Prefetch miss indicator for cache coherence directory misses on external caches

BACKGROUND OF THE INVENTION

00011. Field of the Invention

0002This invention relates to the management of cache memory in data processing systems employing directory-based cache coherence techniques. More particularly, the invention is directed to the reduction of memory access latency associated with cache coherence directory misses on external caches.

00032. Description of the Prior Art

0004By way of background, modern computer systems utilize high-speed cache memory coupled with an associated cache controller (collectively referred to as a “cache”) as a bridge between processors and the relatively slow main memory that holds processor data. As shown in <figref idref="DRAWINGS">FIG. 1</figref>, a cache typically resides in close proximity to the processor it serves, with one or more cache levels often being situated on the processor die itself. The function of a cache is to temporarily store selected subsets of main memory, particularly memory blocks that are frequently accessed. The cached information is thus available to quickly satisfy memory accesses without the latency associated with access requests to main memory.

0005In multiprocessor systems, such as that shown in <figref idref="DRAWINGS">FIG. 2</figref>, each processor typically has its own cache, and each cache may independently store a copy of the same memory block from a main memory shared by all processors via a common system bus. This situation requires that a cache coherence scheme be used in order to ensure that data consistency is maintained between the several processors. As is well known in the art, a “bus-snooping” protocol is commonly used for this purpose. Bus snooping is premised on the idea that the bus is the broadcast medium for all processor-initiated read and write requests to memory. Each processor's cache is a bus agent that can thus listen or “snoop” on the bus in order to apprise itself of bus-related actions taken by other caches with respect to shared memory blocks. When a processor wants to update a memory block and a memory request to write the block is placed on the bus by its cache, all other caches holding the same memory block will know to invalidate their copy. The cache associated with the block-writing processor will now have the only valid copy of the memory block in the system (until the block is written to main memory). When a processor requests a memory block and its cache places a read request on the bus, another cache holding a valid copy of the requested block can respond. If the main memory has the only valid copy, the request will be satisfied from this memory. If a processor cache has the only valid copy, the request must be satisfied by that cache.

0006In larger-scale multiprocessor systems, such as that shown in <figref idref="DRAWINGS">FIG. 3</figref>, the main memory of the system is often distributed among plural processing nodes that are interconnected by a network. Each node typically comprises a small-scale multiprocessor system as described above relative to <figref idref="DRAWINGS">FIG. 2</figref> (i.e., several processors accessing a main memory over a shared bus). The local memory of each node provides a portion of the overall system memory. A processor at any given node can access its own local memory as well as the memories of other nodes. For a given node, the memory at any other node in the system is typically referred to as remote memory or external memory.

0007A distributed directory-based cache coherence scheme is commonly used to maintain coherence between the caches of different nodes, all of which could theoretically hold a copy of the same memory block. Each node maintains a cache coherence directory to keep track of which processors in the system have cached memory blocks from that node's local memory. Each directory entry typically contains a tag corresponding to the address of a given memory block, identifying information for locating all processors that are caching the block, and a status field indicating whether the cached copies are valid. A node's directory information is used to evaluate read and write requests pertaining to the node's memory blocks and to send out coherency messages to all caches that maintain copies. When a processor in the system updates a shared memory block, the directory having jurisdiction over the memory block is consulted to determine which caches hold copies of the block. Before the write operation can proceed, invalidation messages are sent to the identified caches and invalidation acknowledgements must be returned to verify that all cached copies have been invalidated. In similar fashion, when a processor requests read access to a shared memory block, the directory having jurisdiction over the block is consulted to identify the location and status of all cached copies. Based on the information in the directory, the requested block can be provided to the requestor from one of the caches holding a valid copy, or from the main memory of the node that stores the block.

0008Within each node, the job of managing the cache coherence directory and coordinating the exchange of coherency messages is performed by an intelligent processing agent known as a “coherence controller.” As shown in <figref idref="DRAWINGS">FIG. 3</figref>, each coherence controller is connected so that it can communicate concurrency messages with its peers on other nodes by way of the system interconnection network. Each coherence controller also sits as a snooping agent on the local memory bus of it host node. The coherence controllers can thus keep track of all external and local caching of memory blocks under their jurisdiction.

0009Because cache coherence directories are sometimes large, and are usually stored in relatively low-speed memory, it is common for coherence controllers to implement a high-speed directory cache in order to temporarily store subsets of relevant directory entries. This can greatly reduce the latency associated with directory lookups. In order to populate the directory cache, a coherence controller will perform prefetches (speculative lookups) of directory entries prior to receiving actual lookup requests for particular memory blocks. Conventional algorithms based on principles of spatial locality can be used to select optimal prefetch candidates. For example, following a directory lookup of a particular memory block requested by some processor as part of a read or write operation, the caching algorithm may attempt to prefetch into the directory cache some number of additional directory entries corresponding to memory blocks whose addresses are proximate to that of the requested block.

0010One issue that arises when using a directory prefetching scheme is that a prefetch operation may result in a directory “miss” on the candidate directory entry. A directory miss signifies that a memory block associated with the prefetch attempt is not cached anywhere in the system outside of the local node, i.e., there is no copy present in any external cache. In that case, there will be either no directory entry for the memory block, or a directory entry will exist but will be marked invalid. Such a directory entry will not be placed in the directory cache insofar as the caching algorithm is designed to discard invalid entries to make room for a new cache entries and adding an invalid entry could potentially replace a valid entry, thus consuming time and system resources to replace that entry.

0011Subsequently, when the memory block associated with the prefetch miss is actually requested for reading or writing by a local processor, the cache coherence directory will again be accessed and a directory miss will again occur. Note that because the requestor is a local processor, the second directory lookup is entirely unwarranted. The fact that there is a directory miss condition signifies there are no external cached copies of the memory block and that a local copy of the memory block can be used without having to notify other nodes. Insofar as the coherence controller is a local bus snooping agent whose snoop response must be awaited before a memory block request from a local requestor can be satisfied, the local node experiences undue processing delay. Had it been known that the directory lookup would miss, the requesting processor could have obtained the memory block locally many cycles earlier, without waiting on the coherence controller.

0012It would be desirable to provide a solution to the foregoing problem whereby such latency can be avoided, particularly since local memory requests tend to predominate at any given node whereas requests to remote or external memories on other nodes are more infrequent.

SUMMARY OF THE INVENTION

0013The present invention provides a system, method and article of manufacture for reducing the latency that occurs following a cache coherence directory prefetch miss on the external caches of a shared distributed memory data processing system. In exemplary embodiments of the invention, a cache coherence directory is evaluated for possible prefetching of a directory entry into a directory cache. A prefetch miss indicator is set if the prefetch evaluation results in a directory miss. The prefetch miss indicator is consulted during subsequent processing of a memory block request corresponding to the directory entry that missed during the prefetch evaluation. An accelerated snoop response action is taken if the prefetch miss indicator is set. In this way, the latency of a second lookup into the cache coherence directory, which would otherwise be required, is avoided.

0014The prefetch miss indicator can be implemented in several ways, including by establishing a prefetch miss buffer for storage of the tag associated with the directory prefetch miss. Another option is to establish a new status flag for directory cache entries that signifies the occurrence of a directory prefetch miss on the associated tag.

BRIEF DESCRIPTION OF THE DRAWINGS

0015The foregoing and other features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying Drawings, in which:

0016<figref idref="DRAWINGS">FIG. 1</figref> is a functional block diagram showing a prior art data processing system containing a single processor, a cache and a main memory;

0017<figref idref="DRAWINGS">FIG. 2</figref> is a functional block diagram showing another prior art data processing system containing a multiple processors, multiple associated caches, a shared main memory and a common system bus, and employing a bus-snooping cache coherence protocol;

0018<figref idref="DRAWINGS">FIG. 3</figref> is a functional block diagram showing another prior art data processing system containing a network of processing nodes accessing a shared memory space, each node containing a data processing system according to <figref idref="DRAWINGS">FIG. 2</figref> together with functionality for implementing a directory-based cache coherence protocol;

0019<figref idref="DRAWINGS">FIG. 4</figref> is a functional block diagram showing a data processing system representing an improvement to the data processing system of <figref idref="DRAWINGS">FIG. 3</figref> according to the principles of the present invention;

0020<figref idref="DRAWINGS">FIG. 5</figref> is a functional block diagram showing a coherence controller, a directory cache, a directory, and a prefetch miss buffer in accordance with one exemplary embodiment of the invention;

0021<figref idref="DRAWINGS">FIG. 6</figref> is a flow diagram showing exemplary prefetch processing that may be performed in accordance with the invention;

0022<figref idref="DRAWINGS">FIG. 7</figref> is a flow diagram showing exemplary protocol logic processing in accordance with the invention;

0023<figref idref="DRAWINGS">FIG. 8</figref> is a functional block diagram showing a coherence controller, a directory cache, and a directory in accordance with another exemplary embodiment of the invention; and

0024<figref idref="DRAWINGS">FIG. 9</figref> is a diagrammatic illustration of storage media that can be used to store a computer program to provide an article of manufacture for implementing functions in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

0025Turning now to the Drawings, in which like reference numerals indicate like elements in all of the several views, <figref idref="DRAWINGS">FIG. 4</figref> illustrates a data processing system <b>2</b> that provides an exemplary environment in which the present invention may be implemented. The data processing system <b>2</b> is configured as a network of data processing nodes sharing a distributed memory space. A cc-NUMA architecture, which stands for “cache coherent-Non Uniform Memory Access,” is one species of shared distributed memory system that could be used as a model for the data processing system <b>2</b>. Other types of shared distributed memory architectures could potentially also be used.

0026The data processing system <b>2</b> comprises “N” processing nodes where N can be any desired number according to processing requirements, cost and other factors. For clarity, <figref idref="DRAWINGS">FIG. 2</figref> only shows the 1^stand the N^thnodes, which are designated as “Node <b>1</b>” and “Node N,” respectively. An interconnection network <b>4</b> interconnects Node <b>1</b> through Node N to form what is commonly referred to in the art as a “cluster.” The interconnection network <b>4</b> can be implemented by way of a switching fabric topology, a ring topology, or any other suitable arrangement, including a combination of ring and fabric topologies.

0027Each node in the data processing system <b>2</b> includes <b>1</b> through N processors <b>6</b>, a cache <b>8</b> associated with each processor, and a common main memory <b>10</b> interconnected by a system memory bus <b>12</b>. The memory of the data processing system <b>2</b> is distributed across the main memory <b>10</b> of all of the nodes of the system, such that the main memory of each node provides a portion of the overall system memory. There is a single system-wide address space and each node's main memory <b>10</b> is assigned a unique address range within the address space. Because of the interconnectivity between nodes, a processor <b>6</b> at any given node can access its own local memory <b>10</b> as well as the memories (and caches) of other nodes (the latter being considered remote or external memory relative to the given node). Which node will service a processor memory access will depend on the address of the memory block being requested.

0028Cache coherence between the processors <b>6</b> within a single node of the data processing system <b>2</b> is supported in part by a conventional bus-snooping protocol as described by way of background above. Thus, each of the caches <b>8</b> within a node will snoop activity on the local bus <b>12</b> (i.e., via a conventional cache controller that is not separately shown). The data processing system <b>2</b> further employs a directory-based cache coherence protocol to maintain consistency among the caches <b>8</b> of different nodes. To that end, and as described by way of background above, each node includes a cache coherence directory <b>14</b> that is managed by a coherence controller <b>16</b>. Each coherence controller <b>16</b> snoops its own local bus <b>12</b> and also communicates with its peers at other nodes.

0029The directories <b>14</b> can be configured using any of the well known techniques that are conventionally found in directory-based cache coherence architectures. One option is to configure the directories <b>14</b> as full mapped directories in which every memory block in the local main memory <b>10</b> associated with a given directory is represented by a directory entry that lists all processors caching the block. Sparse directory formats can also be used, in which case only a fraction of the memory blocks in a directory's local main memory <b>10</b> will be represented at any given time. Any suitable memory technology may be used to implement the directories <b>14</b>, with external DRAM modules being the conventional choice for full directories, and embedded DRAM being the technology most commonly used for sparse directories.

0030Except to the extent they are modified to implement the functions of the invention, the coherence controllers <b>16</b> are all conventional in design. As such, they can be implemented according to well known techniques using programmable protocol processors, custom hardware designs implemented with ASICs (Application Specific Integrated Circuits), or by any other suitable means. As described by way of background above, the principal function of the coherence controllers <b>16</b> is to maintain coherence for local memory blocks stored in external caches on other nodes. Access requests involving such memory blocks come from the interconnection network side of the coherence controllers <b>16</b> as messages from peer coherence controllers. On the local bus side, access requests for local memory blocks arrive as conventional read/write requests on the local bus <b>12</b>. Another function of the coherence controllers <b>16</b> is to forward local requests for external memory blocks (with addresses outside the range of the local main memory) through the interconnection network <b>4</b>.

0031The present invention concerns local memory block requests from local processors, and the remainder of the discussion will thus focus on this aspect of coherence controller operation. When a local processor <b>6</b> requests access to a local memory block, it asserts the requested address on the bus <b>12</b>, and the local coherence controller <b>16</b> is notified via its conventional bus snooping functionality. At the same time, all local caches <b>8</b> will likewise snoop the request insofar as they are also agents on the bus <b>12</b>. The requesting processor <b>6</b> will then wait for some number of cycles to receive snoop responses from the various bus agents. For example, if the requesting processor <b>6</b> seeks to read a memory block, the bus agents on the bus <b>12</b> will provide snoop responses advising whether or not they have a valid copy of the requested block. In this context, the only difference between the snoop response of the coherence controller <b>16</b> and the responses of the other bus agents on the bus <b>12</b> is that the coherence controller <b>16</b> collectively speaks on behalf of all external caches holding copies of the requested memory block at other nodes. The coherence controller <b>16</b> may thus be thought of as a bus snooping proxy for the external caches.

0032Each coherence controller <b>16</b> uses its directory <b>14</b> to provide snooping responses to local processors <b>6</b>. If one or more copies of a local memory block are being cached externally at other nodes, there will be a directory entry in the directory <b>14</b> that contains information for identifying the external processors <b>6</b> at the other nodes that are caching the block, and a status field indicating whether the external copies are valid. If a sparse directory format is being used, and there are no externally cached copies of a requested local memory block, there will be no directory entry for that block. If a full directory format is being used, there will be a directory entry that is marked invalid, and the processor-identifying information will show no external processors holding the block. This condition is called a directory miss. The coherence controller's snoop response in this situation is to advise the requesting processor <b>6</b> that the controller is not caching the requested memory block.

0033As discussed by way of background above, among the various functions that may be conventionally performed by each coherence controller <b>16</b> is the establishment and management of a directory cache, as shown by reference numeral <b>18</b> in <figref idref="DRAWINGS">FIG. 4</figref>. Each directory cache <b>18</b> is used to hold a subset of directory entries for rapid access by its coherence controller <b>16</b> as the latter services memory block access requests. A fast memory technology, such as SRAM (Static Random Access Memory) is conventionally used to implement such caches. As also mentioned by way of background, a coherence controller <b>16</b> conventionally implements a prefetching algorithm to preload anticipated directory entries from its directory <b>14</b> into its directory cache <b>18</b>.

0034<figref idref="DRAWINGS">FIG. 5</figref> illustrates a typical prefetch operation resulting from a coherence controller <b>16</b> evaluating a directory cache entry with tag T<b>1</b> for some memory block. Caching logic <b>20</b> in the coherence controller <b>16</b> speculates that additional directory entries having tags T<b>2</b> and T<b>3</b> (corresponding to contiguous memory blocks) should be prefetched from the directory <b>14</b> into the directory cache <b>18</b>. The caching logic <b>20</b> thus accesses directory entry T<b>2</b> in the directory <b>14</b> and copies it into the directory cache <b>18</b>. Caching logic <b>20</b> then accesses directory entry T<b>3</b> in the directory <b>14</b>. In a sparse directory, the caching logic <b>20</b> would find that there is no entry for T<b>3</b>. In a full directory, the caching logic <b>20</b> will discover that T<b>3</b>'s status is invalid and that its processor identification field shows no external processors holding copies of the memory block associated with this entry (referred to as a NULL field in <figref idref="DRAWINGS">FIG. 5</figref>). The T3 directory entry thus represents a directory miss occurring on the directory prefetch attempt.

0035As discussed by way of background above, a conventional coherence controller encountering the T3 directory entry during a directory prefetch operation would ignore it. In contrast, the present invention recognizes that a prefetch directory miss condition provides useful information that can be used to avoid subsequent processing latency when the same memory block is involved in an actual (non-speculative) request. To that end, the invention proposes the setting of a prefetch miss indicator to represent the fact that a directory miss occurred during the directory prefetch operation. In <figref idref="DRAWINGS">FIG. 5</figref>, the prefetch miss indicator is set as a copy of the T3 tag in a prefetch miss buffer <b>22</b> that is implemented using high-speed data storage hardware in the node on which it resides (see below). The T3 tag is placed in the prefetch miss buffer <b>22</b> when the caching logic <b>20</b> determines that the T3 directory entry has generated a directory miss during the prefetch operation on that tag. The entire T3 directory entry could be alternatively stored as the prefetch miss indicator in the prefetch miss buffer <b>22</b>. However, no information other than the tag itself should be needed to indicate a prefetch miss.

0036Subsequently, when a non-speculative request for the T3 memory block is received, the coherence controller's protocol logic <b>24</b> performs a lookup in the prefetch miss buffer <b>22</b> in addition to its normal lookup in the directory cache <b>18</b>. Assuming no intervening access to the T3 memory block has occurred on a remote node, the prefetch buffer lookup will result in a match on the T3 tag. This, in turn, terminates further processing on that tag. The coherence controller <b>16</b> is able to promptly return a snoop response to the local bus <b>12</b> advising that it has no cached copies of the T3 memory block. This snoop response is accelerated when compared to the snoop response that would be generated if a second lookup for the T3 tag had to be performed in the directory <b>14</b>. By way of example, the latency for a directory lookup in an external DRAM directory would be on the order of 40-60 clock cycles. The latency for a directory lookup in an embedded DRAM directory would be on the order of 8 cycles. By comparison, the prefetch miss buffer <b>22</b> can provide single cycle access if it is implemented in SRAM or as a storage register, as discussed below.

0037If the assumption above does not hold, and there is an intervening access to the T3 memory block between the time of the prefetch operation and the non-speculative lookup, the T3 tag in the prefetch miss buffer <b>22</b> will be invalidated by the coherence controller <b>16</b>. Thereafter, when the non-speculative lookup is made, the T3 tag will not be found in the prefetch miss buffer <b>22</b> and conventional tag lookup processing will result.

0038<figref idref="DRAWINGS">FIGS. 6 and 7</figref> summarize the exemplary processing logic that may be respectively implemented by caching logic <b>20</b> and the protocol logic <b>24</b> of the coherence controller <b>16</b> to support the aforementioned features of the present invention. Beginning in step <b>30</b> of <figref idref="DRAWINGS">FIG. 6</figref>, the caching logic <b>20</b> accesses a candidate directory entry in the directory <b>14</b> for possible prefetching into the directory cache <b>18</b>. In step <b>32</b>, the caching logic <b>20</b> tests for a prefetch miss condition. If a miss condition is not present, the caching logic <b>20</b> copies the directory entry into the directory cache <b>18</b> in step <b>34</b>. If a miss condition is present, the caching logic <b>20</b> sets a prefetch miss indicator, as by copying the directory entry tag into the prefetch miss buffer <b>22</b>.

0039In step <b>40</b> of <figref idref="DRAWINGS">FIG. 7</figref>, the protocol logic <b>24</b> receives a memory block access request. In step <b>42</b>, it initiates parallel lookups of the directory cache <b>18</b> and the prefetch miss buffer <b>22</b>. If, in step <b>44</b>, the protocol logic <b>24</b> locates a corresponding tag for the memory block in the prefetch miss buffer <b>22</b>, it generates a snoop response in step <b>46</b>. Otherwise, in step <b>48</b>, the protocol logic <b>24</b> will either find the corresponding tag in the directory cache <b>18</b>, or subsequently in the directory <b>14</b> if there is no directory cache entry for the tag.

0040The prefetch miss buffer <b>22</b> can be implemented using any suitable data storage hardware that operates at relatively high speed. SRAM is one choice. Another option is to use one or more storage registers associated with the processor or ASIC used for the coherence controller <b>16</b>. By using fast storage hardware for the prefetch miss buffer <b>22</b>, the latency associated with a memory block request is accelerated from 40-60 cycles for a lookup in a full external DRAM directory, and from 8 cycles for a lookup in a sparse embedded DRAM directory, to the single cycle access latency that is characteristic of SRAM modules and storage registers. As mentioned above, without the use of a prefetch miss indicator in accordance with the invention, such a lookup into the directory <b>14</b> would be performed for any directory entry even though that entry previously generated a prefetch miss.

0041It should be understood that the prefetch miss buffer <b>22</b> is not the only way a prefetch miss indicator can be set for consideration during a subsequent non-speculative memory block request. As shown in <figref idref="DRAWINGS">FIG. 8</figref>, one possible alternative technique would be to implement a prefetch miss indicator by adding a new status flag field to directory cache entries. This flag is exemplified by the “I/M” indicator in <figref idref="DRAWINGS">FIG. 8</figref>, which stands for “Invalid/Miss.” The coherence controller's protocol logic <b>24</b> would then encounter the prefetch miss indicator during its normal lookup in the directory cache <b>18</b>. The caching logic in the coherence controller <b>16</b> would also use the new status flag to distinguish directory cache entries generated as a result of prefetch misses from normal invalid directory cache entries. The former should be preserved in the cache for some specified time period, while the latter can be flushed.

0042Accordingly, a technique has been disclosed for reducing local memory access latency resulting from cache coherence directory misses on external caches. It will be appreciated that the foregoing concepts may be variously embodied in any of a data processing system, a machine implemented method, and an article of manufacture in which programming means are recorded on one or more data storage media for use in programming a device, such as the coherence controller <b>16</b>, to perform the required functions. Exemplary data storage media for storing such programming means are shown by reference numeral <b>100</b> in <figref idref="DRAWINGS">FIG. 9</figref>. The media <b>100</b> are shown as being portable optical storage disks of the type that are conventionally used for commercial software sales. Such media can store the programming means of the invention. The programming means could also be stored on portable magnetic media (such as floppy disks, flash memory sticks, etc.) or on magnetic media combined with drive systems (e.g. disk drives) incorporated in computer platforms.

0043While several embodiments of the invention have been shown and described, it should be apparent that many variations and alternative embodiments could be implemented. It is understood, therefore, that the invention is not to be in any way limited except in accordance with the spirit of the appended claims and their equivalents.