Patent Publication Number: US-6704843-B1

Title: Enhanced multiprocessor response bus protocol enabling intra-cache line reference exchange

Description:
RELATED APPLICATIONS 
     The present invention is related to the subject matter of commonly assigned, U.S. patent applications: Ser. No. 09/696,888, now U.S. Pat. No. 6,629,210 issued Sep. 29, 2003, entitled “INTELLIGENT CACHE MANAGEMENT MECHANISM VIA PROCESSOR ACCESS SEQUENCE ANALYSIS”; Ser. No. 09/696,912, now U.S. Pat. No. 6,601,144 issued Jul. 29, 2003, entitled “DYNAMIC CACHE MANAGEMENT IN A SYMMETRIC MULTIPROCESSOR SYSTEM VIA SNOOP OPERATION SEQUENCE ANALYSIS”; Ser. No. 09/696,890 entitled “ENHANCED CACHE MANAGEMENT MECHANISM VIA AN INTELLIGENT SYSTEM BUS MONITOR”; Ser. No. 09/696,910 entitled “HIGH PERFORMANCE CACHE INTERVENTION MECHANISM FOR SYMMETRIC MULTIPROCESSOR SYSTEMS”; and Ser. No. 09/696,889, now U.S. Pat. No. 6,631,450 issued Oct. 7, 2003, entitled “SYMMETRIC MULTIPROCESSOR ADDRESS BUS PROTOCOL WITH INTRA-CACHE LINE ACCESS INFORMATION”. The content of the above-identified applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention generally relates to an improved data processing system and in particular to improved memory management in a data processing system. Still more particularly, the present invention relates to improved cache memory management in a data processing system, which an improved system bus response protocol. 
     2. Description of the Related Art 
     Most data processing systems are controlled by one or more processors and employ various levels of memory. Typically, programs and data are loaded into a data processing system&#39;s memory storage areas for execution or reference by the processor, and are stored in different portions of the memory storage depending on the processor&#39;s current need for such programs or data. A running program or data referenced by a running program must be within the system&#39;s main memory (primary or main storage, which is typically random access memory). Programs or data which are not needed immediately may be kept in secondary memory (secondary storage, such as a tape or disk drive) until needed, and then brought into main storage for execution or reference. Secondary storage media are generally less costly than random access memory components and have much greater capacity, while main memory storage may generally be accessed much faster than secondary memory. 
     Within the system storage hierarchy, one or more levels of high-speed cache memory may be employed between the processor and main memory to improve performance and utilization. Cache storage is much faster than the main memory, but is also relatively expensive as compared to main memory and is therefore typically employed only in relatively small amounts within a data processing system. In addition, limiting the size of cache storage enhances the speed of the cache. Various levels of cache memory are often employed, with trade-offs between size and access being made at levels logically further from the processor(s). Cache memory generally operates faster than main memory, typically by a factor of five to ten times, and may, under certain circumstances, approach the processor operational speed. If program instructions and/or data which are required during execution are pre-loaded in high speed cache memory, average overall memory access time for the system will approach the access time of the cache. 
     In order to enhance performance, contemporary data processing systems often utilize multiple processors which concurrently execute portions of a given task. To further enhance performance, such multiple processor or multi-processor (MP) data processing systems often utilize a multi-level cache/memory hierarchy to reduce the access time required to retrieve data from memory. A multi-processor system may include a number of processors each with an associated on-chip, level-one (L1) cache, a number of level-two (L2) caches, and a number of system memory modules. Typically, the cache/memory hierarchy is arranged such that each L2 cache is accessed by a subset of the L1 caches within the system via a local bus. In turn, each L2 cache and system memory module is coupled to a system bus (or interconnect switch) such that an L2 cache within the multi-processor system may access data from any of the system memory modules coupled to the bus. 
     The use of cache memory imposes one more level of memory management overhead on the data processing system. Logic must be implemented to control allocation, deallocation, and coherency management of cache content. When space is required, instructions or data previously residing in the cache must be “swapped” out, usually on a “least-recently-used” (LRU) basis. Accordingly, if there is no room in the cache for additional instructions or data, then the information which has not been accessed for the longest period of time will be swapped out of the cache and replaced with the new information. In this manner, the most recently used information, which has the greatest likelihood of being again required, is available in the cache at any given time. 
     As noted, previous cache management techniques mostly depend on least-recently-used (LRU) algorithms in selecting a cache line victim for eviction and replacement. However, empirical measurements have shown that strict least-recently-used algorithms are unsatisfactory in many cases. 
     Various enhancements to LRU algorithms have been proposed or implemented in recent years, such as software managed LRU, pseudo-random influences, etc. Basic symmetric multi-processor snooping protocols have also been utilized to influence cache management. 
     Even with a cache memory management scheme, there are additional, related problems that can cause system performance to suffer. For example, in data processing systems with several levels of cache/memory storage, a great deal of shuttling of instructions and data between the various cache/memory levels occurs, which consumes system resources such as processor cycles and bus bandwidth which might otherwise be put to more productive processing use. The problem has been exacerbated in recent years by the growing disparity between processor speeds and the operational speeds of the different system components used to transfer information and instructions to the processor. 
     Additionally, only limited amounts of access history information is shared between horizontal processors/caches within conventional multiprocessor systems. Prior art system bus communication of access history is generally limited to coherency information (e.g., indicating in a snoop response that the snooping cache has the line in a shared or modified coherency state). 
     It would be desirable, therefore, to provide a system increasing the “intelligence” of cache management, and in particular to passing dynamic application sequence behavior information between processors and caches and utilizing that information to optimize cache management. 
     SUMMARY OF THE INVENTION 
     It is therefore one object of the present invention to provide an improved data processing system. 
     It is another object of the present invention to provide improved memory management in a data processing system. 
     It is yet another object of the present invention to provide improved cache memory management in a multiprocessor data processing system, which includes an improved system bus response protocol. 
     The foregoing objects are achieved as is now described. System bus snoopers within a multiprocessor system in which dynamic application sequence behavior information is maintained within cache directories append the dynamic application sequence behavior information for the target cache line to their snoop responses. The system controller, which may also maintain dynamic application sequence behavior information in a history directory, employs the available dynamic application sequence behavior information to append “hints” to the combined response, appends the concatenated dynamic application sequence behavior information to the combined response, or both. Either the hints or the dynamic application sequence behavior information may be employed by the bus master and other snoopers in cache management. 
     The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 depicts a multi-processor data processing system in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is a block diagram of a portion of a specific implementation of a data processing system in accordance with a preferred embodiment of the present invention; 
     FIGS. 3A-3C depict comparative diagrams of cache and history directory entries in accordance with the prior art and in accordance with a preferred embodiment of the present invention; 
     FIG. 4 is a diagram of convergence in dynamic cache management to fixed algorithm cache management utilizing historical processor access, snoop operation, and system controller hint information in accordance with a preferred embodiment of the present invention; 
     FIG. 5 depicts a high level flow chart for a process of generating processor access history and passing processor access, snoop operation, and system controller hint history for cache lines in accordance with a preferred embodiment of the present invention; 
     FIG. 6 is a high level flow chart for a process of generating snoop operation history for cache lines in accordance with a preferred embodiment of the present invention; 
     FIG. 7 depicts a high level flow chart for a process of generating and transmitting system controller hints in accordance with a preferred embodiment of the present invention; and 
     FIG. 8 is a timing diagram illustrating passing of dynamic application sequence behavior among bus participants in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference now to the figures and in particular with reference to FIG. 1, a high level block diagram of a multi-processor data processing system in accordance with a preferred embodiment of the present invention is depicted. Data processing system  102  includes a plurality of processors  104   a - 104   n  (where n is any positive integer). Data processing system  102  also includes a number of caches  106   a - 106   n  between the processors  104   a - 104   n  and a system bus  108 , which couples processors  104   a - 104   n  and caches  106   a - 106   n  to system memory  110  under the control of system controller  116 . 
     Caches  106   a - 106   n  each include a cache memory and cache directory, and may be shared or private, logically in-line or look-aside, and inclusive or noninclusive caches within the present invention. Processors  104   a - 104   n  are each coupled to system memory  110  via system bus  108 . In a tightly coupled symmetric multiprocessor system such as data processing system  102  in the exemplary embodiment, each processor  104   a - 104   n  may be utilized to read from and write to memory  110 . Thus, systems and interlocks must be utilized to ensure that the data and instructions within memory  110  remain coherent. 
     Each processor  104   a - 104   n  within multiprocessor data processing system  102  includes a cache  106   a - 106   n  which may be utilized to efficiently and temporarily access and store selected instructions or data from system memory  110 . Since each cache memory  112   a - 112   n  constitutes a memory space, coherency should be maintained among each cache memory  112   a - 112   n  and system memory  110  in order to assure accurate operation. Each cache  106   a - 106   n  is therefore connected to system bus  108  and snoops transactions on system bus  108 . 
     Each cache  106   a - 106   n  also has an associated cache directory  114   a - 114   n , which each store a plurality of cache directory entries corresponding to memory locations within cache memory  112   a - 112   n . In the present invention, cache directory entries each contain historical processor access, snoop operation, and system controller hint information as described in further detail below. 
     Also connected to system bus  108  is system controller  116 , which may control arbitration for system bus  108  and monitor transactions on system bus  108 . System controller  116  includes an associated history directory  118 , which contains data relating to the present and past status of each cache  106   a - 106   n , as described in more detail below. 
     Referring to FIG. 2, a block diagram of a portion of a specific implementation of a data processing system in accordance with a preferred embodiment of the present invention is illustrated. The particular implementation of data processing system  102  illustrated in FIG. 2 includes a multilevel cache  106   a  including a level one (L1) cache  202  and a level two (L2) cache  204 . Although depicted as logically discrete, L1 and L2 caches  202  and  204  may be formed within a single integrated circuit die; however, L1 cache  202  is integrally formed with the functional elements of processor  104   a  while L2 cache  204  is a logically separate component. 
     L2 cache  204  includes a cache memory  206  and a cache (address tag) directory  208 , as well as processor interface control logic  210  controlling communications transactions between cache  204  and processor  104   a  and bus interface control logic  212  controlling communications transactions between cache  204  and system bus  108 . L2 cache  204  also includes a read/write queue  214  controlling processing of data access transactions received from processor  104   a  within cache  204  and a snoop queue  216  controlling processing of data access transactions snooped from system bus  108  within cache  204 . Processor interface control logic  210 , bus interface control logic  212 , read/write queue  214 , and snoop queue  216  may be collectively viewed as a cache controller for L2 cache  204 . 
     With reference now to FIGS. 3A through 3C, comparative diagrams of cache and history directory entries in accordance with the prior art and in accordance with the present invention are depicted. The data structures depict cache directory entries within L2 cache directory  204  or history directory  118  within system controller  116 . 
     FIG. 3A depicts a prior art cache directory entry format  302  for each cache line within a cache directory  208 , which includes: an address tag  304 , typically some subset of address bits from the system memory address for the corresponding cache line; coherency state bits  306 , which identify a coherency state for the corresponding cache line; and, optionally, LRU bits  308 , which identify, within a set-associative cache, the LRU position of the corresponding cache line within the congruence class indexed by the address tag. 
     Because each processor within a multi-processor system may modify data, such systems must generally employ a protocol to maintain memory coherence. For example, multi-processor systems utilizing processors based upon the PowerPC RISC instruction set architecture (ISA) utilize a coherency protocol having four possible states: modified (M), exclusive (E), shared (S), and invalid (I). The MESI state  306  associated with each cache line (i.e., the line state) informs the data processing system as to what memory operations are required to maintain memory coherence following an access to that cache line. 
     As noted above, conventional LRU bits  308  within a cache directory entry represent the LRU position within a congruence class for the corresponding cache line. These LRU positions are employed by conventional LRU cache management algorithms to select a victim within a congruence class for deallocation. 
     FIG. 3B depicts a cache/history directory entry format  310  for each cache line within cache directory  208  in accordance with the present invention. Cache/history directory entry format  310  includes address tag  304 , coherency state bits  306 , and optional LRU bits  308 . In addition, cache directory entry format  310  includes historical processor access, snoop operation, and system controller hint information  312 , collected each individual cache controller and passed on to each other. 
     The historical processor access, snoop operation, and system controller hint information  312  allows the cache controllers to track the operations performed on that cache line by the processors and caches. Historical processor access, snoop operation, and system controller hint information  312  includes an integer number m of subentries each containing information about a processor&#39;s access and/or snooping of the corresponding cache line, as well as system controller hints relating to the corresponding cache line. Subentry T 0  contains information regarding the most recent processor to access (and thereby gain either shared or exclusive “ownership” of) the corresponding cache line; subentry T −1  contains information regarding the next most recent processor to access the corresponding cache line; subentry T −2  contains information regarding the processor which accessed the corresponding cache line just prior to the processor of subentry T −1 ; and so on, with subentry T −m  containing information regarding the oldest available processor access information for any particular processor which previously accessed the corresponding cache line. 
     The number m of subentries T 0 , T −1 , T −2 , . . . T −m  within historical processor access and snoop operation information  312  may or may not correspond to the number of processors which share access to the corresponding cache  204 . Each subentry T 0 , T −1 , T −2 , . . . T −m  includes multiple segments: segments  314   a ,  314   b , and  314   c  for subentry T 0 ; segments  316   a ,  316   b , and  316   c  for subentry T −1 ; segments  318   a ,  318   b , and  318   c  for subentry T −2 ; and segments  320   a ,  320   b , and  320   c  for subentry T −m . The first segments  314   a ,  316   a ,  318   a  and  320   a  contain historical processor access information regarding the corresponding cache line; the second segments  314   b ,  316   b ,  318   b  and  320   b  contain historical snoop operation information regarding the corresponding cache line; and the third segments  314   c ,  316   c ,  318   c  and  320   c  contain historical system controller hint information regarding the corresponding cache line. 
     The historical processor access information segments  314   a ,  316   a ,  318   a  and  320   a  of each subentry T 0 , T −1 , T −2 , . . . T −m  contains an identification of the processor which accessed the cache line, an identification of each load or store type operation which was performed on the cache line by that processor, and a timestamp for each operation of when that operation was performed. The historical snoop operation information segments  314   b ,  316   b ,  318   b  and  320   b  of each subentry T 0 , T −1 , T −2 , . . . T −m  contains an identification of each operation relating to the corresponding cache line which was snooped off the system bus by the respective processor, an identification the processor which originated the snooped operation, and a timestamp for each snooped operation of when that operation was snooped. The historical system controller hint information segments  314   c ,  316   c ,  318   c  and  320   c  of each subentry T 0 , T −1 , T −2 , . . . T −m  contain an identification of each system controller hint received, and a timestamp for each received hint. 
     Thus, each subentry T 0 , T −1 , T −2 , . . . T −m  may contain information regarding more than one access by the corresponding processor, as well as more than one snoop operation or system controller hint. Each subentry T 0 , T −1 , T −2 , . . . T −m  may contain either a fixed or a variable number of storage locations for recording access (operation) types or snooped operation types and timestamps, with only information regarding the most recent accesses or snooped operations being maintained if a fixed or otherwise limited number of storage locations are provided. 
     With each new direct access of the corresponding cache line by a different processor, the content of all subentries are shifted by one position (e.g., the content of subentry T 0  is shifted into subentry T −1 , the content of subentry T −1  is shifted into subentry T −2 , etc.) for the cache directory of the cache receiving the cache line. Information regarding the new processor access of the corresponding cache line is then stored within segment  314   a  of subentry T 0 . If a processor reacquires a line which it had previously owned and then given up, the prior access is shifted just as if performed by a different processor and a new subentry T 0  is created for the current access by that processor. 
     Within the cache directory  208 , information regarding system bus operations snooped by the same processor is stored within segment  314   b  of subentry T 0 . Within the history directory  118 , system bus operations snooped by the system controller  116  are stored within segment  314   b  of subentry T 0  in the cache directories, and in segment  324  (see FIG. 3C) in the system controller&#39;s history directory  118 . If ownership of the corresponding cache line is shared among multiple processors/caches, snooped system bus operations and/or detected system controller hints may be written to corresponding T 0  subentries within the entry for that cache line in each cache controller which shares the cache line. 
     Within cache directory  208 , processor access, snooped operation, and system controller hint information received from other processors or caches (e.g., via an intervention) are loaded into subentries T −1  through T −m  when the cache line/directory entry is first allocated within the processor/cache corresponding to subentry T 0 . 
     FIG. 3C illustrates the format of directory entries within history table  118  in accordance with the present invention. The system controller (system bus controller) typically performs functions such as combining snoop responses, address bus arbitration, data bus arbitration, etc. In the present invention, the system controller  116  implements a history directory  118  in which historical bus sequences are logged on a per cache line basis. The history directory  118  may be either direct mapped or set associative. 
     The system controller&#39;s directory entry format  322  differs from the cache directory entry format  310 . History directory entry format  322  includes the address tag  304  and a single log field  324 . Just as each cache directory entry within cache directory  208  contains processor accesses and snooped operations detected by the cache controller, each directory entry within history directory  118  contains system a log of all system transactions—or at least those considered important by the system controller—detected by the system controller  116 . 
     System controller  116  maintains within each entry in history directory  118  a log of all system bus transactions relating to the corresponding cache line, or at least those system bus transactions which are deemed important. Each recorded transaction includes: 
     a. the operation type (read, read with intent to modify, dclaim, etc); 
     b. the processor identifier for the processor which issued the transaction; 
     c. the individual responses to this transaction by snoopers; 
     d. the “hint” or guidance provided by the system controller within the combined response, if any; and 
     f. “hint” generation algorithm updates (e.g., previous guesses determined to be wrong), if any. 
     Entries  322  within history directory  118  contain different information than entries  310  within cache directory  208 . There exists some overlapping information, such as operation type, processor identifier, time stamps, and the hint provide. The system controller  116 , however, has visibility to the individual snoop responses from each snooper, while snoopers do not see each other&#39;s responses, only the combined response. This additional information aids the system controller  116  in formulating the hints which it provides. 
     Additionally, history directory  118  may maintain its history of a given cache line when all of the caches have since evicted the line (assuming that the system controller&#39;s history directory  118  is able to hold many more entries than just the individual cache directories, taken alone or in combination). When the line is eventually requested by one of the caches, the system controller  116  provides its hint history to the requestor, which may then be employed as a starting point to guide cache management as described in further detail below. 
     When a transaction occurs on the system bus, such as a read, read with intent to modify (rwitm), dclaim, etc., the system controller  116  snoops the operation (together with other bus participants) and looks up the cache line within its “transcation sequence log” or history directory  118 . Based on the information within the transaction log for a particular cache line, the system controller  116  may elect to append some hint (guidance) information to the combined snoop response for the transaction. 
     One example of such system controller hint information relates to read operations. During a read operation, snoopers which have the subject cache line in a shared state may, through hints appended to the combined response by the system controller  116 , be induced to go ahead and invalidate their copies when it is likely that a dclaim will soon follow. By invalidating early, it is less likely that a snooper will have to retry the dclaim (due to busy snoop queues) when the dclaim does occur. 
     Similarly, the master which initiates a read operation may be induced by hints from the system controller  116  to forward the data to the requesting processor without caching the data internally if it is likely that another cache will seek to modify the line soon. By not caching the line to begin with, snoopers will not have to do anything when the operation requiring invalidation of the line occurs, and will not tie up a snoop queue for the invalidating operation. The general goal, in both instances, is to reduce system bus traffic and reduce the average number of times which bus transactions get retried. 
     If a cache has the target cache line in a modified coherency state and needs to evict the cache line, the cache will pass that cache line&#39;s dynamic application sequence behavior (historical processor access, snoop operation, and system controller hint) information  312  to the system controller  116  at the same time as the modified data is being written back to memory. The system controller  116  need not directly store this information within the entry in history directory  118  for the corresponding cache line. Instead, the system controller  116  evaluates this information (in light of the history logged within the corresponding entry of history table  118 ) to determine whether the hints generation algorithms for this cache line need to be updated. 
     If a cache controller requests ownership of a cache line and no other caches have a copy of the line, then the data will be sourced from system memory and the system controller  116  will source the hint history log information to the requester. The requesting cache controller then stores this system controller guidance history in the system controller hint history segment of the T −1  subentry for the cache line. Unlike processor access and snoop operation history, where different history segments are stored based on the accessing processor, received system controller hint history is stored only in the T −1  segment. In this case, since no other cache supplied any history information to the requestor, the system controller&#39;s hint log is the only history field information with which the requesting cache controller starts out. 
     By tracking the snooped operations and system controller hints for each cache line, as well as “ownership” changes of each cache line, the cache management algorithms employed by the cache controller may be dynamically influenced. Although potentially less effective for larger caches, such dynamic cache management algorithms should help smaller cache approach the effectiveness of a larger cache through more intelligent cache management. Historical access, snoop operation, and system controller hint information may be utilized to influence victim selection, coherency state transitions, LRU state transitions, deallocation timing, and other cache management functions. The historical processor access, snoop operation, and system controller hint information enables the cache controller to engage in dynamic application sequence behavior, cache management which varies on a per cache line, per processor basis. 
     The cache controllers and the system controller need not have a set algorithm with respect to the particular actions for employing dynamic application sequence behavior information  312 / 324  in the different situations described above. Instead, the cache management algorithms may be dynamic and may start with a trial (guess) based on the historical processor access, snoop operation, and system controller hint information and then determine, over time, whether the guess was correct. If the initial guess was incorrect, the system controller may alter the algorithm the next time a similar situation arises. Over time, as these correct and incorrect decisions are made to change various behaviors, the frequency of incorrect decisions will decrease and the overall cache behavior will converge to a fixed algorithm. This algorithm will eventually be upset when the current software application (process) stops and another is started, and/or when a change occurs in the virtual to real memory address translation. 
     Referring to FIG. 4, a diagram of convergence in dynamic cache management to fixed algorithm cache management utilizing historical processor access, snoop operation, and system controller hint information in accordance with a preferred embodiment of the present invention is illustrated. A fixed algorithm  402  will result in constant behavior, while dynamic cache management based on historical processor access, snoop operation, and system controller hint information as described above will result in changing behavior  404   a - 404   b  which gradually approaches a constant. Some events, such as a change in the current software application being run or a change in the virtual to real memory address translation, will cause the behavior, which had previously achieved a steady state, to again change with a gradual approach to a constant. Not shown, however, are the efficiency and/or performance measures achieved for the fixed algorithm region  402 , which should exceed those of corresponding dynamic regions  404   a  and  404   b.  Note that the fixed algorithm which is converged upon may be unique to a particular cache line. Management of different cache lines may thus converge to different fixed algorithms. 
     With reference now to FIG. 5, a high level flow chart for a process of generating processor access history and passing processor access, snoop operation, and system controller hint history for cache lines in accordance with a preferred embodiment of the present invention is depicted. The process, which is initially idle, begins at step  502 , which depicts a processor access request being received at a cache such as cache  204  depicted in FIG.  2 . The process first passes to step  504 , which illustrates a determination of whether a cache hit for the requested cache line occurs within the cache receiving the processor access request. 
     If a cache hit occurs within the cache receiving the access request, the process proceeds to step  506 , which depicts sending the requested cache line to the processor (or other device) which requested access and logging the access into the processor access segment of the T 0  history field in the cache directory entry for the corresponding cache line. From step  506 , the process then passes to step  524 , described below. 
     If a cache miss occurs at step  504 , the process proceeds instead to step  508 , which illustrates selecting a victim (within the congruence class to which the requested cache line is indexed), which may be based, in part, on the history information, if any, of cache lines within the subject congruence class. The selected victim&#39;s history log is transferred to the system controller in conjunction with the write operation (cast-out) for the victim cache line to system memory, and may be employed by the system controller to update hint generation algorithms for this cache line. The process then passes to step  510 , which depicts requesting the new cache line via the system bus. 
     The process then passes to step  512 , which illustrates a determination of whether the requested cache line has been returned on the system bus. The requested cache line may be sourced from either system memory or, in the case of an intervention, from another cache within the system. If the requested cache line has not been received, the process returns to step  512  to continue awaiting return of the requested cache line. If so, however, the process proceeds instead to step  514 , which depicts sending the cache line to the requesting device and caching the cache line within the cache. The process then passes to step  516 , which illustrates a determination of whether any T −1  (processor access, snoop operation and system controller hint) history was received for the requested cache line via the combined response extensions. If so, the process proceeds to step  518 , which depicts updating the T −1  history field for the requested cache line within the cache directory. 
     The process passes next to step  520 , which illustrates a determination of whether any T −2  history was received for the requested cache line via the combined response extensions. No T −2  history will exist without some T −1  history. If T −2  history was received for the requested cache line, the process proceeds to step  522 , which depicts updating the T −2  history field for the requested cache line within the cache directory. The process then passes to step  524 , which illustrates updating the coherency state for the cache line within the directory (which may be updated based at least in part on T −1  history or T −2  history information, if any). The process then passes to step  526 , which depicts the process again becoming idle until another access request is received. 
     Referring to FIG. 6, a high level flow chart for a process of generating snoop operation history for cache lines in accordance with a preferred embodiment of the present invention is illustrated. The process, which is initially idle, is actively initiated at step  602 , which illustrates an operation being snooped off the system bus by a cache. The process passes first to step  604 , which illustrates a determination of whether the snooped operation is a cache directory hit within the snooping cache. If so, the process proceeds to step  606 , which depicts logging the snooped operation into the snoop operation segment of the T 0  history field of the corresponding cache directory entry. If the snooped operation is a cache miss, or once the snooped operation is logged, the process proceeds to step  608 , which illustrates updating the coherency state for the respective cache line within the cache directory, if necessary. The process then passes to step  610 , which depicts the process again becoming idle until another operation is snooped off the system bus by the cache. 
     With reference now to FIG. 7, a high level flow chart for a process of generating and transmitting system controller hints in accordance with a preferred embodiment of the present invention is depicted. The process, which is executed within the system controller and is initially idle, begins at step  702 , which depicts a system bus transaction being detected. The process first passes to step  704 , which illustrates a determination of whether the cache line which is the subject of the system bus transaction is a match for an entry within the history directory. If not the process proceeds to step  706 , which depicts selecting a victim within history directory for replacement. The manner of selecting a victim is not important, and may be performed utilizing a simple least-recently-used algorithm. A new entry is allocated in place of the selected victim. 
     Once a new entry has been allocated, or if the cache line address matched an entry within the history directory, the process proceeds to step  708 , which illustrates logging the detected transaction, the corresponding processor identifier, and other information into the allocated or matching history directory entry. The process then passes to step  710 , which depicts analysis of the transaction history for the subject cache line by the system controller. The process next passes to step  712 , which illustrates a determination of whether any information (e.g., “hints”) can be derived from the transaction history to send to the bus master initiating the detected system controller hint and/or to snoopers of the detected system controller hint. 
     If the analysis of the cache line transaction history produces information which might be useful to the bus master and/or any snoopers, the process proceeds to step  714 , which depicts appending the “hint” information to the combined response which is returned as a result of the system bus transaction. Otherwise, or once the hint information is appended, the process proceeds to step  716 , which illustrates the process becoming idle until another system controller hint is detected. 
     Referring to FIG. 8, a timing diagram illustrating passing of dynamic application sequence behavior among bus participants in accordance with a preferred embodiment of the present invention is illustrated. In the present invention, when a processor or cache (“Device A”) initiates a system bus operation  802  such as a read or read with intent to modify (rwitm), other (horizontal) processors or caches (“Device B” through “Device n”) transmit a snoop response  804  to the system controller. Along with the usual coherency response information, snoop responses  804  preferably include any historical access information associated with the target cache line within the corresponding processor or cache. 
     Individual snoop responses  804  are received by the system controller, together with any historical access or system transaction information within the history directory for the target cache line, to generate a combined response  806  which is returned to the requestor and all snoopers. In addition to the typical coherency response information, combined response  806  includes information or “hints” for the bus master or requester (Device A) and/or snoopers (Device B through Device n) such as: 
     information for the requester (and snoopers) regarding the access history for cache line by the requestor; 
     a suggestion that the requester NOT cache the data (on read operations); 
     a suggestion that the requester cache the data but then allow the cache line to be aged out sooner than others (i.e., make the cache line appear to be the least recently used); 
     a suggestion that the requester cache the data, but then allow the cache line to be aged out sooner than other AFTER a threshold number of loads/stores have been performed by the requestor (or a processor coupled to the requester); 
     a suggestion to the snoopers to go ahead and invalidate the cache line if they have it cached even though the issued request (e.g., read) would not require invalidation of the cache line. 
     Additionally, or alternatively, the dynamic application sequence behavior concatentated from among all bus participants may be appended to the combined response. 
     The present invention provides a system and method for communicating dynamic application sequence behavior information among bus participants within a multi-processor system. The dynamic application sequence behavior information may then be employed to more effectively manage cache contents through intelligent victim selection, caching decisions, and coherency settings. 
     While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.