Abstract:
Methods and apparatus relating to a fairness mechanism for starvation prevention in directory-based cache coherence protocols are described. In one embodiment, negatively-acknowledged (nack&#39;ed) requests from a home agent may be tracked (e.g., using distributed linked-lists). In turn, the tracked requests may be served in a fair order. Other embodiments are also disclosed.

Description:
FIELD 
     The present disclosure generally relates to the field of electronics. More particularly, an embodiment of the invention relates to a fairness mechanism for starvation prevention in directory-based cache coherence protocols. 
     BACKGROUND 
     When an electronic system includes multiple local memories, such as local cache memories, servicing of access requests to the multiple local memories needs to be maintained. This servicing of access requests typically includes sending a message indicating that a request to access a particular location in a memory device cannot be serviced and should be retried at a future time. As the number of local memory devices and/or processors increases, the problem of these “retried” access requests also increases. 
     The cache memory in multiprocessor systems may be kept coherent using a snoopy bus or a directory based protocol. In either case, a memory address is associated with a particular location in the system. This location is known as the “home node” of the memory address. In a directory based protocol, multiple processing/caching agents may concurrently make requests to the home node for access to the same memory address with which the home agent is associated. “Target node” refers to a node which is the target of a request to access a location in memory associated with the node. A node sending such a request is a “source node”. The mechanism for handling the request at a node is the agent of the node. The target agent processes such requests using a finite number of resources, which are occupied while processing a request and released when processing the request is completed. If there is no resource available at the target agent, the request is “retried” by the source agent in response to a message sent indicating the need to retry the access request later. The request may also be retried if there is a conflicting request for the same memory address being processed at the target agent. 
     For those cache coherency protocols that allow retries of requests, it is possible that a request from one source agent encounters either a conflict or an unavailability of appropriate resources every time it is retried to the target. The result is that the request from that source agent is never serviced by the target agent. This failure to service a request is referred to as “starvation”, and may result in a livelock in the system. In a livelock situation, some agents are either unable to complete their transactions or keep repeating the same operation without making progress. In the interest of system performance, it is critical to have a fair servicing mechanism that ensures forward progress in processing requests from multiple agents. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description is provided with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The use of the same reference numbers in different figures indicates similar or identical items. 
         FIGS. 1 ,  3 - 4 , and  7  illustrate block diagrams of embodiments of computing systems, which may be utilized to implement various embodiments discussed herein. 
         FIG. 2  data structure information retained in requester tracker (source) and home agent (target), accordingly to some embodiments. 
         FIGS. 5-6  illustrate flow diagrams according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth in order to provide a thorough understanding of various embodiments. However, some embodiments may be practiced without the specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to obscure the particular embodiments. 
     In accordance with some embodiments, in system interface protocols that rely on retry for resource management and/or conflict resolution, a fair and/or scalable solution is provided. On example would be a cache coherence protocol in a shared memory multiprocessor system. In an embodiment, busy retries may be eliminated. Moreover, some embodiments may provide for strong fairness among competing requests. 
     In one embodiment, negatively-acknowledged (nack&#39;ed) requests from a home agent may be tracked using distributed linked-lists, which may in turn serve the requests in a fair order as described in more detail below. For example, each request in the lists will eventually be invited to the home when it may be processed by the home agent. In an embodiment, the requesting agent may wait for an invitation from the home agent for any nack&#39;ed request to resend the request; hence, there are no busy retries from requesting agents. Moreover, the requests for the same memory address may be invited in the same order as they had arrived at the home, e.g., providing fair service (e.g., in order) to the competing requesters. Accordingly, in some embodiments, a set of linked lists are applied to each resource at the home agent, e.g., to allow reservation of a special resource to control the fairness. 
     Various computing systems may be used to implements embodiments, discussed herein, such as the systems discussed with reference to  FIGS. 1 ,  3 - 4 , and  7 . More particularly,  FIG. 1  illustrates a block diagram of a computing system  100 , according to an embodiment of the invention. The system  100  may include one or more agents  102 - 1  through  102 -M (collectively referred to herein as “agents  102 ” or more generally “agent  102 ”). In an embodiment, one or more of the agents  102  may be any of components of a computing system, such as the computing systems discussed with reference to  FIG. 3-4  or  7 . 
     As illustrated in  FIG. 1 , the agents  102  may communicate via a network fabric  104 . In one embodiment, the network fabric  104  may include a computer network that allows various agents (such as computing devices) to communicate data. In an embodiment, the network fabric  104  may include one or more interconnects (or interconnection networks) that communicate via a serial (e.g., point-to-point) link and/or a shared communication network. For example, some embodiments may facilitate component debug or validation on links that allow communication with fully buffered dual in-line memory modules (FBD), e.g., where the FBD link is a serial link for coupling memory modules to a host controller device (such as a processor or memory hub). Debug information may be transmitted from the FBD channel host such that the debug information may be observed along the channel by channel traffic trace capture tools (such as one or more logic analyzers). 
     In one embodiment, the system  100  may support a layered protocol scheme, which may include a physical layer, a link layer, a routing layer, a transport layer, and/or a protocol layer. The fabric  104  may further facilitate transmission of data (e.g., in form of packets) from one protocol (e.g., caching processor or caching aware memory controller) to another protocol for a point-to-point or shared network. Also, in some embodiments, the network fabric  104  may provide communication that adheres to one or more cache coherent protocols. 
     Furthermore, as shown by the direction of arrows in  FIG. 1 , the agents  102  may transmit and/or receive data via the network fabric  104 . Hence, some agents may utilize a unidirectional link while others may utilize a bidirectional link for communication. For instance, one or more agents (such as agent  102 -M) may transmit data (e.g., via a unidirectional link  106 ), other agent(s) (such as agent  102 - 2 ) may receive data (e.g., via a unidirectional link  108 ), while some agent(s) (such as agent  102 - 1 ) may both transmit and receive data (e.g., via a bidirectional link  110 ). 
     Additionally, at least of the agents  102  may be a home agent and one or more of the agents  102  may be requesting agents as will be further discussed with reference to  FIG. 2 . For example, in an embodiment, one or more of the agents  102  may maintain entries in a storage device (e.g., implemented as a table, queue, buffer, linked list, etc.) to track information about requests. 
     More particularly,  FIG. 2  illustrates data structure information retained in requester tracker (source) and home agent (target), accordingly to some embodiments. The request naming utilized in  FIG. 2  is such that the first character indicates the address and the second character indicates the requester; for example, B 2  corresponds to a request from Requester  2  to address B. 
     In one embodiment, each processing entry in the home agent maintains an associated linked list of nack&#39;ed requests. One entry is reserved as special in order to track requests arrived when all other regular entries are occupied. However, more than one entry may be reserved as special in some embodiments. If a request is nack&#39;ed due to a conflicting request being processed for a same address, the nack&#39;ed request is appended to the list associated with the same processing entry. If a request is nack&#39;ed because there is no processing entry available, the request is appended to the list associated to the special entry. 
     In some embodiments, to maintain distributed linked lists of nack&#39;ed requests, each processing entry in the home agent (target) maintains two pointers, Head and Tail in  FIG. 2 , to record head and tail requests tracked by the associated linked list. The requesting agents (source) also maintains a pointer in each request entry, Next in  FIG. 2 , to point to a next request in the distributed linked list. 
     Various types of computing systems may be used to implement the embodiments discussed herein (such as those discussed with reference to  FIGS. 1-2 ). For example,  FIG. 3  illustrates a block diagram of portions of a computing system  300 , according to an embodiment. In one embodiment, various components of the system  300  may be implemented by one of the agents  102 - 1  and/or  102 -M discussed with reference to  FIG. 1 . Further details regarding some of the operation of the computing system  300  will be discussed herein with reference to  FIGS. 5-6 . 
     The system  300  may include one or more processors  302 - 1  through  302 -N (collectively referred to herein as “processors  302 ” or more generally “processor  302 ”). Each of the processors  302 - 1  through  302 -N may include various components, such as private or shared cache(s)  303 , execution unit(s), one or more cores, etc. Moreover, the processors  302  may communicate through a bus  304  with other components such as an interface device  306 . In an embodiment, the interface device  306  may be a chipset or a memory controller hub (MCH). Moreover, as will be further discussed with reference to  FIG. 7 , the processors  302  may communicate via a point-to-point (PtP) connection with other components. Additionally, the interface device  306  may communicate with one or more peripheral devices  308 - 1  through  308 -P (collectively referred to herein as “peripheral devices  308 ” or more generally “device  308 ”). The devices  308  may be a peripheral device that communicates in accordance with the PCIe specification in an embodiment. 
     As shown in  FIG. 3 , a switching logic  312  may be coupled between a variety of agents (e.g., peripheral devices  308  and the interface device  306 ). The switching logic  312  may include a storage unit such as a cache that is maintained coherent (e.g., such as discussed with reference to  FIGS. 2 ,  5 , and/or  6 ) with the cache(s)  303 , or caches present elsewhere in system  300  such as in one or more of the devices  308 , interface device  306 , switching logic  312 , etc. Furthermore, cache(s) discussed herein (such as cache  303 ) may be shared or private. Also, such cache(s) may be any type of cache, such a level 1 (L1) cache, a level 2 (L2) cache, a level 3 (L3), a mid-level cache, a last level cache (LLC), etc. to store electronic data (e.g., including instructions) that is utilized by one or more components of the computing systems discussed herein. 
       FIG. 4  illustrates a block diagram of an embodiment of a computing system  400 . One or more of the agents  102  of  FIG. 1  and/or the system  300  of  FIG. 3  may comprise one or more components of the computing system  400 . The computing system  400  may include one or more central processing unit(s) (CPUs)  402  (which may be collectively referred to herein as “processors  402 ” or more generically “processor  402 ”) coupled to an interconnection network (or bus)  404 . The processors  402  may be any type of processor such as a general purpose processor, a network processor (which may process data communicated over a computer network  405 ), etc. (including a reduced instruction set computer (RISC) processor or a complex instruction set computer (CISC)). Moreover, the processors  402  may have a single or multiple core design. The processors  402  with a multiple core design may integrate different types of processor cores on the same integrated circuit (IC) die. Also, the processors  402  with a multiple core design may be implemented as symmetrical or asymmetrical multiprocessors. 
     The processor  402  may include one or more caches  303 , which may be private and/or shared in various embodiments. Generally, a cache stores data corresponding to original data stored elsewhere or computed earlier. To reduce memory access latency, once data is stored in a cache, future use may be made by accessing a cached copy rather than refetching or recomputing the original data. The cache(s) may be any type of cache, such a level 1 (L1) cache, a level 2 (L2) cache, a level 3 (L3), a mid-level cache, a last level cache (LLC), etc. to store electronic data (e.g., including instructions) that is utilized by one or more components of the system  400 . Also, as discussed herein, cache(s)  303  may be maintained such as discussed with reference to  FIGS. 2 ,  5 , and/or  6 . Additionally, such cache(s) may be located in various locations (e.g., inside other components to the computing systems discussed herein, including systems of  FIGS. 1 ,  3 - 4 , and  7 ). 
     A chipset  406  may additionally be coupled to the interconnection network  404 . In an embodiment, the chipset  406  may be the same as or similar to the interface device  306  of  FIG. 3 . Further, the chipset  406  may include a memory control hub (MCH)  408 . The MCH  408  may include a memory controller  410  that is coupled to a memory  413 . The memory  413  may store data, e.g., including sequences of instructions that are executed by the processor  402 , or any other device in communication with components of the computing system  400 . In an embodiment, the memory  413  may be the same or similar to the memory  311  of  FIG. 3 . Also, in one embodiment of the invention, the memory  413  may include one or more volatile storage (or memory) devices such as random access memory (RAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), static RAM (SRAM), etc. Nonvolatile memory may also be utilized such as a hard disk. Additional devices may be coupled to the interconnection network  404 , such as multiple processors and/or multiple system memories. 
     The MCH  408  may further include a graphics interface  414  coupled to a display device  416  (e.g., via a graphics accelerator in an embodiment). In one embodiment, the graphics interface  414  may be coupled to the display device  416  via an accelerated graphics port (AGP). In an embodiment of the invention, the display device  416  (such as a flat panel display) may be coupled to the graphics interface  414  through, for example, a signal converter that translates a digital representation of an image stored in a storage device such as video memory or system memory (e.g., memory  413 ) into display signals that are interpreted and displayed by the display  416 . 
     As shown in  FIG. 4 , a hub interface  418  may couple the MCH  408  to an input/output control hub (ICH)  420 . The ICH  420  may provide an interface to input/output (I/O) devices coupled to the computing system  400 . The ICH  420  may be coupled to a bus  422  through a peripheral bridge (or controller)  424 , such as a peripheral component interconnect (PCI) bridge that may be compliant with the PCIe specification, a universal serial bus (USB) controller, etc. The bridge  424  may provide a data path between the processor  402  and peripheral devices. Other types of topologies may be utilized. Also, multiple buses may be coupled to the ICH  420 , e.g., through multiple bridges or controllers. Further, the bus  422  may comprise other types and configurations of bus systems. Moreover, other peripherals coupled to the ICH  420  may include, in various embodiments of the invention, integrated drive electronics (IDE) or small computer system interface (SCSI) hard drive(s), USB port(s), a keyboard, a mouse, parallel port(s), serial port(s), floppy disk drive(s), digital output support (e.g., digital video interface (DVI)), etc. 
     The bus  422  may be coupled to an audio device  426 , one or more disk drive(s)  428 , and a network adapter  430  (which may be a NIC in an embodiment). In one embodiment, the network adapter  430  or other devices coupled to the bus  422  may communicate with the chipset  406  via the switching logic  412  (which may be the same or similar to the logic  312  of  FIG. 3  in some embodiments). Other devices may be coupled to the bus  422 . Also, various components (such as the network adapter  430 ) may be coupled to the MCH  408  in some embodiments of the invention. In addition, the processor  402  and the MCH  408  may be combined to form a single chip. In an embodiment, the memory controller  410  may be provided in one or more of the CPUs  402 . Further, in an embodiment, MCH  408  and ICH  420  may be combined into a Peripheral Control Hub (PCH). 
     Additionally, the computing system  400  may include volatile and/or nonvolatile memory (or storage). For example, nonvolatile memory may include one or more of the following: read-only memory (ROM), programmable ROM (PROM), erasable PROM (EPROM), electrically EPROM (EEPROM), a disk drive (e.g.,  428 ), a floppy disk, a compact disk ROM (CD-ROM), a digital versatile disk (DVD), flash memory, a magneto-optical disk, or other types of nonvolatile machine-readable media capable of storing electronic data (e.g., including instructions). 
     The memory  413  may include one or more of the following in an embodiment: an operating system (O/S)  432 , application  434 , and/or device driver  436 . The memory  413  may also include regions dedicated to Memory Mapped I/O (MMIO) operations. Programs and/or data stored in the memory  413  may be swapped into the disk drive  428  as part of memory management operations. The application(s)  434  may execute (e.g., on the processor(s)  402 ) to communicate one or more packets with one or more computing devices coupled to the network  405 . In an embodiment, a packet may be a sequence of one or more symbols and/or values that may be encoded by one or more electrical signals transmitted from at least one sender to at least on receiver (e.g., over a network such as the network  405 ). For example, each packet may have a header that includes various information which may be utilized in routing and/or processing the packet, such as a source address, a destination address, packet type, etc. Each packet may also have a payload that includes the raw data (or content) the packet is transferring between various computing devices over a computer network (such as the network  405 ). 
     In an embodiment, the application  434  may utilize the O/S  432  to communicate with various components of the system  400 , e.g., through the device driver  436 . Hence, the device driver  436  may include network adapter  430  specific commands to provide a communication interface between the O/S  432  and the network adapter  430 , or other I/O devices coupled to the system  400 , e.g., via the chipset  406 . 
     In an embodiment, the O/S  432  may include a network protocol stack. A protocol stack generally refers to a set of procedures or programs that may be executed to process packets sent over a network  405 , where the packets may conform to a specified protocol. For example, TCP/IP (Transport Control Protocol/Internet Protocol) packets may be processed using a TCP/IP stack. The device driver  436  may indicate the buffers in the memory  413  that are to be processed, e.g., via the protocol stack. 
     The network  405  may include any type of computer network. The network adapter  430  may further include a direct memory access (DMA) engine, which writes packets to buffers (e.g., stored in the memory  413 ) assigned to available descriptors (e.g., stored in the memory  413 ) to transmit and/or receive data over the network  405 . Additionally, the network adapter  430  may include a network adapter controller, which may include logic (such as one or more programmable processors) to perform adapter related operations. In an embodiment, the adapter controller may be a MAC (media access control) component. The network adapter  430  may further include a memory, such as any type of volatile/nonvolatile memory (e.g., including one or more cache(s) and/or other memory types discussed with reference to memory  413 ). 
       FIG. 5  illustrates a flow diagram of a method  500  to be performed at a home agent (target), according to an embodiment. In one embodiment, various components discussed with reference to  FIGS. 1-4  and  7  may be utilized to perform one or more of the operations discussed with reference to  FIG. 5 . 
     Referring to  FIGS. 1-5 , at an operation  502 , when a new request arrives at the home agent, processing entries in the home are searched  504  to find a conflicting request to the same address  506  being processed based on the received request of operation  502 . If a conflicting entry for the same address is found, a wait response is sent  508  to the requester and the request is appended to the list associated with the processing entry. In an embodiment, if head pointer is empty, then the request is recorded as head pointer. Otherwise, if tail pointer is empty, then the request is recorded as tail pointer and a next message to the requester is sent in the head pointer (so that the old request occupying head pointer can record its next pointer to the new request). Otherwise, the tail pointer is replaced with the new request and a next message to the requester is sent in the old tail pointer (so that the old request occupying tail pointer can record its next pointer to the new request). 
     If a conflicting entry is not found at operation  506  and if a regular entry is available at operation  510 , the request of operation  502  is accepted into a regular entry  512 . In an embodiment, if a regular entry is unavailable  510 , then a special entry is used if available at operation  512 . If no regular or special entries are available  510 , a wait response is sent to the requester and the request is appended to the list associated with the special entry following the process discussed with reference to operation  508 . 
     In an embodiment, once a processing entry completes a request and becomes available, the home agent sends a resend invitation to the request in the head pointer if any, with an indication of existence of a next request (e.g., false if its Tail pointer is empty; true otherwise). The indication allows the requester to wait for a possible next message in transit before resending its nack&#39;ed request. Further, in one embodiment, when a resent request invited to a regular entry arrives, the request may be accepted and the Head pointer may be replaced with the next pointer in the resent message if any. If there is no next pointer, then the head pointer may be cleared. If the next pointer matches to the Tail pointer, then the tail pointer may be cleared. 
     In one embodiment, when a resent request invited to a special entry arrives, the same operations as for a new request discussed above may be followed. In this case, however, the situation in operation  508  may not occur again because a special entry has been reserved for this request. The request should be accepted into one of the processing entries, or appended to a list associated with a regular entry, which will be guaranteed to be accepted next time. 
       FIG. 6  illustrates a flow diagram of a method  600  to be performed at a requesting agent (source), according to an embodiment. In one embodiment, various components discussed with reference to  FIGS. 1-4  and  7  may be utilized to perform one or more of the operations discussed with reference to  FIG. 6 . 
     Referring to  FIGS. 1-6 , at an operation  602 , when a nack&#39;ed request receives a resend invitation, if the resend message is without a next pointer  604 , the original request is resent at operation  605 ; otherwise, if the resend message indicates existence of a next pointer  604 , the requester resends  606  the request to the home with its next pointer. In an embodiment, the requester may wait for a next message before resending the request at operation  606 . As shown in  FIG. 6 , after operations  605  and  606 , the method  600  resumes at operation  602 . 
     In some embodiments, if the cache coherence protocol allows some requests to be cancelled while waiting for a resend invitation, e.g., a write-back request voided after an implicit write-back response provided, the requester may still follow the same operations by resending a void request with the next pointer if any. 
       FIG. 7  illustrates a computing system  700  that is arranged in a point-to-point (PtP) configuration, according to an embodiment of the invention. In particular,  FIG. 7  shows a system where processors, memory, and input/output devices are interconnected by a number of point-to-point interfaces. The operations discussed with reference to  FIGS. 1-6  may be performed by one or more components of the system  700 . 
     As illustrated in  FIG. 7 , the system  700  may include several processors, of which only two, processors  702  and  704  are shown for clarity. The processors  702  and  704  may each include a local memory controller hub (MCH)  706  and  708  to enable communication with memories  710  and  712 . The memories  710  and/or  712  may store various data such as those discussed with reference to the memory  413  of  FIG. 4 . As shown in  FIG. 7 , the processors  702  and  704  (or other components of system  700  such as chipset  720 , I/O devices  743 , etc.) may also include one or more cache(s) such as those discussed with reference to  FIGS. 1-6 . 
     In an embodiment, the processors  702  and  704  may be one of the processors  402  discussed with reference to  FIG. 4 . The processors  702  and  704  may exchange data via a point-to-point (PtP) interface  714  using PtP interface circuits  716  and  718 , respectively. Also, the processors  702  and  704  may each exchange data with a chipset  720  via individual PtP interfaces  722  and  724  using point-to-point interface circuits  726 ,  728 ,  730 , and  732 . The chipset  720  may further exchange data with a high-performance graphics circuit  734  via a high-performance graphics interface  736 , e.g., using a PtP interface circuit  737 . 
     In at least one embodiment, the switching logic  412  may be coupled between the chipset  720  and other components of the system  700  such as those communicating via a bus  740 . Other embodiments of the invention, however, may exist in other circuits, logic units, or devices within the system  700  of  FIG. 7 . Furthermore, other embodiments of the invention may be distributed throughout several circuits, logic units, or devices illustrated in  FIG. 7 . 
     The chipset  720  may communicate with the bus  740  using a PtP interface circuit  741 . The bus  740  may have one or more devices that communicate with it, such as a bus bridge  742  and I/O devices  743 . Via a bus  744 , the bus bridge  742  may communicate with other devices such as a keyboard/mouse  745 , communication devices  746  (such as modems, network interface devices, or other communication devices that may communicate with the computer network  405 ), audio I/O device, and/or a data storage device  748 . The data storage device  748  may store code  749  that may be executed by the processors  702  and/or  704 . 
     In various embodiments of the invention, the operations discussed herein, e.g., with reference to  FIGS. 1-7 , may be implemented as hardware (e.g., circuitry), software, firmware, microcode, or combinations thereof, which may be provided as a computer program product, e.g., including a machine-readable or computer-readable medium having stored thereon instructions (or software procedures) used to program a computer to perform a process discussed herein. Also, the term “logic” may include, by way of example, software, hardware, or combinations of software and hardware. The machine-readable medium may include a storage device such as those discussed with respect to  FIGS. 1-7 . Additionally, such computer-readable media may be downloaded as a computer program product, wherein the program may be transferred from a remote computer (e.g., a server) to a requesting computer (e.g., a client) through data signals provided in a carrier wave or other propagation medium via a communication link (e.g., a bus, a modem, or a network connection). 
     Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment may be included in at least an implementation. The appearances of the phrase “in one embodiment” in various places in the specification may or may not be all referring to the same embodiment. 
     Also, in the description and claims, the terms “coupled” and “connected,” along with their derivatives, may be used. In some embodiments of the invention, “connected” may be used to indicate that two or more elements are in direct physical or electrical contact with each other. “Coupled” may mean that two or more elements are in direct physical or electrical contact. However, “coupled” may also mean that two or more elements may not be in direct contact with each other, but may still cooperate or interact with each other. 
     Thus, although embodiments of the invention have been described in language specific to structural features and/or methodological acts, it is to be understood that claimed subject matter may not be limited to the specific features or acts described. Rather, the specific features and acts are disclosed as sample forms of implementing the claimed subject matter.