Abstract:
A method and apparatus for implementing a combined data/coherency cache for a shared memory multi-processor. The combined data/coherency cache includes a system cache with a number of entries. The method includes building a system cache directory with a number of entries equal to the number of entries of the system cache. The building includes designating a number of sub-entries for each entry which is determined by a number of sub-entries operable for performing system cache coherency functions. The building also includes providing a sub-entry logic designator for each entry, and mapping one of the sub-entries for each entry to the system cache via the sub-entry logic designator.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to data caches, and particularly to a method and apparatus for implementing a combined data/coherency cache.  
         [0003]     2. Description of Background  
         [0004]     In a large shared-memory multiprocessor, providing a system-level cache of the recently accessed contents of memory, along with an efficient means to handle system-wide cache coherency, can be accomplished with a single system cache directory array by requiring the contents of the respective processor-level caches to be a subset of the system cache. Unfortunately, when the combined size of the processor caches is sufficiently large, this subset rule can become impractical because of the resulting size of the system cache required to work effectively becomes too big. While one possible solution to this is to maintain two directories (one for the system cache, one for all the processor cache contents), this complicates the design significantly. Using two separate directories to accomplish the same task requires more logic, both to synchronize the contents of the two directories (either to keep them distinct, or to manage them if allowed to overlap), as well as to carry out any system memory access (which requires looking up both directories and taking the appropriate action in each).  
         [0005]     What is needed, therefore, is a single system cache directory that is large enough to contain all the processor cache directory entries, but with only sufficient system cache to back the most recent fraction of data accessed by the processors.  
       SUMMARY OF THE INVENTION  
       [0006]     The shortcomings of the prior art are overcome and additional advantages are provided through the provision of a method and apparatus for implementing a combined data/coherency cache for a shared memory multi-processor. The combined data/coherency cache includes a system cache with a number of entries.  
         [0007]     The method includes building a system cache directory with a number of entries equal to the number of entries of the system cache. The building includes designating a number of sub-entries for each entry, which is determined by a number of sub-entries operable for performing system cache coherency functions. The building also includes providing a sub-entry logic designator for each entry, and mapping one of the sub-entries for each entry to the system cache via the sub-entry logic designator.  
         [0008]     System and computer program products corresponding to the above-summarized methods are also described and claimed herein.  
         [0009]     Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with advantages and features, refer to the description and to the drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:  
         [0011]      FIG. 1  is a system diagram of a shared memory multi-processor utilizing a system cache structure in the prior art;  
         [0012]      FIG. 2  is a modified congruence class utilized by the combined data/coherency cache in exemplary embodiments; and  
         [0013]      FIG. 3A-3B  is a flow diagram describing a process for performing look-up and install procedures via the combined data/coherency cache structure in exemplary embodiments. 
     
    
       [0014]     The detailed description explains the preferred embodiments of the invention, together with advantages and features, by way of example with reference to the drawings.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     In accordance with exemplary embodiments, a single system cache directory and system level cache for a shared memory multi-processor is provided. The system cache directory is sufficient in size to contain the entries of all underlying processor cache directories. The system cache is sized commensurate given limitations such as desired access time and reasonable chip area. The most recent fraction of the system cache directory entries are mapped into the system cache. This structure behaves both as a cache (for recently accessed memory data needed by the processors) and a system-wide cache coherency controller (by maintaining a record of which processors have copies of which portions of memory and what state they are in). By using a single directory, the job of both cache management and cache coherency management can be maintained with one look-up and set of resulting actions.  
         [0016]     For background purposes, a shared-memory multiprocessor system  100  including system cache and directory components utilized in the prior art will now be described with respect to  FIG. 1 . In the shared memory multi-processor environment of  FIG. 1 , each of the processors P 1  and P 2  has its own set of processor caches (C 1  and C 2  for P 1 , and C 3  and C 4  for P 2 ), which contain copies of the instructions and data of recent work performed on the processor. It will be understood that any number of processors and processor caches may be included in the system  100  of  FIG. 1 . In order to work efficiently in such an environment, a system control structure is utilized for controlling resource access, ensure cache coherency, etc. This system control structure may be multi-level; however, a single level is described herein for purposes of illustration. It will be understood that extensions to multiple levels may be implemented in a manner analogous to the single level implementation.  
         [0017]     System control structures include two elements: a system cache and system cache coherency. A system cache, similar to the processor caches, is a cache of all the recent instructions and data of all the processors under control of this system. It performs two key roles: resupplies data to a processor cache when such data ages out or is otherwise removed from the processor cache, and provides data to other processors (possibly via other system caches), when they need access to it.  
         [0018]     System cache coherency involves recordkeeping. Oftentimes, memory cannot be accessed every time it is needed or changed because the access time is too great relative to the speed of the processors. Consequently, the job of maintaining a single, coherent, view of memory to the processors (e.g., P 1 , P 2 ) via their respective processor caches (e.g., C 1 -C 4 ) falls on the system control structure via the system cache directory  102 . By maintaining a record of which processors (P 1 , P 2 ) have cached which portions of memory they are actively using, the system control can take the appropriate action when another processor needs to access the same parts of memory.  
         [0019]     Obviously, if the system cache can be built sufficiently large enough to accomplish both tasks (by requiring all processor cache contents to be part of the system cache contents, otherwise known as the subset rule), there is no problem. But if such a cache is too large for practical reasons, a redesign of the system may be required or perhaps a system cache is not utilized. Alternatively, a subset rule may be utilized that effectively limits a significant portion of the processor caches available to the processors, or having two separate system cache directories (i.e., one to manage the cache and one to manage the cache coherency) as well as the resulting complexity required to make it work.  
         [0020]     By way of example, suppose the system cache is comprised of N=C*A entries, where C represents the number of congruence classes (where each of the congruence classes represents a set of addresses of memory corresponding to one of C possible values), and A represents the associativity of each congruence class (the number of cache entries that can share the same address mapping used to select the congruence class).  
         [0021]     Typical computer cache designs have such a structure, along with a corresponding cache directory (e.g.,  102 ) with a similar C*A structure, whereby each directory entry represents the corresponding cache entry and contains such information as the memory address of the cache entry, the last processor to access the data, and whether the data has been changed with respect to the memory contents. Lastly, some form of LRU (least recently used) logic (e.g., logic column  106 ) is present for each congruence class to manage the entries within that congruence class from least recently used to most recently used (MRU). The two most prevalent uses of LRU logic are update and install.  
         [0022]     An update is performed by looking up and finding an address in a directory, typically making it the MRU entry and displacing all those entries that stand in between it and the MRU position. An install is performed when an address is not found in a directory, whereby a place is cleared for it. This time the LRU entry is chosen and replaced with the new entry, then making the new entry the MRU.  
         [0023]     In accordance with exemplary embodiments, a combined data/coherency cache structure is provided that utilizes a single system cache of practical size, where only the most recent entries in the directory have corresponding entries in the cache. Turning now to  FIG. 2 , a modified congruence class  200  for implementing the combined data/coherency cache will now be described. Continuing with the example cache system  100  described in  FIG. 1 , it is assumed that a number of entries ‘N’ are insufficient to provide efficient system cache coherency, although  2 *N entries are sufficient. The system cache may be kept to N entries, however each system cache directory entry may be widened to contain two separate addresses (e.g., sub  1  or sub  2 ) along with corresponding logic (referred to herein as a sub-entry logic designator  202 ) for determining which address represents the corresponding cache entry for this system cache directory entry. In a sense, there are two sub-entries (e.g., Sub-entries  1  and  2 ) per directory entry (e.g., Entry  1 ) as shown in the modified congruence class  200  of  FIG. 2 . Only one of these sub-entries for each entry will have a corresponding cache entry. It will be understood that, with a little more logic, this concept may be extended to any multiple M*N entries, essentially adding a simple M-way LRU logic (a sub-LRU logic) to each entry (M sub-entries) in the cache directory.  
         [0024]     The single cache directory of the combined data/coherency cache system performs both roles updates and installs as will now be described in the flow diagram of  FIG. 3A-3B . When looking up a particular address, all sub-entries in the appropriate congruence class (e.g.,  200 ) are examined to find a possible match at step  302 . If found (step  304 ), and if that particular sub-entry is one that has a corresponding cache entry (step  306 ), this indicates that its data is available in the cache. Appropriate action is taken (e.g., supply data to a processor cache) and the corresponding congruence class has its LRU logic (e.g., logic column  204 ) updated accordingly at step  308 . If found (step  304 ), but the sub-entry is not one with a corresponding cache entry (step  306 ), the directory information may be used to do whatever processing is needed at step  310 . If the data has to be put in the corresponding cache entry (and sub-LRU logic designator updated), the current MRU sub-entry has its data replaced (along with any other necessary processing, such as writing the data back to memory if it differs from the memory copy) at step  312 , and the entire congruence class has its LRU logic (e.g., logic in column  204 ) updated at step  314  (e.g., as with a typical cache install). Turning back to step  304 , if the address does not match any sub-entry, this indicates that either sufficient information is provided at step  316 , or the data needs to be installed and the process, proceeds to  FIG. 3B .  
         [0025]     At step  318 , the LRU entry (e.g., one of entries  1 - 3 ) in the congruence class  200  is identified. Within this entry, the MRU sub-entry and LRU sub-entry (e.g., one of sub-entries  1 - 2 ) are identified at step  320 . The MRU sub-entry is to have its corresponding data removed from the cache directory at step  322 . Appropriate action is taken for this cache data (e.g., writing it back to memory if it differs from the memory copy).  
         [0026]     The LRU sub-entry (e.g., one of sub-entries  1 - 2  associated with the sub-entry logic designator identified above) is removed from the directory and any appropriate action is taken in the underlying processor caches (e.g., invalidate their copies) at step  324 . The LRU sub-entry is replaced with the new address and the new address is made to be the MRU sub-entry at step  326 .  
         [0027]     At step  328 , the corresponding cache contents are replaced with that of the new address. The LRU entry (one of entries associated with the LRU in column  204 ) in the congruence class is made the MRU entry to complete the install procedure at step  330 .  
         [0028]     As described above, a means for constructing a single structure that performs both the role of system cache (data) and system control (coherency) is provided in cases where the size of the system cache is insufficient to contain the contents of all of the underlying caches. While other solutions exist for this problem, such as multiple structures (extra complexity), or relaxed subset rules (extra overhead on the underlying caches to manage the coherency), or no data cache (and thus, no fast central access to recently accessed data), none are able to deliver the advantages and simplicity of the combined data/coherency cache.  
         [0029]     The capabilities of the present invention can be implemented in software, firmware, hardware or some combination thereof.  
         [0030]     As one example, one or more aspects of the present invention can be included in an article of manufacture (e.g., one or more computer program products) having, for instance, computer usable media. The media has embodied therein, for instance, computer readable program code means for providing and facilitating the capabilities of the present invention. The article of manufacture can be included as a part of a computer system or sold separately.  
         [0031]     Additionally, at least one program storage device readable by a machine, tangibly embodying at least one program of instructions executable by the machine to perform the capabilities of the present invention can be provided.  
         [0032]     The flow diagrams depicted herein are just examples. There may be many variations to these diagrams or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order, or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention.  
         [0033]     While the preferred embodiment to the invention has been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.