Patent Publication Number: US-7721047-B2

Title: System, method and computer program product for application-level cache-mapping awareness and reallocation requests

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   The present application is related to the following U.S. patent application filed on even date herewith, and incorporated herein by reference in its entirety: 
   Ser. No. 11/006,127, filed on Dec. 7, 2004, entitled “SYSTEM, METHOD AND COMPUTER PROGRAM PRODUCT FOR APPLICATION-LEVEL CACHE-MAPPING AWARENESS AND REALLOCATION”. 
   BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The present invention relates generally to data processing and more specifically to cache access mechanisms in data processing systems. 
   2. Description of the Related Art 
   A conventional multiprocessor data processing system may comprise a system bus to which a system memory and a number of processing units that may each include a processor and one or more levels of cache memory are coupled. Caches are temporary storage facilities utilized to store subsets of the overall memory of a data processing system at varying latencies. At the various levels of a cache hierarchy, a tradeoff is made between the size and the access latency of the cache at the given hierarchy level. The cache most directly coupled to a processing unit, typically referred to as the level one or “L1” cache, usually has the lowest latency but is the smallest of the various caches. Likewise, the cache at the lowest level of the hierarchy usually has a larger storage capacity, often one or two orders of magnitude larger that the L1 cache, but at a higher access latency. 
   It is often the case, though not required, that the cache at a lower level of the cache hierarchy contains a copy of all the data contained in the caches at higher levels of the cache hierarchy. This property is known as “inclusion” and necessarily leads to the condition that a cache at a lower level of the cache hierarchy be at least as large as the cache at the next higher level of the hierarchy in order to allow the lower level cache to include the contents of memory cached at the next higher level. Those skilled in the art are familiar with the notion of constructing a multi-level cache hierarchy that optimizes the access latency and size characteristics of the various cache hierarchy levels according to available implementation technologies, leading to optimal system performance. 
   A cache, at a given level of hierarchy, is typically comprised of a number of components often including a cache directory array, a cache data array, and those functional logic units necessary to update and manage the cache. The data array portion of a cache is a set of data storage elements utilized to store copies of portions of main memory. The data array is divided into a series of so called “cache blocks”. These cache blocks are storage regions utilized to hold copies of contiguous portions of the main memory within the data processing system. These blocks are typically on the order of 128 bytes in size and are further arranged into groups, known as “sets”, of usually 8 to 16 blocks. The overall data array contains of a number of these sets. When placing a portion of memory within the cache, some number of the bits of the address of the block of memory are typically utilized to index into the various cache sets to determine a set within which to place the block of memory. That is to say, each contiguous aligned portion of main memory within the data processing system maps to a particular set. Within the cache set, various allocation policies are utilized to pick which member among the members within the set to place the block. In summary, the data array is divided into multiple cache sets which contain multiple cache blocks. Any given block in memory is typically allocated to some selected block within a particular set chosen by a mapping function of some of the address bits corresponding to the address of the block in main memory. 
   The cache further typically includes a cache directory array. This array consists of bookkeeping information detailing which portions of the overall data processing system memory and their processing states that are currently present within the cache. Typically, each block within the cache data array also has a corresponding entry within the cache directory array detailing which portion of main memory and its processing state is present in that cache data block. Each directory entry usually includes a number of fields possibly including a TAG field, a STATE field, an LRU field, an INCLUSION field, and an ECC field, which provides error correction and detection. 
   The TAG field within the directory entry corresponds to those high order address bits necessary to determine which block within the main memory is present within the cache data array entry associated with this directory entry. The TAG field typically represents the majority of the bits within a cache directory entry. The STATE field typically indicates the processing state of the cache line. For example, this field is often used to maintain the cache coherence state of the cache block according to some cache coherence protocol such as the well known “MESI” protocol. The LRU field typically contains information about recent accesses to the cache line and is used to guide the cache block replacement policy when cache blocks of new addresses are allocated within the cache set. Finally, the inclusion field often indicates whether or not the current cache block is present in a higher level cache. Those skilled in the art will appreciate that the format and contents of the directory entry discussed here is but one representative format possible. 
   In order to allow for larger lower level caches without dramatically adding to cache directory array overhead, a technique known as “sectoring” is often employed. In sectoring, the cache blocks in a lower level cache often consist of a number of different “sectors”. That is to say, in the lower level cache, the cache blocks as described above are further divided into two or more like-sized sub-regions. These sectors are typically equal in size to the cache block size of the cache immediately above the current cache in the cache hierarchy. 
   Furthermore, each of the sectors can typically be manipulated and managed individually. For example, one sector of a cache block could be present in the lower level cache and the other sector could be not present. To support independent processing of the various sectors, the directory entry is usually formatted to include STATE fields for each individual sector. Importantly, the single TAG field within the cache directory entry, which dominates the size of the cache directory entry, now corresponds to a larger cache block. In other words, a similar number of directory entries with additional STATE fields per sector can support a larger cache in the same cache directory area than would be possible with a non-sectored implementation that would require an additional TAG field for each sector. 
   Finally, the cache also contains functional logic queues that consist of the functional logic necessary to update the cache, provide data to higher level caches or the processing unit(s), and honor snooped requests from either the system interconnect or lower level caches. These functional queues are typically divided into two classes of queues: Read Queues and Snoop queues, which process requests from higher level caches or the processing unit(s) or from the system interconnect or lower level caches, respectively. As part of their function, these queues are responsible for updating the cache data and directory arrays. 
   The methods used today to optimize cache behavior include alignment and cache-line padding. Large pages can also be used to provide a uniform distribution in the cache. Each of these three approaches presents frustrating problems. Alignment in the cache, while providing object separation (e.g., two blocks separated on two cache lines to avoid conflicts), provides poor utilization of an available cache resource through large amounts of unused space. Similar issues exist with cache-line padding. Large pages provide better distribution, because real addresses within the large page sequentially map into congruence class sets. However, multiple large pages cause conflicts in the cache when large page mappings become identical. In addition, any application&#39;s access pattern may not be totally ideally suited to large pages (e.g., an application may benefit from interleaving objects within the cache). 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, the shortcomings of the prior art cache optimization techniques, the present invention provides an improved method, system, and computer program product that can optimize cache utilization. In one embodiment, an application requests a kernel cache map from a kernel service and the application receives the kernel. The application designs an optimum cache footprint for a data set from said application. The objects, advantages and features of the present invention will become apparent from the following detailed description. 
   In one embodiment of the present invention, the application transmits a memory reallocation order to a memory manager. 
   In one embodiment of the present invention, the step of the application transmitting a memory reallocation order to the memory manager further comprises the application transmitting a memory reallocation order containing the optimum cache footprint to the memory manager. 
   In one embodiment of the present invention, the step of the application transmitting a memory reallocation order to a memory manager further comprises the application transmitting the memory reallocation order containing to a reallocation services tool within the memory manager. 

   
     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 an illustrative embodiment of a multiprocessor data processing system with which the present invention may advantageously be utilized; 
       FIG. 2  is a high level block diagram of a sectored cache in accordance with the present invention; 
       FIG. 3  is a software layer diagram illustrating kernel services in accordance with one embodiment of the present invention; 
       FIG. 4  is a high-level logical flowchart of a process for request and receipt of a kernel-generated cache map and design of a cache reallocation scheme in accordance with a preferred embodiment of the present invention; 
       FIG. 5  is a high-level logical flowchart of a process for a kernel service creating and transmitting to an application a cache map according to one embodiment of the present invention; and 
       FIG. 6  is a high-level logical flowchart of a process for a kernel service reallocating cache resources in response to a request from an application according to one embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT 
   With reference now to the figures and in particular with reference to  FIG. 1 , there is illustrated a high level block diagram of a multiprocessor data processing system in accordance with the present invention. As depicted, data processing system  100  includes a number of processing units  110   a - 110   c  communicatively coupled to a system interconnect  112 . Each of processing units  110   a - 110   c  is an integrated circuit including one or more processors  114   a - 114   c . In addition to the registers, instruction flow logic and execution units utilized to execute program instructions, each of processors  114   a - 114   c  also includes an associated level one (L1) cache  116   a - 116   c , which temporarily stores instructions and data that are likely to be accessed by the associated processors  114   a - 114   c . Although L1 caches  116   a - 116   c  are illustrated in  FIG. 1  as unified caches that store both instruction and data (both referred to hereinafter simply as data), those skilled in the art will appreciate that each of L1 caches  116   a - 116   c  could alternatively be implemented as bifurcated instruction and data caches. 
   As further illustrated in  FIG. 1 , the memory hierarchy of data processing system  100  also includes distributed system memories  122   a - 122   c , which form the lowest level of volatile data storage in the memory hierarchy, and one or more lower levels of cache memory, such as on-chip level two (L2) caches  118   a - 118   c  and off-chip L3 caches  120   a - 120   c , which are utilized to stage data from system memories  122   a - 122   c  to processors  114   a - 114   c . Additionally, each of processors  114   a - 114   c  includes a translation lookaside buffer (TLB)  128   a - 128   c  for caching copies of entries from a distributed page frame table  126   a - 126   c , which is distributed throughout system memories  122   a - 122   c.    
   As understood by those skilled in the art, each succeeding lower level of the memory hierarchy is typically capable of storing a larger amount of data than higher levels, but at a higher access latency. For example, in an exemplary embodiment, L1 caches  116   a - 116   c  may each have 512 64-byte cache lines for a total storage capacity of 32 kilobytes (kB), all at single cycle latency. L2 caches  118   a - 118   c  may each have 2048 128-byte cache lines for a total storage capacity of 256 kB at approximately 10 cycle latency. L3 caches  120   a - 120   c  may each have 4096 256-byte cache lines for a total storage capacity of 1 MB, at a latency of approximately 40-60 cycles. Finally, each system memory  122   a - 122   c  can store tens or hundreds of megabytes of data at an even longer latency, for example, 300-400 cycles. Given the large disparity in access latencies between the various levels of the memory hierarchy, it is advantageous to reduce accesses to lower levels of the memory hierarchy and, in particular, to system memories  122   a - 122   c.    
   System interconnect  112 , which can comprise one or more buses or a cross-point switch, serves as a conduit for communicating transactions between processing units  110   a - 110   c  and other snoopers (e.g., L3 caches  120   a - 120   c ) coupled to system interconnect  112 . A typical transaction on system interconnect  112  begins with a request, which may include a transaction field indicating the type of transaction, source and destination tags indicating the source and intended recipient(s) of the transaction, respectively, and an address and/or data. Each device connected to system interconnect  112  preferably snoops all transactions on system interconnect  112  and, if appropriate, responds to the request with a snoop response. Such snoop responses are received and compiled by response logic  124 , which provides a combined response indicating what action, if any, each snooper is to take in response to the request. These actions may include sourcing data on system interconnect  112 , storing data provided by the requesting snooper, etc. 
   Those skilled in the art will appreciate that data processing system  100  can include many additional components, such as bridges to additional interconnects, I/O devices, non-volatile storage, and ports for connection to networks or attached devices, etc. Because such additional components are not necessary for an understanding of the present invention, they are not illustrated in  FIG. 1  or discussed further herein. 
   With reference now to  FIG. 2 , there is depicted a more detailed block diagram of an illustrative embodiment of a cache  230  that may be utilized to implement any of L1 caches  116   a ,  116   b  and  116   c , L2 caches  118   a ,  118   b  and  118   c , and L3 caches  120   a ,  120   b  and  120   c , in accordance with the present invention. In the illustrative embodiment, cache  230  is a four-way set associative cache including a directory array  232 , a data array  234 , and a cache controller  236 . Accordingly, data array  234  of cache  230  comprises a number of congruence classes (or sets) that each contain four ways for storing cache lines. As in conventional set associative caches, memory locations in system memories  222   a ,  222   b  and  222   c , are mapped to particular congruence classes (or sets) within data array  234  utilizing predetermined index bits within the system memory address. 
   As further shown in  FIG. 2 , each cache line, such as first cache line  280  and second cache line  282  within data array  234  is sectored into one or more sectors  238   a - 238   n  that can be individually accessed and modified. Although not required by the present invention, it is convenient if the sector size utilized by each cache is the same as the cache line size of the associated higher level cache, if any. For example, if L1 caches  116   a ,  116   b  and  116   c  have 64-byte cache lines, L2 caches  118   a ,  116   b  and  116   c  and L3 caches  120   a ,  120   b  and  120   c  preferably implement 256-byte (four 64-byte sectors) and 512-byte (four 128-byte sectors) cache lines, respectively. 
   The cache lines stored within data array  234  are recorded in cache directory array  232 , which contains one directory entry for each cache block in data array  234 . Each directory entry comprises a tag field  240 , one or more status fields  242   a - 242   n , a least recently used (LRU) field  244 , an inclusion field  246 , and an ECC field  268 . Tag field  240  specifies which cache line is stored in the corresponding block of data array  234  by storing the tag bits of the system memory address of the cache line. Status field  242  separately indicates the coherency and/or consistency status of each sector of the cache line stored in the corresponding block of data array  234  utilizing predefined bit combinations. LRU field  244  indicates how recently the corresponding block of data array  234  has been accessed relative to the other blocks within its congruence class, thereby indicating which cache line should be evicted from the congruence class in case of a cache miss. Token field  269  holds token data as described below with respect to  FIG. 3 . 
   Inclusion field  246  indicates whether or not each sector of the cache line stored in the corresponding way of data array  234  is also stored in the local memory unit (i.e., cache or system memory) at the next lowest level of the memory hierarchy. Finally, ECC field  268  provides check bits to correct and/or detect soft bit errors within the cache directory entry. It should be noted that an update to any field or subfield within the directory entry requires the re-computation of the ECC field value based on the new values of all the fields in the directory entry. 
   Still referring to  FIG. 2 , cache controller  236  manages storage and retrieval of data within data array  234  and updates to cache directory  232  in response to signals received from the associated components of the memory hierarchy and transactions snooped on system interconnect  112   a ,  112   b  and  112   c . As illustrated, cache controller  236  maintains a read queue  250  and a snoop queue  252  from which cache controller  236  performs updates to cache directory  232  and accesses to data array  234 . 
   In response to a received operation, a snoop or read queue typically performs a number of subtasks, only one of which is updating, if necessary, cache directory array  232 . These subtasks can include invalidating higher level cache directories, reading cache data from cache data array  234 , and intervening, delivering, or pushing cache data, among others. Certain higher level dependencies often require that these subtasks, including the directory array update, be performed in a certain order with some subtasks not overlapping one another. 
   Turning now to  FIG. 3 , a block diagram representation of a set of data processing operations interacting in accordance with one embodiment of the present invention is depicted. As shown in  FIG. 3 , the software configuration includes firmware  300 , which interacts with the underlying hardware of data processing system  100 . The software configuration further includes an operating system  302 , a set of application program interfaces (APIs)  306  and applications  308   a - 308   c.    
   Within operating system  302 , a kernel  304  provides a set of kernel services  310 - 318 . The first of these kernel services is a clock service  310  providing an internal clock. An interrupt kernel service  312  services interrupts. A task management kernel service  314  balances resources between tasks. A streams and I/O kernel service  316  provides interaction with I/O units. Memory manager  318  allocates memory resources, such as distributed system memories  122   a - 122   c , L2 caches  118   a - 118   c  and L3 caches  120   a - 120   c  to various tasks. One skilled in the art will quickly realize that other kernel services, not shown, may also be included. 
   Memory manager  318  interacts with firmware  300  through defined interfaces, such as messages  320  and  322 . Among the functions of memory manager  318  is a set of cache mapping services  324 . Memory manager  318  provides services necessary to translate virtual addresses, used by applications  308   a - 308   c  to physical addresses used by distributed system memories  122   a - 122   c , L2 caches  118   a - 118   c  and L3 caches  120   a - 120   c . The conversion between physical and virtual addresses is called address translation. Pinning memory is the construct of fixing the association between a virtual address and a physical address for translation purposes. Memory manager  318  uses token data  369  to provide cache-mapping services indicates the token range for a given directory entry. Each cache congruence class set will typically be assigned a unique token value. The token value will be within a range of 0 to ‘n’ where ‘n’ corresponds to the highest token value. The highest token value is a product of the last congruence class set. For example, in a cache with 2048 congruence class sets, where each congruence class contains 16 cache lines, the token values may range from 0 to 32,767. The maximum token value is implementation dependent, and could range to 2048 in an embodiment where there are 2048 congruence classes and no token assignment difference on the basis of cache lines. The token value could also range from 0 to 131,071 in an embodiment where there are 2048 congruence classes and a different token value for each cache sector. The present invention is adaptable to any method by which the cache is divided or partitioned, because it assigns unique tokens to each significant division within the cache. Memory manager  318  also provides reallocation services  327 . 
   The present invention provides for an application  308   c  to receive a cache map by sending a cache map request  326  to cache mapping services  324  within memory manager  318  of kernel  304 . Cache mapping services  324  within memory manager  318  then send messages  320  to firmware  300  and receive messages  320  detailing the mapping of cache memory, as is well known in the art. Cache mapping services  324  within memory manager  318  of kernel  304  then sends a cache map  328  to application  308   c.    
   Likewise, the present invention allows application  308   c  to send a reallocation request  330  to reallocation services  327  on memory manager  318  of kernel  304 . Reallocation services  327  can then send messages  322  to firmware  300 , reallocating virtual addresses to different parts of the physical RAM, and can respond to application  308   c  by sending a reallocation response  332  to application  308   c.    
   With reference now to  FIG. 4 , a high-level logical flowchart of a process on an application for request and receipt of a kernel generated cache map and design of a cache reallocation scheme in accordance with a preferred embodiment of the present invention is depicted. The process starts at step  400 . The process next moves to step  402 , which depicts an application  308   c  requesting a cache map by sending a cache map request  326  to cache mapping services  324  of memory manager  318  of kernel  304 . The process then proceeds to step  404 . Step  404  depicts application  308   c  receiving a cache map  328  from cache mapping services  324  on memory manager  318  of kernel  304 . An application  308   c  is free to request any cache description for any pages allocated at any time. Such a cache description will include token field  369  values associated with the appropriate cache line. 
   The process then moves to step  412 . At step  412 , application  308   c  determines whether any problem exists in the current cache mapping for application  308   c  that requires the design of a new cache map. If so, the process moves to step  406 , which is described below. If the current cache map is acceptable, then the process ends at step  414 . 
   The process then proceeds to step  406 , which depicts application  308   c  designing an optimum cache footprint. In the present invention, an application can use cache token values to evaluate a range of or all of the allocated pinned memory for its objects and decide if an appropriate cache distribution has been allocated. The method by which an application would decide if an appropriate cache distribution has been allocated is application-dependent. Some applications will need an allocation scheme where all cache tokens in the applicable range are used. Others will want an allocation wherein the statistical distribution of the tokens follows a given pattern. Optimization of the reallocation routine could include a list of recently reallocated pages that are not candidate pages. The listing could occur at the thread, process, or system scope. One example of an optimized cache footprint is represented by concentrating data within a single congruence class or a limited set of congruence classes, such as first cache line  280  and second cache line  282 . Such an example would prove appropriate where an application delivers temporally disjoint data streams, and the data streams would occupy the same congruence class and be supported by the same token  269 . Conversely, data sets from other applications, such as those performing matrix transformations, may be distributed (to maximize the likelihood of high cache access availability and efficiency) across multiple congruence classes. As is well known in the art, multiple consecutive accesses to data in the same congruence class are less efficiently serviced by many cache designs. 
   The process next proceeds to step  408 . At step  408 , application  308   c  transmits a memory reallocation request  330  to reallocation services  327  of memory manager  318  of kernel  304 . Individual applications will tailor the decision as to when allocation is suitable and the number of acceptable attempts to reallocate pages. An application will eventually receive an acceptable cache mapping or abandon the attempt to reallocate. Reallocation services  327  of memory manager  318  of kernel  304  then sends a message  322  to firmware  300  reallocating memory and response to reallocation request. 
   The process next moves to step  410 , which depicts application  308   c  receiving reallocation request response  332  from reallocation services  327 , of memory manager  318  of kernel  304 . The process then returns to step  402 , which is described above. 
   Turning now to  FIG. 5 , a high level logical flow chart of a process for a kernel service creating and transmitting to an application a cache map according to one embodiment of the present invention is illustrated. The process starts at step  500 . The process then moves to step  502 , which depicts cache mapping services  324  on memory manager  318  of kernel  304  receiving cache map request  326  from application  308   c . The process next proceeds to step  504 . At step  504 , cache mapping services  324  of memory manager  318  of kernel  304  extract from cache map request  326  a virtual pointer. The process next proceeds to step  506 , which depicts cache mapping services  324  translating the virtual pointer extracted in step  504  into a physical address. 
   The process then moves to step  508 , at which cache mapping services  324  of memory manager  318  of kernel  304  generate a cache map structure describing the physical address token range from token field  269  in directory array  232  of cache  230 . Cache mapping services  324  then generates a cache map structure through interaction with firmware  300  using messages  320 . The process then moves to step  510 , which depicts cache mapping service  324  on memory manager  318  of kernel  304  transmitting a cache map  328  to the requesting application  308   c . The process then ends at step  512 . 
   With reference now to  FIG. 6 , a high level logical flow chart of a process for a kernel service reallocating cache resources in response to a request from an application according to one embodiment of the present invention is depicted. The process starts at step  600  and then moves to step  602 . At step  602 , reallocation services  327  of memory manager  318  of kernel  304  receives a cache reallocation request  330  from application  308   c.    
   The process next moves to step  604 , which depicts reallocation services  327  of memory manager  318  of kernel  304  determining whether cache reallocation request  330  requests access to restricted resources, such as a restricted address range. An application is free to reallocate any unrestricted page irrespective of whether the page has previously been reallocated. If reallocation services  327  on memory manager  318  of kernel  304  determines that a cache reallocation request  330  from application  308   c  requests access to restricted resources, then the process proceeds to step  618 , which depicts reallocation services  327  sending a reallocation request response  332  containing an error message to application  308   c . The process then ends at step  620 . 
   Returning to step  604 , if reallocation services  327  on memory manager  318  of kernel  304  determines that the reallocation request  330  sent by application  308   c  does not request restricted resources, then the process proceeds to step  606 . Step  606  depicts reallocation services  327  unpinning a physical page within memory manager  318 . The process then moves to step  608 , which depicts memory manager  318  removing the page frame table entry affecting the reallocated RAM from page frame table  126   a - 126   c . The process next proceeds to step  610 , which depicts reallocation services  327  on memory manager  318  of kernel  304  allocating a different physical page to the page represented by reallocation request  306 . Reallocation of a physical page leaves a virtual address in place, and does not change the semantics of the application objects&#39; access to page objects. Thus, a multi-page object which has contiguous pages from the application&#39;s point of view will continue to have contiguous pages. Allocating a different physical page will necessitate update of the translation look-aside buffers  128   a - 128   c . The process then moves to step  612 , which depicts reallocation services  320  on memory manager  318  of kernel  304  adding a new page frame table entry to page frame tables  126   a - 126   c  reflecting the reallocation indicated in reallocation request  330 . 
   The process next moves to step  614 , which depicts reallocation services  320  and memory manager  318  of kernel  304  determining whether the reallocation requested in reallocation request  330  was successfully executed. If the reallocation request contained in reallocation request  330  was successfully executed by reallocation services  320  on memory manager  318  of kernel  304 , then the process proceeds to step  616 , which illustrates sending to application  308   c  a reallocation request response to  332  containing confirmation that the reallocation request  330  was successfully executed. The process then ends at step  620 . If the reallocation request  330  was not successfully executed, then the process moves to step  618 , which is described above. 
   The present invention provides a method for a kernel service to provide information to an application about caching properties for a particular memory resource. The present invention allows an application to use information provided by the kernel service to optimize its cache footprint for the lifetime of the application. The anticipated use of this kernel service would be long running computation workloads that can afford the higher cost to improve an optimized cache layout. These applications can be very sensitive to cache efficiency, and, in the case of a scientific workload, throughput is paced by the longest latency. By optimizing cache footprint, the present invention allows the long-running applications to improve their performance. 
   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.