExOS and Ultrix; times are in microseconds. The times for appel1 and appel2 areper page.

OS
ExOS/ASH
ExOS
UltrixUltrix/FRPC

3500

Roundtrip latency
259
320
3400
340

Roundtrip Latency (microseconds)

Machine
DEC5000/125
DEC5000/125
DEC5000/125
DEC5000/200

Table 11: Roundtrip latency of a 60-byte packet over Ethernet using
ExOS with ASHs, ExOS without ASHs, Ultrix, and FRPC; times
are in microseconds.
conversion into the data transfer engine itself. This integration is done at the level of pipes. A pipe is a computationthat acts on streaming data. Pipes contain sufficient semantic
information for the ASH compiler to integrate several pipes
into the message transfer engine at runtime, providing a large
degree of flexibility and modularity. Pipe integration allows
message traversals to be modularly aggregated to a single
point in time. To the best of our knowledge, ASH-based
ILP is the first to allow either dynamic pipe composition orapplication-extended in-kernel ILP.

3250

ExOS with ASH
ExOS without ASH

3000
2750
2500
2250
2000
1750
1500
1250
1000
750
500
250
0
1

2

3

4

5

6

7

8

9

10

Number of Processes
Figure 2: Average roundtrip latency with increasing number ofactive processes on receiver.

3. Message initiation. ASHs can initiate message sends, allowing for low-latency message replies.
Despite being measured on a slower machine, ExOS/ASH is
81 microseconds faster than a high-performance implementation of
RPC for Ultrix (FRPC) running on DECstation5000/200s and using
a specialized transport protocol [49]. In fact, ExOS/ASH is only 6
microseconds slower than the lower bound for cross-machine communication on Ethernet, measured on DECstation5000/200s [49].

4. Control initiation. ASHs perform general computation. Thisability allows them to perform control operations at message
reception time, implementing such computational actions as
traditional active messages [51] or remote lock acquisition.
It is important to note the power of the ASH computationalmodel. It allows the vectoring process to be completely dynamic:
the application does not have to pre-specify that it is waiting fora particular message, nor does it have to pre-bind buffer locations
for the message. Instead, it can defer these decisions until message
reception and use application-level data structures, system state,
and/or the message itself to determine where to place the message.
Capturing the same expressiveness within a statically defined protocol is difficult.

ASHs can be used to decouple latency-critical operations suchas message reply from the scheduling of processes. To measure
the impact of this decoupling on average message roundtrip latency
we performed the same experiment as above while increasing thenumber of active processes on the receiving host (see Figure 2).
With ASHs, the roundtrip latency stays constant. Without them,
the latency increases, since the reply can be sent only when the application is scheduled. As the number of active processes increases,
it becomes less likely that the process is scheduled. Since processes
are scheduled in “round-robin” order, latency increases linearly. On
Ultrix, the increase in latency was more erratic, ranging from .5 to
4.5 milliseconds with 10 active processes. While the exact rate that
latency increases will vary depending on algorithm used to schedule processes, the implication is clear: decoupling actions such as
message reception from scheduling of a process can dramatically
improve performance.

Table 11 shows the roundtrip latency over Ethernet of ASHbased network messaging and compares it to ExOS without ASHs,
Ultrix, and FRPC [49] (the fastest RPC in the literature on comparable hardware). Roundtrip latency for Aegis and Ultrix was measuredby ping-ponging a counter in a 60-byte UDP/IP packet 4096 times
between two processes in user-space on DECstation5000/125s. The
FRPC numbers are taken from the literature [49]. They were measured on a DECstation5000/200, which is approximately 1.2 times
faster than a DECstation5000/125 on SPECint92.

The message processing at each node consisted of reading the60-byte message, incrementing the counter, copying the new value
and a precomputed message header into a transmission buffer, andthen sending the reply message. In comparison to a complete
application-level implementation, ASHs save 61 microseconds.

     




%



Library operating systems, which work above the exokernel interface, implement higher-level abstractions and can define specialpurpose implementations that best meet the performance and functionality goals of applications. We demonstrate the flexibility of the

12

lrpc
13.9
10.4
6.3

2500

tlrpc
8.6
6.4
2.9

Cumulative Iterations (thousands)

Machine
DEC2100
DEC3100
DEC5000

Table 12: Time to perform untrusted (lrpc) and trusted (tlrpc)
LRPC extensions; times are in microseconds.
exokernel architecture by showing how fundamental operating system abstractions can be redefined by simply changing applicationlevel libraries. We show that these extensions can have dramatic
performance benefits. These different versions of ExOS can coexist on the same machine and are fully protected by Aegis.

 


%



   




Ideal
Measured

2250
2000
1750
1500
1250
1000
750
500
250
0
0

Most RPC systems do not trust the server to save and restore
registers [27]. We implemented a version of lrpc (see Section 6.1)
that trusts the server to save and restore callee-saved registers. We
call this version tlrpc (trusted LRPC). Table 12 compares tlrpc to
ExOS’s more general IPC mechanism,lrpc, which saves all generalpurpose callee-saved registers. Both implementations assume that
only a single function is of interest (e.g., neither uses the RPC
number to index into a table) and do not check permissions. Both
implementations are also single-threaded. The measurements showthat this simple optimization can improve performance by up to afactor of two.




%

   




 

$%'



%

   




#$



%

30

40

50

60

70

80

90 100

It is important to note that there is nothing special about thisscheduler either in terms of privileges (any application can perform
identical actions) or in its complexity (the entire implementation isless than 100 lines of code). As a result, any application can easily
manage processes. An important use of such fine-grained control
is to enhance the modularity of application design: previously,
applications that had subtasks of different/fluctuating priorities had
to internalize them in the form of schedulable threads. As a result,the likelihood of software errors increased, and the complexityof the design grew. By constructing a domain-specific scheduler,these applications can now effectively and accurately schedule subprocesses, greatly improving fault isolation and independence.

   

We made a new version of ExOS that supports inverted pagetables. Applications that have a dense address space can use linear
page tables, while applications with a sparse address space can use
inverted ones. Table 13 shows the performance for this new version
of ExOS. The inverted page-table trades the performance of modifying protection on memory regions for the performance of fasterlookup. On the virtual memory benchmarks of Section 6.2, it isover a factor of two more efficient on dirty, 37% faster on appel1,
and 17% faster on appel2. Because VM is implemented at application level, applications can make such tradeoffs as appropriate.
This experiment emphasizes the degree of flexibility offered by an
exokernel architecture.



20Time (quanta)
Figure 3: Application-level stride scheduler.




10

   $

Many early operating system papers discussed the need for extendible, flexible kernels [32, 42]. Lampson’s description of CALTSS [31] and Brinch Hansen’s microkernel paper [24] are two classic rationales. Hydra was the most ambitious early system to havethe separation of kernel policy and mechanism as one of its central
tenets [55]. An exokernel takes the elimination of policy one stepfurther by removing “mechanism” wherever possible. This process
is motivated by the insight that mechanism is policy, albeit with one
less layer of indirection. For instance, a page-table is a very detailed
policy that controls how to translate, store and delete mappings andwhat actions to take on invalid addresses and accesses.



Aegis includes a yield primitive to donate the remainder of a process’ current time slice to another (specific) process. Applications
can use this simple mechanism to implement their own schedulingalgorithms. To demonstrate this, we have built an application-level
scheduler that implements stride scheduling [54], a deterministic,
proportional-share scheduling mechanism that improves on recent
work [53]. The ExOS implementation maintains a list of processes
for which it is responsible, along with the proportional share they
are to receive of its time slice(s). On every time slice wakeup, thescheduler calculates which process is to be scheduled and yields toit directly.

VM/370 [17] exports the ideal exokernel interface: the hardware interface. On top of this hardware interface, VM/370 supports
a number of virtual machines on top of which radically different
operating systems can be implemented. However, the importantdifference is that VM/370 provides this flexibility by virtualizing
the entire base-machine. Since the base machine can be quite complicated, virtualization can be expensive and difficult. Often, thisapproach requires additional hardware support [23, 40]. Additionally, since much of the actual machine is intentionally hidden fromapplication-level software, such software has little control over theactual resources and may manage the virtual resources in a counterproductive way. For instance, the LRU policy of pagers on top of
the virtual machine can conflict with the paging strategy used by
the virtual machine monitor [23]. In short, while a virtual machinecan provide more control than many other operating systems, application performance can suffer and actual control is lacking in key
areas.

We measure the effectiveness of this scheduler by creating three
processes that increment counters in shared memory. The processes
are assigned a 3:2:1 relative allocation of the scheduler’s time slice
quanta. By plotting the cumulative values of the shared counters,
we can determine how closely this scheduling allocation is realized.
As can be seen in Figure 3, the achieved ratios are very close toidealized ones.

13

Machine
DEC2100
DEC2100
DEC3100
DEC3100

MethodOriginal page-table
Inverted page-table
Original page-table
Inverted page-table

dirty
17.5
8.0
13.1
5.9

prot1
32.5
23.1
24.4
17.7

prot100
213.
253.
156.
189.

unprot100
275.
325.
206.
243.

trap
13.9
13.9
10.1
10.1

appel1
74.4
54.4
55.0
40.4

appel2
45.9
38.8
34.0
28.9

Table 13: Time to perform virtual memory operations on ExOS using two different page-table structures; times are in microseconds.
Modern revisitations of microkernels have argued for kernel extensibility [2, 43, 48]. Like microkernels, exokernels are designed
to increase extensibility. Unlike traditional microkernels, an exokernel pushes the kernel interface much closer to the hardware, whichallows for greater flexibility. An exokernel allows application-level
libraries to define virtual memory and IPC abstractions. In addition,
the exokernel architecture attempts to avoid shared servers (especially trusted shared servers), since they often limit extensibility.
For example, it is difficult to change the buffer management policyof a shared file server. In many ways, servers can be viewed as fixedkernel subsystems that run in user-space. Some newer microkernels push the kernel interface closer to the hardware [34], obtaining
better performance than previous microkernels. However, sincethese systems do not employ secure bindings, visible resource revocation, and abort protocols, they give less control of resources toapplication-level software.

   
#In the exokernel architecture, an exokernel securely multiplexes
available hardware resources among applications. Library operating systems, which work above the low-level exokernel interface,
implement higher-level abstractions and can define special-purpose
implementations that best meet the performance and functionality goals of applications. The exokernel architecture is motivatedby a simple observation: the lower the level of a primitive, themore efficiently it can be implemented, and the more latitude it
grants to implementors of higher-level abstractions. To achievea low-level interface, the exokernel separates management fromprotection. To make this separation efficient it uses secure bindings, implemented using hardware mechanisms, software caches,or downloading code.
Experiments using our Aegis and ExOS prototypes demonstrate
our four hypotheses. First, the simplicity and limited number of exokernel primitives allows them to be implemented very efficiently.
Measurements of Aegis show that its basic primitives are substantially more efficient than the general primitives provided by Ultrix.
In addition, Aegis’s performance is better than or on par with recent high-performance implementations of exceptions dispatch andcontrol transfer primitives.

The SPIN project is building a microkernel system that allows
applications to make policy decisions [9] by safely downloading
extensions into the kernel. Unlike SPIN, the focus in the exokernel architecture is to obtain flexibility and performance by securely
exposing low-level hardware primitives rather than extending a traditional operating system in a secure way. Because the exokernel
low-level primitives are simple compared to traditional kernel interfaces, they can be made very fast. Therefore, the exokernel has
less use for kernel extensions.

Second, because exokernel primitives are fast, low-level secure
multiplexing of hardware resources can be implemented efficiently.
For example, Aegis multiplexes resources such as the processor,
memory, and the network more efficiently than state-of-the-art implementations.

Scout [25] and Vino [46] are other current extensible operating
systems. These systems are just beginning to be constructed, so it
is difficult to determine their relationship to exokernels in generaland Aegis in particular.

Third, traditional operating system abstractions can be implemented efficiently at application level. For instance, ExOS’s
application-level VM and IPC primitives are much faster than Ultrix’s corresponding primitives and than state-of-the-art implementations reported in the literature.

SPACE is a “submicro-kernel” that provides only low-level kernel abstractions defined by the trap and architecture interface [41].
Its close coupling to the architecture makes it similar in many ways
to an exokernel, but we have not been able to make detailed comparisons because its design methodology and performance have notyet been published.

Fourth, applications can create special-purpose implementations of abstractions by merely modifying a library. We implemented several variations of fundamental operating system abstractions such as interprocess communication, virtual memory, andschedulers with substantial improvements in functionality and performance. Many of these variations would require substantial kernel
alternations on today’s systems.

Anderson [3] makes a clear argument for application-specificlibrary operating systems and proposes that the kernel concentratesolely on the adjudication of hardware resources. The exokernel
design addresses how to provide secure multiplexing of physicalresources in such a system, and moves the kernel interface to a lower
level of abstraction. In addition, Aegis and ExOS demonstrate that
low-level secure multiplexing and library operating systems canoffer excellent performance.

Based on the results of these experiments, we conclude that theexokernel architecture is a viable structure for high-performance,
extensible operating systems.

Like Aegis, the Cache Kernel [13] provides a low-level kernelthat can support multiple application-level operating systems. To
the best of our knowledge ExOS and the Cache Kernel are thefirst general-purpose library operating systems implemented in a
multiprogramming environment. The difference between the Cache
Kernel and Aegis is mainly one of high-level philosophy. The
Cache Kernel focuses primarily on reliability, rather than securely
exporting hardware resources to applications. As result, it is biased
towards a server-based system structure. For example, it supports
only 16 “application-level” kernels concurrently.



$

 




)



We thank Henri Bal, Robert Bedichek, Matthew Frank, Greg Ganger,
Bob Gruber, Sandeep Gupta, Wilson Hsieh, Kirk Johnson, Butler Lampson, Ulana Legedza, Hank Levy (our shepherd), David
Mosberger-Tang, Massimiliano Poletto, Robbert van Renesse, Satya(M. Satyanarayanan), Raymie Stata, Carl Waldspurger, and Deborah Wallach for insightful discussions and careful reading of earlier
versions of this paper. We also thank the anonymous referees fortheir valuable feedback. We thank Jochen Lietdke for his aid in14

[13] D. Cheriton and K. Duda. A caching model of operating
system kernel functionality. In Proceedings of the First Symposium on Operating Systems Design and Implementation,
pages 179–193, November 1994.

comparing the IPC mechanisms of Aegis and L3. We thank Ken
Mackenzie for many insights and discussions. In addition, we thank
Hector Briceno for porting NFS and SUN RPC; Sandeep Gupta fordeveloping an inverted page-table; Robert Grimm for porting adisk driver; and Tom Pinckney for porting gdb and developing anapplication-level file system. Finally, we thank Deborah Wallach
for her input on the design of ASHs, the software she developed toevaluate them, and porting Aegis to the DECstation5000. Her help
was invaluable.



'

(

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