Abstract:
A system and method for dynamic preloading of classes through memory space cloning of a master runtime system process is presented. A master runtime system process is executed. A representation of at least one class is obtained from a source definition provided as object-oriented program code. The representation is interpreted and instantiated as a class definition in a memory space of the master runtime system process. The memory space is cloned as a child runtime system process responsive to a process request and the child runtime system process is executed, inheriting the memory state of the parent, which reflects the data structures and state corresponding to the preloaded classes.

Description:
FIELD OF THE INVENTION 
   The invention relates in general to class preloading and, in particular, to a system and method for dynamic preloading of classes through memory space cloning of a master runtime system process. 
   BACKGROUND OF THE INVENTION 
   Recent advances in microprocessor design and component integration have enabled a wide range of devices to offer increasingly complex functionality and “soft” features. Soft features include software applications that enhance and customize the operation of a device. These devices include standard computing devices, such as desktop and laptop computers, portable computing devices, such as personal data assistants, and consumer devices, such as cellular telephones, messaging pagers, gaming consoles, and set top boxes. Most devices now include an operating system to support the soft features and other extensions. 
   The increased capabilities offered by these software-upgradeable devices have also created certain user expectations. Often, users are not technically savvy and are intolerant of performance compromises occasioned by architectural challenges, such as slow or inconsistent application performance. Similarly, users generally expect to be able to access a host of separate applications, which are implemented at the system level through multitasking. For users, widely available software applications assure a positive experience through consistency and increased exposure across multiple platforms. However, for software developers, engineering software applications for disparate computing platforms entails increased development costs and on-going support and upgrade commitments for each supported architecture. 
   Managed code platforms provide one solution to software developers seeking to support multiple platforms by presenting a machine-independent and architecture-neutral operating environment. Managed code platforms include programming language compilers and interpreters executed by an operating system as user applications, but which provide virtual runtime environments within which compatible applications can operate. For instance, applications written in the Java programming language, when combined with a Java virtual machine (JVM) runtime environment, can operate on heterogeneous computer systems independent of machine-specific environment and configuration settings. An overview of the Java programming language is described in P. van der Linden, “Just Java,” Ch. 1, Sun Microsystems, Inc. (2d ed. 1997), the disclosure of which is incorporated by reference. JVMs are a critical component to the overall Java operating environment, which can be ported to the full range of computational devices, including memory-constrained consumer devices. 
   Managed code platforms are generally designed for the monotonic execution of a single application instance. Multiple instances of a managed code platform are executed to simulate multitasking behavior. Such forced concurrency, however, creates several performance problems. First, each instance incurs a startup transient. Executable and startup data must be read from slow persistent storage, which results in slow initial application performance. Similarly, memory is not shared between instances and each additional instance increases the overall memory footprint of the platform by separately loading and instantiating classes, generally problematic in memory-constrained systems. Moreover, data dependencies and deferred initialization of system state can result in non-deterministic execution patterns. Finally, each instance independently determines the relative importance of executing methods and compiles machine code on an ad hoc basis, often causing inconsistent application performance. 
   Deferred class loading is sometimes necessitated by the dynamic nature of the object oriented languages involved. Dynamic class loading can also adversely affect performance and cause nondeterministic execution behavior. To help improve runtime performance, managed code platforms lazily defer class loading until a class is actually referenced. Deferred class loading conserves the time required to load a class by delaying class loading and compilation until, and if, the class is actually needed. Deferred class loading sacrifices runtime performance for improved application startup. However, for near real time applications, deferred class loading causes non-deterministic execution behavior that increases worst case performance by the longest class loading execution thread. Similarly, deferred class loading exacerbates the resource usage of multiple application instances that each requires the same classes by duplicatively performing identical operations and needlessly consuming memory that could be conserved, if the memory state were shared. 
   Static preloading of classes and interfaces is currently supported in many Java virtual machines, which allows a build-time tool to pre-process and preload classes and to link the classes into the JVM static executable image before JVM startup. However, static preloading can result in large executable sizes and can be problematic for resource constrained devices, where boot startup time is critical and a combination of slower processor and persistent storage and modest memory can cause significant boot times. 
   Therefore, there is a need for an approach to providing class preloading in a managed code platform, such as the Java operating environment, to provide concurrently executable applications that share warmed up memory state and to minimize worst case performance. 
   SUMMARY OF THE INVENTION 
   A managed code platform is executed in an application framework that supports the spawning of multiple and independent isolated user applications. Preferably, the application framework supports the cloning of the memory space of each user application using copy-on-write semantics. The managed code platform includes a master runtime system process, such as a virtual machine, to interpret machine-portable code defining compatible applications. An application manager also executes within the application framework and is communicatively interfaced to the master runtime system process through an inter-process communication mechanism. The application framework logically copies the master runtime system process context upon request by the application framework to create a child runtime system process through process cloning. The context of the master runtime system process stored in memory is inherited by the child runtime system process as prewarmed state and cached code. When implemented with copy-on-write semantics, the process cloning creates a logical copy of references to the master runtime system process context. Segments of the referenced master runtime system process context are lazily copied only upon an attempt by the child runtime system process to modify the referenced context. During initialization, the master runtime system process preloads classes and interfaces likely to be required by user applications at runtime. The classes and interfaces are identified through profiling by ranking a set of classes according to a predetermined criteria, such as described in commonly-assigned U.S. patent application Ser. No. 09/970,661, filed Oct. 5, 2001, pending, the disclosure of which is incorporated by reference. An example of a suitable managed code platform and runtime system process are the Java operating environment and Java virtual machine (JVM) architecture, as licensed by Sun Microsystems, Inc., Palo Alto, Calif. 
   One embodiment provides a system and method for dynamic preloading of classes through memory space cloning of a master runtime system process. A master runtime system process is executed. A representation of at least one class is obtained from a source definition provided as object-oriented program code. The representation is interpreted and instantiated as a class definition in a memory space of the master runtime system process. The memory space is cloned as a child runtime system process responsive to a process request and the child runtime system process is executed. 
   The use of the process cloning mechanism provided by the underlying application framework provides several benefits in addition to resolving the need for efficient concurrent application execution of machine portable code. The inheritance of prewarmed state through the cloning of the master runtime process context provides inter-process sharing of preloaded classes. Similarly, each child runtime system process executes in isolation of each other process, thereby providing strong resource control through the system level services of the application framework. Isolation, reliable process invocation and termination, and resource reclamation are available and cleanly provided at an operating system level. In addition, process cloning provides fast user application initialization and deterministic runtime behavior, particularly for environments providing process cloning with copy-on-write semantics. Finally, for non-shareable segments of the master runtime system process context, actual copying is deferred until required through copy-on-write semantics, which avoids impacting application performance until, and if, the segment is required. 
   Still other embodiments of the invention will become readily apparent to those skilled in the art from the following detailed description, wherein are described embodiments of the invention by way of illustrating the best mode contemplated for carrying out the invention. As will be realized, the invention is capable of other and different embodiments and its several details are capable of modifications in various obvious respects, all without departing from the spirit and the scope of the invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a functional block diagram showing, by way of example, runtime environments implemented on a plurality of heterogeneous devices. 
       FIG. 2  is a block diagram showing a system for dynamic preloading of classes through memory space cloning of a master runtime system process, in accordance with the invention. 
       FIG. 3  is a block diagram showing, by way of example, a master JVM process mapped into memory. 
       FIG. 4  is a block diagram showing, by way of example, a master JVM process and a cloned JVM process mapped into memory through memory space cloning. 
       FIGS. 5A-B  are block diagrams showing, by way of example, a master JVM process and a cloned JVM process mapped into memory through memory space cloning with copy-on-write semantics. 
       FIG. 6  is a flow diagram showing a method for dynamic preloading of classes through memory space cloning of a master runtime system process, in accordance with the invention. 
       FIG. 7  is a flow diagram showing the routine for loading a master JVM process for use in the method of  FIG. 6 . 
       FIG. 8  is a flow diagram showing the routine for cloning a process without copy-on-write semantics for use in the method of  FIG. 6 . 
       FIG. 9  is a flow diagram showing the routine for cloning a process with copy-on-write semantics for use in the method of  FIG. 6 . 
       FIG. 10  is a flow diagram showing the routine for preloading a class for use in the routine of  FIG. 7 . 
   

   DETAILED DESCRIPTION 
   System Overview 
     FIG. 1  is a functional block diagram  10  showing, by way of example, runtime environments (RTEs)  14 ,  22 ,  24 ,  26  implemented on a plurality of heterogeneous devices  11 . Each heterogeneous device  11  provides a managed code platform, such as the Java operating environment, executing in a runtime environment  14 ,  22 ,  24 ,  26 , as further described below with reference to  FIG. 2 . The heterogeneous devices  11  include, nonexclusively, a client computer system  13 , such as a desktop or laptop computer system. Each client  13  is operatively coupled to a storage device  15  and maintains a set of classes  16  and class libraries  17 , which respectively define code modules that specify data structures and sets of methods that operate on the data, and shareable collections of the modules. The heterogeneous devices  11  also include portable computing devices, including personal data assistants  21 , and consumer devices, such as cellular telephones  23  and set top boxes (STB)  25 . Finally, a server  18  is operatively coupled to a storage device  19  in which globally shareable class libraries  20  are maintained. Each of the heterogeneous devices  11  can interface via a network  12 , which includes conventional hardwired and wireless network configurations. Other types of heterogeneous devices  11  and various network configurations, arrangements, and topologies are possible. 
   Each heterogeneous device  11  includes an operating system to manage resources, provide access to peripheral devices, allocate memory resources, and control program execution and termination. Each operating system supports a process cloning mechanism that spawns multiple and independent isolated user applications by cloning the memory space of specifiable processes. An example of a process cloning mechanism suitable for use in the present invention is the fork( ) system call provided by the Unix or Linux operating systems, such as described in M. J. Bach, “The Design Of The Unix Operating System,” Ch.  7 , Bell Tele. Labs., Inc. (1986), the disclosure of which is incorporated by reference. The process invoking the fork( ) system call is known as the parent process and the newly created process is called the child process. The operating system assigns a separate process identifier to the child process, which executes as a separate process. The operating system also creates a logical copy of the context of the parent process by copying the memory space of the parent process into the memory space of the child process. In a copy-on-write variant of the fork( ) system call, the operating system only copies references to the memory space and defers actually copying individual memory space segments until, and if, the child process attempts to modify the referenced data of the parent process context. The copy-on-write fork( ) system call is faster than the non-copy-on-write fork( ) system call and implicitly shares any data not written into between the parent and child processes. 
   System for Preloading Classes 
     FIG. 2  is a block diagram  30  showing a system for dynamic preloading of classes through memory space cloning of a master runtime system process  33 , in accordance with the invention. Although described with specific reference to classes, other forms of structured static data could also be preloaded, including data structures, processes, functions, subroutines, interfaces, and the like. The system consists of a runtime environment  31  and individual classes  36  and class libraries  37  that form the overall core managed code platform. By way of example, the system is described with reference to the Java operating environment, although other forms of managed code platforms that execute applications preferably written in an object oriented programming language, such as the Java programming language, could also be used. 
   The exemplary runtime environment  31  includes an application manager  32 , master Java virtual machine (JVM) process  33  and zero or more cloned JVM processes  34 . The master JVM process  33  and cloned JVM processes  34  respectively correspond to a master runtime system process and child runtime system processes. The master runtime system process, preferably provided as a virtual machine, interprets machine-portable code defining compatible applications. The runtime environment  31  need not execute cloned JVM processes  34 , which are only invoked upon request by the application manager  32 . 
   The runtime environment  31  executes an application framework that spawns multiple independent and isolated user application process instances by preferably cloning the memory space of a master runtime system process. The example of an application framework suitable for use in the present invention is the Unix operating system, such as described generally in M. J. Bach, supra at Ch. 2, the disclosure of which is incorporated by reference. 
   The application manager  32  presents a user interface through which individual applications can be selected and executed. The application manager  32  and master JVM process  33  preferably communicate via an inter-process communication (IPC) mechanism, such as a pipe or a socket. The master JVM process  33  is started at device boot time. 
   Upon initialization, the master JVM process  33  reads an executable process image from the storage device  35  and performs bootstrapping operations. These operations include preloading the classes  36  and classes defined in the class libraries  37 , as further described below with reference to  FIG. 10 . Thus, upon completion of initialization, the memory image of the master JVM process  33  resembles that of an initialized, primed and warmed up JVM process with key classes stored in the master JVM process context as prewarmed state  41 . Preferably, the prewarmed state  41  is stored as read only data. 
   Following the initialization, the master JVM process  33  idles, that is, “sleeps” in an inactive state, while awaiting further instructions from the application manager  32 . The master JVM process  33  awakens in response to requests received from the application manager  32  to execute applications. The application manager  32  sends a request to the master JVM process  33 , including standard command line parameters, such as application name, class path, and application arguments. The master JVM process  33  awakens and creates a cloned JVM process  34  as a new cloned process instance of the master JVM process  33  using the process cloning mechanism of the underlying operating system. The context of the master JVM process  33  stored in memory as prewarmed state  41  is inherited by the cloned JVM process  34  as inherited prewarmed state  42 , thereby saving initialization and runtime execution times and providing deterministic execution behavior. Following the “cloning” of the cloned JVM process  34 , the master JVM process  33  records the launched application in an applications launched list  38  and returns to an inactive sleep state. 
   When implemented with copy-on-write semantics, the process cloning creates a logical copy of only the references to the master JVM process context. Segments of the referenced master JVM process context are lazily copied only upon an attempt by the cloned JVM process to modify the referenced context. Therefore, as long as the cloned JVM process does not write into a memory segment, the segment remains shared between parent and child processes. 
   The master JVM process  33  recognizes the following basic commands received from the application manager  32  through the IPC mechanism:
         (1) list: Provides a list of applications launched in response to requests received from the application manager  32 .   (2) jexec: Invokes the process cloning mechanism, parses command line arguments and executes a new instance of the master JVM process  33  as the cloned JVM process  34 . Preferably adopts a syntax compatible to standard JVM processes.   (3) kill: Terminates the application identified by an application handle or process identifier.
 
Other commands are possible, such as described in commonly-assigned U.S. patent application Ser. No. 10/745,164, entitled “System And Method For Performing Incremental Initialization Of A Master Runtime System Process,” filed 22 Dec. 2003, pending, the disclosure of which is incorporated by reference.
       

   During initialization, the master JVM process  33  also preloads classes  36  and classes defined in the class libraries  37  that are likely to be required by applications at runtime. The classes and interfaces are identified through profiling by ranking a set of classes according to a predetermined criteria, such as described in commonly-assigned U.S. patent application Ser. No. 09/970,661, filed Oct. 5, 2001, pending, the disclosure of which is incorporated by reference. A set of core Java foundation classes is specified in a bootstrap class loader  39  and application classes in a system application class loader  40 . Class loading requires identifying a binary form of a class type as identified by specific name, as further described below with reference to  FIG. 10 . Depending upon whether the class was previously loaded or referenced, class loading can include retrieving a binary representation from source and constructing a class object to represent the class in memory. The master JVM process  33  maintains an internal symbol table (not shown) of classes previously loaded to resolve symbolic references. If the internal symbol table does not already contain an entry for the class name or class loader, the class loader responsible for loading the class is identified, invoked and given the name of the class. 
   The master JVM process  33  invokes the bootstrap class loader  39  and system application class loader  40  for every class likely to be requested by the applications. Thus, the prewarmed state  41  includes the class loading for applications prior to actual execution and the initialized and loaded classes are inherited by each cloned JVM process  34  as the inherited prewarmed state  42 . 
   Master JVM Process Mapping 
     FIG. 3  is a block diagram  60  showing, by way of example, a master JVM process  33  mapped into memory  62 . Generally, the context for an executing process includes a data space, user stack, kernel stack, and a user area that lists open files, current directory and supervisory permission settings. Other types of context can also be provided. The context is stored and managed in the memory  62  by the operating system. At device boot time, the operating system instantiates a representation of the executable master JVM process  33  into the memory  62 , possibly in non-contiguous pages  64   a - d , and records the allocation of the memory space as page table entries  63  into the page table  61  prior to commencing execution of the master JVM process  33 . As well, the master JVM process context could similarly be mapped using other memory management systems, such as using demand paging, swapping and similar process memory allocation schemes compatible with process cloning, particularly process cloning with copy-on-write semantics. 
   Cloned JVM Process Mapping 
     FIG. 4  is a block diagram  70  showing, by way of example, a master JVM process  33  and a cloned JVM process  34  mapped into memory  62  through memory space cloning. In a system with process cloning that does not provide copy-on-write semantics, physical copies of the pages  64   a - c  in the memory  62  storing the parent process context are created for each child process. In response to a process cloning request, the operating system instantiates a copy of the representation of the executable master JVM process  33  for the cloned JVM process  34  into the memory  62 , possibly in non-contiguous pages  72   a - d , and records the allocation of the memory space as page table entries  71  into the page table  61  prior to commencing execution of the cloned JVM process  34 . Thus, the cloned JVM process  34  is created with a physical copy of the context of the master JVM process  33 . Since a new, separate physical copy of the master JVM process context is created, the cloned JVM process  34  inherits the prewarmed state  41 , including the preloaded classes of the master JVM process  33 . However, the overall memory footprint of the runtime environment  31  is increased by the memory space required to store the additional copy of the master JVM process context. 
   Cloned JVM Process Mapping with Copy-On-Write 
     FIGS. 5A-B  are block diagrams  80 ,  90  showing, by way of example, a master JVM process  33  and a cloned JVM process  34  mapped into memory  62  through memory space cloning with copy-on-write semantics. In a system with process cloning that provides copy-on-write semantics, only copies of the references, typically page table entries, to the memory space storing the parent process context are created for each child process. Referring first to  FIG. 5A , in response to a process cloning request, the operating system copies only the page table entries  63  referencing the memory space of the executable master JVM process  33  as a new set of page table entries  81  for the cloned JVM process  34 . Thus, the cloned JVM process  34  uses the same references to the possibly non-contiguous pages  64   a - d  storing the master JVM process context as the master JVM process  34 . Initialization and execution of the application associated with the cloned JVM process  34  requires less time, as only the page table entries  62  are copied to clone the master JVM process context. Furthermore, until the cloned JVM process  34  attempts to modify the master JVM process context, the memory space is treated as read only data, which can be shared by other processes. 
   Referring next to  FIG. 5B , the cloned JVM process  34  has attempted to modify one of the pages  82   c  in the memory space of the master JVM process context. In response, the operating system creates a physical copy of the to-be-modified memory space page  82   c  as a new page  91  and updates the allocation in the page table entries  81  for the cloned JVM process  34 . Through copy-on-write semantics, the overall footprint of the runtime environment  31  is maintained as small as possible and only grows until, and if, each cloned JVM process  34  actually requires additional memory space for application-specific context. 
   Method for Preloading Classes 
     FIG. 6  is a flow diagram, showing a method  100  for dynamic preloading of classes through memory space cloning of a master runtime system process, in accordance with the invention. The method  100  is described as a sequence of process operations or steps, which can be executed, for instance, by the runtime environment  31  of  FIG. 2  or other components. 
   Initially, the application manager  32  is loaded (block  101 ). The master JVM process  33  is loaded and initialized at device boot time (block  102 ), as further described below with reference to  FIG. 7 . Following loading and initialization, the master JVM process  33  enters an inactive sleep mode (block  103 ). Upon receiving a request from the application manager  32  (block  104 ), the master JVM process  33  awakens (block  105 ). If necessary, the master JVM process  33  checks the network connection identifier (ID) (block  106 ) for the application manager  32  and determines the type of request (block  107 ). The master JVM process  33  recognizes the commands list, jexec, and kill, as described above with reference to  FIG. 2 . If the request type corresponds to a jexec request, instructing the master JVM process  33  to initiate an execution of an application through process cloning (block  108 ), a cloned JVM process  34  is cloned and executed (block  109 ), as further described below with reference to  FIGS. 8 and 9 . Processing continues indefinitely until the master JVM process  33  and the runtime environment  31  are terminated. 
   Routine for Loading Master JVM Process 
     FIG. 7  is a flow diagram showing the routine  120  for loading a master JVM process  33  for use in the method  100  of  FIG. 6 . One purpose of the routine is to invoke the master JVM process  33  and to preload classes into the prewarmed state  41  for inheritance by cloned JVM processes  34 . 
   Initially, the master JVM process  33  begins execution at device boot time (block  121 ). The master JVM process  33  then preloads classes as a part of the initialization process (block  122 ), as further described below with reference to  FIG. 10 . Briefly, preloading classes involves executing the bootstrap class loader  39  and system application class loader  40  to create and resolve classes likely required by one or more of the applications. The master JVM process  33  completes any other warmup operations (block  123 ) and the routine returns. 
   Routine for Process Cloning without Copy-On-Write 
     FIG. 8  is a flow diagram showing the routine  130  for cloning a process without copy-on-write for use in the method  100  of  FIG. 6 . One purpose of the routine is to create and initiate execution of a cloned JVM process  34  through process cloning that does not provide copy-on-write semantics. 
   Initially, the memory space containing the context of the master JVM process  33  is physically copied into a new memory space for the cloned JVM process  34  (block  131 ). Optionally, the master JVM process  33  can set operating system level resource management parameters over the cloned JVM process  34  (block  132 ), including setting scheduling priorities and limiting processor and memory consumption. Other types of resource management controls are possible. The cloned JVM process  34  is then executed by the runtime environment  31  (block  133 ) using the duplicated master JVM process context. The routine returns upon the completion (block  134 ) of the cloned JVM process  34 . 
   Routine for Process Cloning with Copy-On-Write 
     FIG. 9  is a flow diagram showing the routine  140  for cloning a process with copy-on-write for use in the method  100  of  FIG. 6 . One purpose of the routine is to create and initiate execution of a cloned JVM process  34  through process cloning that provides copy-on-write semantics. 
   Initially, references to the memory space containing the context of the master JVM process  33  are copied for the cloned JVM process  34  (block  141 ). Optionally, the master JVM process  33  can set operating system level resource management parameters over the cloned JVM process  34  (block  142 ), including setting scheduling priorities and limiting processor and memory consumption. Other types of resource management controls are possible. The cloned JVM process  34  is then executed by the runtime environment  31  (block  143 ) using the referenced master JVM process context. Each time the cloned JVM process  34  attempts to write into the memory space referenced to the master JVM process context (block  144 ), the operating system copies the applicable memory segment (block  145 ). Otherwise, the cloned JVM process  34  continues to use the referenced master JVM process context (block  146 ), which is treated as read only data. The routine returns upon the completion (block  147 ) of the cloned JVM process  34 . 
   Routine for Preloading Class 
     FIG. 10  is a flow diagram showing the routine  150  for preloading a class  36  for use in the routine  120  of  FIG. 7 . One purpose of the routine is to find and instantiate prewarmed instances of classes  36  and classes defined in the class libraries  37  as specified in the bootstrap class loader  39  and system application class loader  40  as prewarmed state  41  in the master JVM process  33  for inheritance by a cloned JVM process  34 . 
   Initially, the bootstrap class loader  39  and system application class loader  40  is located and invoked by the master JVM process  33  (block  151 ). Each class  36  and class contained in a class library  37  is then iteratively processed (blocks  152 - 163 ) as follows. First, the master JVM process  33  attempts to locate the class in a system class dictionary (block  153 ). If the class is found (block  154 ), no further class loading need be performed. Otherwise, the master JVM process  33  attempts to locate the class (block  155 ) through standard Java class path location. If the class is found (block  156 ), no further class loading need be performed. Otherwise, the master JVM process  33  attempts to load the bytes for the class from the source associated with the applicable bootstrap class loader  39  and system application class loader  40  (block  157 ). If successful (block  158 ), an instance of the class is created by compiling the source and the class instance is installed in the system class dictionary (block  160 ). If the bytes for the class cannot be loaded from the source (block  158 ), the master JVM process  33  throws a class not found exception (block  159 ). Following the loading or attempted loading of the class, if the class requires resolution with respect to symbolic references (block  161 ), the class is resolved by identifying the applicable class loader for the fully qualified class (block  162 ). Processing continues with the next class (block  163 ), after which the routine returns. 
   While the invention has been particularly shown and described as referenced to the embodiments thereof, those skilled in the art will understand that the foregoing and other changes in form and detail may be made therein without departing from the spirit and scope of the invention.