Classifications

• • G—PHYSICS
  • G06—COMPUTING OR CALCULATING; COUNTING
  • G06F—ELECTRIC DIGITAL DATA PROCESSING
  • G06F12/00—Accessing, addressing or allocating within memory systems or architectures
  • G06F12/02—Addressing or allocation; Relocation
  • G06F12/08—Addressing or allocation; Relocation in hierarchically structured memory systems, e.g. virtual memory systems
• • G—PHYSICS
  • G06—COMPUTING OR CALCULATING; COUNTING
  • G06F—ELECTRIC DIGITAL DATA PROCESSING
  • G06F12/00—Accessing, addressing or allocating within memory systems or architectures
  • G06F12/02—Addressing or allocation; Relocation
  • G06F12/08—Addressing or allocation; Relocation in hierarchically structured memory systems, e.g. virtual memory systems
  • G06F12/0802—Addressing of a memory level in which the access to the desired data or data block requires associative addressing means, e.g. caches
  • G06F12/0875—Addressing of a memory level in which the access to the desired data or data block requires associative addressing means, e.g. caches with dedicated cache, e.g. instruction or stack
• • G—PHYSICS
  • G06—COMPUTING OR CALCULATING; COUNTING
  • G06F—ELECTRIC DIGITAL DATA PROCESSING
  • G06F12/00—Accessing, addressing or allocating within memory systems or architectures
  • G06F12/02—Addressing or allocation; Relocation
  • G06F12/08—Addressing or allocation; Relocation in hierarchically structured memory systems, e.g. virtual memory systems
  • G06F12/0802—Addressing of a memory level in which the access to the desired data or data block requires associative addressing means, e.g. caches
  • G06F12/0864—Addressing of a memory level in which the access to the desired data or data block requires associative addressing means, e.g. caches using pseudo-associative means, e.g. set-associative or hashing
• • G—PHYSICS
  • G06—COMPUTING OR CALCULATING; COUNTING
  • G06F—ELECTRIC DIGITAL DATA PROCESSING
  • G06F12/00—Accessing, addressing or allocating within memory systems or architectures
  • G06F12/02—Addressing or allocation; Relocation
  • G06F12/08—Addressing or allocation; Relocation in hierarchically structured memory systems, e.g. virtual memory systems
  • G06F12/12—Replacement control
  • G06F12/121—Replacement control using replacement algorithms
  • G06F12/126—Replacement control using replacement algorithms with special data handling, e.g. priority of data or instructions, handling errors or pinning
• • G—PHYSICS
  • G06—COMPUTING OR CALCULATING; COUNTING
  • G06F—ELECTRIC DIGITAL DATA PROCESSING
  • G06F12/00—Accessing, addressing or allocating within memory systems or architectures
  • G06F12/02—Addressing or allocation; Relocation
  • G06F12/08—Addressing or allocation; Relocation in hierarchically structured memory systems, e.g. virtual memory systems
  • G06F12/0802—Addressing of a memory level in which the access to the desired data or data block requires associative addressing means, e.g. caches
  • G06F12/0862—Addressing of a memory level in which the access to the desired data or data block requires associative addressing means, e.g. caches with prefetch

Definitions

• the present invention relates in general to the field of processors, and in particular, to a technique of providing a shared cache structure for temporal and non-temporal instructions.
• cache memory with a processor facilitates the reduction of memory access time.
• the fundamental idea of cache organization is that by keeping the most frequently accessed instructions and data in the fast cache memory, the average memory access time will approach the access time of the cache.
• typical processors implement a cache hierarchy, that is, different levels of cache memory.
• the different levels of cache correspond to different distances from the processor core. The closer the cache is to the processor, the faster the data access. However, the faster the data access, the more costly it is to store data. As a result, the closer the cache level, the faster and smaller the cache.
• cache memory The performance of cache memory is frequently measured in terms of its hit ratio.
• the processor refers to memory and finds the word in cache, it is said to produce a hit. If the word is not found in cache, then it is in main memory and it counts as a miss. If a miss occurs, then an allocation is made at the entry indexed by the access. The access can be for loading data to the processor or storing data from the processor to memory. The cached information is retained by the cache memory until it is no longer needed, made invalid or replaced by other data, in which instances the cache entry is de-allocated.
• the faster and smaller LI cache is located closer to the processor than the L2 cache.
• the processor requests cacheable data, for example, a load instruction
• the request is first sent to the LI cache. If the requested data is in the LI cache, it is provided to the processor. Otherwise, there is an LI miss and the request is transferred to the L2 cache. Likewise, if there is an L2 cache hit, the data is passed to the LI cache and the processor core. If there is an L2 cache miss, the request is transferred to main memory. The main memory responds to the L2 cache miss by providing the requested data to the L2 cache, the LI cache, and to the processor core.
• the type of data that is typically stored in cache includes active portions of programs and data. When the cache is full, it is necessary to replace existing lines of stored data in the cache memory to make room for newly requested lines of data.
• One such replacement technique involves the use of the least recently used (LRU) algorithm, which replaces the least recently used line of data with the newly requested line.
• LRU least recently used
• the L2 cache since the L2 cache is larger than the LI cache, the L2 cache typically stores everything in the LI cache and some additional lines that have been replaced in the LI cache by the LRU algorithm.
• U.S. Patent Application serial number 08/767,950 filed December 17, 1996, entitled “Cache Hierarchy Management” discloses a technique for allocating cache memory through the use of a locality hint associated with an instruction.
• a processor accesses memory for transfer of data between the processor and the memory, that access can be allocated to the various levels of cache, or not allocated to cache memory at all, according to the locality hint associated with the instruction.
• Certain instructions are used infrequently.
• non-temporal prefetch instructions preload data which the processor does not require immediately, but which are anticipated to be required in the near future. Such data is typically used only once or will not be reused in the immediate future, and is termed “non-temporal data”. Instructions that are frequently used are termed "temporal data".
• U.S. Application serial number 08/767,950 solves this problem by providing a buffer, separate from the cache memory, for storing the infrequently used data, such as non-temporal prefetched data.
• a buffer separate from the cache memory, for storing the infrequently used data, such as non-temporal prefetched data.
• the use of an extra, separate buffer is expensive both in terms of cost and space.
• a method and system for providing cache memory management comprises a main memory, a processor coupled to the main memory, and at least one cache memory coupled to the processor for caching of data.
• the at least one cache memory has at least two cache ways, each comprising a plurality of sets. Each of the plurality of sets has a bit which indicates whether one of the at least two cache ways contains non-temporal data.
• the processor accesses data from one of the main memory or the at least one cache memory.
• Figure 1 illustrates a circuit block diagram of one embodiment of a computer system which implements the present invention, in which a cache memory is used for data accesses between a main memory and a processor of the computer system.
• Figure 2 is a circuit block diagram of a second embodiment of a computer system which implements the present invention, in which two cache memories are arranged into cache memory levels for accessing data between a main memory and a processor(s) of the computer system.
• Figure 3 is a block diagram illustrating one embodiment of the organizational structure of the cache memory in which the technique of the present invention is implemented.
• Figure 4 is a table illustrating the cache management technique, according to one embodiment of the present invention.
• Figures 5A and 5B illustrate one example of the organization of a cache memory prior to and after temporal instruction hits way 2 of cache set 0, according to one embodiment of the present invention.
• Figures 6A and 6B illustrate another example of the organization of a cache memory prior to and after temporal instruction hits way 2 of cache set 0, according to one embodiment of the present invention.
• Figures 7A - 7D illustrate a example of the organization of a cache memory prior to and after a non-temporal instruction hits way 2 of cache set 0, according to one embodiment of the present invention.
• Figures 8A - 8D illustrate another example of the organization of a cache memory prior to and after a non-temporal instruction hits way 2 of cache set 0, according to one embodiment of the present invention.
• Figures 9A and 9B illustrate one example of the organization of a cache memory prior to and after a temporal instruction miss to cache set 0, according to one embodiment of the present invention.
• Figures 10 A - 10D illustrate an example of the organization of a cache memory prior to and after a non-temporal instruction miss to cache set 0, according to one embodiment of the present invention.
• a technique for providing management of cache memories, in which cache allocation is determined by data utilization.
• numerous specific details are set forth, such as specific memory devices, circuit diagrams, processor instructions, etc., in order to provide a thorough understanding of the present invention.
• the present invention may be practiced without these specific details.
• well known techniques and structures have not been described in detail in order not to obscure the present invention. It is to be noted that a particular implementation is described as a preferred embodiment of the present invention, however, it is readily understood that other embodiments can be designed and implemented without departing from the spirit and scope of the present invention.
• a typical computer system is shown, wherein a processor 10, which forms the central processing unit (CPU) of the computer system is coupled to a main memory 11 by a bus 14.
• the main memory 11 is typically comprised of a random-access- memory and is usually referred to as RAM.
• the main memory 11 is generally coupled to a mass storage device 12, such as a magnetic or optical memory device, for mass storage (or saving) of information.
• a cache memory 13 (hereinafter also referred simply as cache) is coupled to the bus 14 as well.
• the cache 13 is shown located between the CPU 11 and the main memory 11, in order to exemplify the functional utilization and transfer of data associated with the cache 13. It is appreciated that the actual physical placement of the cache 13 can vary depending on the system and the processor architecture. Furthermore, a cache controller 15 is shown coupled to the cache 13 and the bus 14 for controlling the operation of the cache 13. The operation of a cache controller, such as the controller 15, is known in the art and, accordingly, in the subsequent Figures, cache controllers are not illustrated. It is presumed that some controller(s) is/are present under control of the CPU 10 to control the operation of cache(s) shown.
• information transfer between the memory 11 and the CPU 10 is achieved by memory accesses from the CPU 10.
• cacheable data is currently or shortly to be accessed by the CPU 10, that data is first allocated in the cache 13. That is, when the CPU 10 accesses a given information from the memory 11, it seeks the information from the cache 13. If the accessed data is in the cache 13, a "hit” occurs. Otherwise, a "miss” results and cache allocation for the data is sought.
• most accesses (whether load or store) require the allocation of the cache 13. Only uncacheable accesses are not allocated in the cache. Referring to Figure 2, a computer system implementing a multiple cache arrangement is shown.
• the CPU 10 is still coupled to the main memory 11 by the bus 14 and the memory 11 is then coupled to the mass storage device 12.
• two separate cache memories 21 and 22 are shown.
• the caches 21-22 are shown arranged serially and each is representative of a cache level, referred to as Level 1 (LI) cache and Level 2 (L2) cache, respectively.
• the LI cache 21 is shown as part of the CPU 10, while the L2 cache 22 is shown external to the CPU 10.
• This structure exemplifies the current practice of placing the LI cache on the processor chip while lower level caches are placed external to it, where the lower level caches are further from the processor core.
• the actual placement of the various cache memories is a design choice or dictated by the processor architecture.
• the LI cache could be placed external to the CPU 10.
• CPU 10 includes an execution unit 23, register file 24 and fetch/decoder unit 25.
• the execution unit 23 is the processing core of the CPU 10 for executing the various arithmetic (or non-memory) processor instructions.
• the register file 24 is a set of general purpose registers for storing (or saving) various information required by the execution unit 23. There may be more than one register file in more advanced systems.
• the fetch/decoder unit 25 fetches instructions from a storage location (such as the main memory 11) holding the instructions of a program that will be executed and decodes these instructions for execution by the execution unit 23.
• a bus interface unit (BIU) 26 provides an interface for coupling the various units of CPU 10 to the bus 14.
• the LI cache is coupled to the internal bus 27 and functions as an internal cache for the CPU 10.
• the LI cache could reside outside of the CPU 10 and coupled to the bus 14.
• the caches can be used to cache data, instructions or both. In some systems, the LI cache is actually split into two sections, one section for caching data and one section for caching instructions.
• the computer system may be comprised of more than one CPU (as shown by the dotted line in Figure 2). In such a system, it is typical for multiple CPUs to share the main memory 11 and/or mass storage unit 12. Accordingly, some or all of the caches associated with the computer system may be shared by the various processors of the computer system. For example, with the system of Figure 2, LI cache 21 of each processor would be utilized by its processor only, but the main memory 11 would be shared by all of the CPUs of the system. In addition, each CPU has an associated external L2 cache 22. The invention can be practiced in a single CPU computer system or in a multiple CPU computer system.
• DMA direct memory accessing
• Figure 3 is a block diagram illustrating one embodiment of the organizational structure of the cache memory in which the technique of the present invention is implemented.
• the invention provides an LRU lock bit which indicates whether any one of the ways within that set contains non-temporal (NT) data. If so, the regular or pseudo LRU bits will be updated to point to the NT data.
• NT non-temporal
• the number of regular or pseudo-LRU bits per set varies depending on the number of ways per set and the LRU (regular or pseudo) technique implemented.
• the cache 50 is organized as a four- way set associative cache.
• each page is shown as being equal to one-fourth the cache size.
• the cache 50 is divided into four ways (for example, way 0 (52), way 1 (54), way 2 (56) and way 3 (58)) of equal size and main memory 11 (see also Figures 1 and 2) is viewed as divided into pages (e.g., page 0 - page n).
• each page may be larger or smaller than the cache size.
• the organizational structure of cache 50 (as shown in Figure 3) may be implemented within the cache 13 of Figure 1, the LI cache and /or L2 cache 22 of Figure 2.
• the cache 50 also includes an array of least recently used (LRU) bits 60o - 60n each of which points to the way within a set with the least recently used data (or NT data, if a biased LRU technique is implemented). Such listing is performed in accordance with an LRU technique under the control of the cache controller 15, to determine which cache entry to overwrite in the event that a cache set is full.
• the LRU logic (not shown) keeps track of the cache locations within a set that have been least recently used.
• an LRU technique that strictly keeps track of the least-recently used directory algorithm may be implemented.
• a pseudo-LRU algorithm which makes a best attempt at keeping track of the least recently used directory element, is implemented.
• the cache 50 further includes an array of LRU lock bits 70o - 70n which indicates whether any of the ways 52, 54, 56, 58 within a given set contains data that should not pollute the cache 50 (i.e., data with infrequent usage), as described in detail in the following sections.
• FIG 4 is a table illustrating the cache management technique in accordance with the principles of the present invention.
• the invention utilizes the array of LRU lock bits 70o - 70n to indicate whether any of the corresponding cached data is streaming or non- temporal, and as such, would be the first entry to be replaced upon a cache miss to the corresponding set.
• the LRU lock bit 70 when set to 1, indicates that the corresponding set has an entry that is non-temporal. If the LRU lock bit 70 is cleared, upon a cache hit by a temporal instruction, the corresponding LRU bit(s) 60 is(are) updated in accordance with the LRU technique implemented (see item 1 of Figure 4) and the associated LRU lock bit is not updated. However, if the LRU lock bit 70 is already set to 1 (indicating that the corresponding set has a non-temporal instruction), the LRU lock bit 70 is not updated, and the LRU bit 60 is not updated (see item 2 ).
• the LRU bit 60 and the LRU lock bit 70 are not updated, regardless of the status of the LRU lock bit 70 (see item 3).
• cache hits by a streaming or non-temporal instructions force the LRU bits to the way that was hit (see item 4).
• the LRU lock bit 70 is set to 1.
• the data hit by the streaming or non-temporal instruction will be the first to be replaced upon a cache miss to the corresponding set.
• the LRU lock bit Upon a cache miss by a temporal instruction, the LRU lock bit is cleared and the LRU bit 60 is updated (item 5) based on a pseudo LRU technique. However, upon a cache miss by a streaming or non- temporal instruction, the LRU lock bit 70 is set to 1 and the corresponding LRU bit 60 is not updated (item 6).
• Figures 5 A and 5B illustrate one example of the organization of a cache memory prior to and after temporal instruction hits way 2 of cache set 0.
• This example corresponds to item 1 of Figure 4.
• LRU lock bit 70o had been previously cleared for cache set 0, and since the cache set 0 was hit by a temporal instruction, the LRU lock bit 70o is not updated.
• the LRU bit 60o is updated in accordance with the LRU technique implemented. In the example, it is assumed that the pseudo LRU technique indicates that way 3 is the least recently used entry.
• Figures 6A and 6B illustrate another example of the organization of a cache memory prior to and after temporal instruction hits way 2 of cache set 0. This example corresponds to item 2 of Figure 4.
• LRU lock bit 70o had been previously set for cache set 0, indicating that the corresponding set contains non-temporal data. Accordingly, neither the LRU lock bit 70o nor the LRU bit 60o is updated.
• Figures 7A - 7D illustrate an example of the organization of a cache memory prior to and after a non-temporal instruction hits way 2 of cache set 0.
• This example corresponds to item 3 of Figure 4 and may be implemented by setting a mode bit located in the LI cache controller to zero (see Figure 4).
• LRU lock bit 700 had been previously cleared for cache set 0.
• a non-temporal cache hit does not update the LRU lock bit 70.
• LRU lock bit 70Q nor the LRU bit 60Q is updated.
• Figures 7C and 7D LRU lock bit 70o had been previously set for cache set 0, indicating that the corresponding set contains non-temporal data. Accordingly, neither the LRU lock bit 700 nor the LRU bit 60o is updated.
• Figures 8A - 8D illustrate another example of the organization of a cache memory prior to and after a non-temporal instruction hits way 2 of cache set 0.
• This example corresponds to item 4 of Figure 4 and may be implemented by setting the mode bit located in the LI cache controller to one (see Figure 4).
• LRU lock bit 70o had been previously cleared for cache set 0.
• a non-temporal cache hit updates the LRU lock bit 70. Accordingly, as shown in Figure 8A, since the cache set 0 was hit by a non-temporal instruction, the LRU lock bit 70o is updated (set to 1), as shown in Figure 8B.
• the LRU bits 60o are updated to indicate the way that was hit. In the case where LRU lock bit 70 ⁇ had been previously set for cache set 0 ( Figures 8C and 8D), the LRU lock bit 70o remains set to 1. In addition, the LRU bits 60o are forced to point to the way within the set that was hit.
• Figures 9A and 9B illustrate one example of the organization of a cache memory prior to and after a temporal instruction miss to cache set 0.
• This example corresponds to item 5 of Figure 4.
• LRU lock bit 70 ⁇ had been previously set for cache set 0, and since there is a miss by a temporal instruction targeting set 0, the LRU lock bit 70 ⁇ is cleared for that set, upon replacing the temporal miss in the cache.
• the LRU bit 60o is updated in accordance with the LRU technique implemented.
• the pseudo LRU technique indicates that way 3 is the least recently used entry.
• Figures 10A - 10B illustrate an example of the organization of a cache memory prior to and after a non-temporal instruction miss to cache set 0. This example corresponds to item 6 of Figure 4.
• LRU lock bit 70o had been previously cleared for cache set 0. Since there is a non-temporal miss to cache set 0, the LRU lock bit 70o is set and the LRU bits 60o remain the same, in order to point to the non-temporal data in the corresponding set 0.
• a shared cache structure for managing temporal and non- temporal instructions, which minimizes data pollution in cache or cache hierarchy is provided.
• Implementation of the present invention also eliminates the use of a separate buffer, making its implementation both cost effective and efficient.

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• 1998
  • 1998-03-31 US US09/053,386 patent/US6202129B1/en not_active Expired - Lifetime
• 1999
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