Multiprocessor system bus with system controller explicitly updating snooper cache state information

[0002] The present invention is related to the subject matter of commonly assigned, copending United States patent applications: Ser. No. 09/368,222 (Docket No. AT9-98-813) entitled “MULTIPROCESSOR SYSTEM BUS WITH READ/CASTOUT (RCO) ADDRESS TRANSACTION”; Ser. No. 09/368,221 (Docket No. AT9-98-814) entitled “SYSTEM BUS DIRECTORY SNOOPING MECHANISM FOR READ/CASTOUT (RCO) ADDRESS TRANSACTION”; Ser. No. 09/368,225 (Docket No. AT9-98-815) entitled “PRECISE INCLUSIVITY MECHANISM FOR SYSTEM BUS WITH READ/DEALLOCATE (RDA) ADDRESS TRANSACTION”; Ser. No. 09/368,224 (Docket No. AT9-98-816) entitled “MULTIPROCESSOR SYSTEM BUS WITH CACHE STATE AND LRU SNOOP RESPONSES FOR READ/CASTOUT (RCO) ADDRESS TRANSACTION”; Ser. No. 09/368,223 (Docket No. AT9-98-817) entitled “UPGRADING OF SNOOPER CACHE STATE MECHANISM FOR SYSTEM BUS WITH READ/CASTOUT (RCO) ADDRESS TRANSACTIONS”; Ser. No. 09/368,227 (Docket No. AT9-98-818) entitled “MULTIPROCESSOR SYSTEM BUS WITH COMBINED SNOOP RESPONSES IMPLICITLY UPDATING SNOOPER LRU POSITION”; Ser. No. 09/368,229 (Docket No. AT9-98-820) entitled “MULTIPROCESSOR SYSTEM BUS WITH SYSTEM CONTROLLER EXPLICITLY UPDATING SNOOPER LRU INFORMATION”; Ser. No. 09/368,228 (Docket No. AT9-98-821) entitled “MULTIPROCESSOR SYSTEM BUS WITH COMBINED SNOOP RESPONSES EXPLICITLY CANCELLING MASTER VICTIM SYSTEM BUS TRANSACTION”; Ser. No. 09/368,230 (Docket No. AT9-98-822) entitled “MULTIPROCESSOR SYSTEM BUS WITH COMBINED SNOOP RESPONSES EXPLICITLY CANCELLING MASTER ALLOCATION OF READ DATA”; and Ser. No. 09/368,231 (Docket AT9-98-823) entitled “MULTIPROCESSOR SYSTEM BUS WITH COMBINED SNOOP RESPONSES EXPLICITLY INFORMING SNOOPERS TO SCARF DATA”. The content of the above-identified applications is incorporated herein by reference.

[0003] 1. Technical Field

[0004] The present invention relates in general to data storage management in data processing systems and in particular to coherency state information maintained for data storage management. Still more particularly, the present invention relates to upgrading cache coherency states in response to related data access and cast out or deallocate operations in a data processing system.

[0005] 2. Description of the Related Art

[0006] High performance data processing systems typically include a number of levels of caching between the processor(s) and system memory to improve performance, reducing latency in data access operations. When utilized, multiple cache levels are typically employed in progressively larger sizes with a trade off to progressively longer access latencies. Smaller, faster caches are employed at levels within the storage hierarchy closer to the processor or processors, while larger, slower caches are employed at levels closer to system memory. Smaller amounts of data are maintained in upper cache levels, but may be accessed faster.

[0007] Within such systems, when data access operations frequently give rise to a need to make space for the subject data. For example, when retrieving data from lower storage levels such as system memory or lower level caches, a cache may need to overwrite other data already within the cache because no further unused space is available for the retrieved data. A replacement policy—typically a least-recently-used (LRU) replacement policy—is employed to decide which cache location(s) should be utilized to store the new data.

[0008] Often the cache location (commonly referred to as a “victim”) to be overwritten contains only data which is invalid or otherwise unusable from the perspective of a memory coherency model being employed, or for which valid copies are concurrently stored in other devices within the system storage hierarchy. In such cases, the new data may be simply written to the cache location without regard to preserving the existing data at that location.

[0009] At other times, however, the cache location selected to receive the new data contains modified data, or data which is otherwise unique or special within the storage hierarchy. In such instances, the replacement of data within a selected cache location (a process often referred to as “updating” the cache) requires that any modified data associated with the cache location selected by the replacement policy be written back to lower levels of the storage hierarchy for preservation. The process of writing modified data from a victim to system memory or a lower cache level is generally called a cast out or eviction.

[0010] When a cache initiates a data access operation—for instance, in response to a cache miss for a READ operation originating with a processor—typically the cache will initiate a data access operation (READ or WRITE) on a bus coupling the cache to lower storage levels. If the replacement policy requires that a modified cache line be over-written, compelling a cast out for coherency purposes, the cache will also initiate the cast out bus operation.

[0011] Even when the selected victim contains data which is neither unique nor special within the storage hierarchy (i.e. invalid data), an operation to lower levels of the storage hierarchy may still be required. For instance, the cache organization may be “inclusive,” meaning that logically vertical in-line caches contain a common data set. “Precise” inclusivity requires that lower level caches include at least all cache lines contained within a vertically in-line, higher level cache, although the lower level cache may include additional cache lines as well. Imprecise or “pseudo-precise” inclusivity relaxes this requirement, but still seeks to have as much of the data within the higher level cache copied within the lower level cache as possible within constraints imposed by bandwidth utilization tradeoffs. Within an inclusive, hierarchical cache system, even if the cache line to be replaced is in a coherency state (e.g., “shared”) indicating that the data may be simple discarded without writing it to lower level storage, an operation to the lower level storage may be required to update inclusivity information. The storage device within which the cache line is to be overwritten (or “deallocated” and replaced) initiates an operation notifying lower level, in-line storage devices of the deallocation, so that the lower level devices may update internal inclusivity information associated with the cache line. This requires an operation in addition to the data access operation necessitating replacement of the cache line.

[0012] The data access and cast out/deallocate bus operations represent opportunities for global data storage management. In particular, the coherency state of the cast out or deallocate victim in horizontal storage devices may be updated based on the change in the storage device initiating the data access and cast out/deallocate operations. However, due to the disjoint nature of the related operations in the prior art, such opportunities are not generally exploited. Additionally, a lack of sufficient information from other horizontal storage devices may prevent exploitation of data storage management opportunities in related data access and replacement operations.

[0013] It would be desirable, therefore, to take advantage of data storage management opportunities represented by related data access and cast out or deallocate bus operations. It would further be advantageous to support upgrade of coherency state information for cast out or deallocate victims in horizontal storage devices.

[0014] It is therefore one object of the present invention to provide improved data storage management in data processing systems.

[0015] It is another object of the present invention to provide improved management of coherency state information maintained for data storage.

[0016] It is yet another object of the present invention to provide upgrading of cache coherency states in response to related data access and cast out or deallocate operations in a data processing system.

[0017] The foregoing objects are achieved as is now described. Combined response logic for a bus receives a combined data access and cast out/deallocate operation initiating by a storage device within a specific level of a storage hierarchy with a coherency state of the cast out/deallocate victim appended. Snoopers on the bus drive snoop responses to the combined operation with the coherency state and/or LRU position of locally-stored cache lines corresponding to the victim appended. The combined response logic determines, from the coherency state information appended to the combined operation and the snoop responses, whether a coherency upgrade is possible. If so, the combined response logic selects a snooper storage device to upgrade the coherency state of a respective cache line corresponding to the victim, and appends an upgrade directive to the combined response. The snooper selected to upgrade the coherency state of a cache line corresponding the victim may be randomly chosen or, as an optimization, be chosen for having the highest LRU position for the respective cache line.

[0018] The above as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed written description.

[0019] 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:

[0020] FIG. 1 depicts a block diagram of a data processing system in which a preferred embodiment of the present invention may be implemented;

[0021] FIG. 2 is a timing diagram for a combined data access and related cast out/deallocate operation in accordance with a preferred embodiment of the present invention;

[0022] FIG. 3 depicts a diagram of a storage device which snoops and responds to the combined operation for related data access and cast out/deallocate operations in accordance with a preferred embodiment of the present invention;

[0023] FIGS. 4A-4C are diagrams of a mechanism for upgrading a cache coherency state as a result of a combined data access and cast out/deallocate operation in accordance with a preferred embodiment of the present invention; and

[0024] FIGS. 5A-5B depict high level flow charts for a process of upgrading a cache coherency state utilizing a combined response to a combined data access and cast out/deallocate operation in accordance with a preferred embodiment of the present invention.

[0025] With reference now to the figures, and in particular with reference to FIG. 1, a block diagram of a data processing system in which a preferred embodiment of the present invention may be implemented is depicted. Data processing system 100 is a symmetric multiprocessor (SMP) system including a plurality of processors 102aathrough 102anand 102mathrough 102mn(where “m” and “n” are integers). Each processor 102aa-102mnincludes a respective level one (L1) cache 104aa-104mn, preferably on chip with processor and bifurcated into separate instruction and data caches. Each processor 102aa-102mnis coupled via a processor bus 106aa-1061 to a level two cache 108a-1081 (where “1” is an integer), which are in-line caches shared by multiple processors in the exemplary embodiment.

[0026] Although in the exemplary embodiment only two processors are depicted as sharing each L2 cache, and only two L2 caches are depicted, those skilled in the art will appreciate that additional processors L2 caches may be utilized in a multiprocessor data processing system in accordance with the present invention. For example, each L2 cache may be shared by four processors, and a total of sixteen L2 caches may be provided.

[0027] Each L2 cache 108a-1081 is connected to a level three (L3) cache 1110a-1101 and to system bus 112. L3 caches l10a-1101 are actually in-line caches rather than lookaside caches as FIG. 1 suggests, but operations received from a vertical L2 cache (e.g., L2 cache 108a) are initiated both within the L3 cache 110aand on system bus 112 concurrently to reduce latency. If the operation produces a cache hit within the L3 cache 110a, the operation is cancelled or aborted on system bus 112. On the other hand, if the operation produces a cache miss within the L3 cache 110a, the operation is allowed to proceed on system bus 112.

[0028] The lower cache levels—L2 caches 108a-1081 and L3 caches 110a-1101—are employed to stage data to the L1 caches 104a-1041 and typically have progressively larger storage capacities but longer access latencies. L2 caches 108a-1081 and L3 caches 110a-1101 thus serve as intermediate storage between processors 102a-1021 and system memory 114, which typically has a much larger storage capacity but may have an access latency many times that of L3 caches 110a-1101. Both the number of levels in the cache hierarchy and the cache hierarchy configuration (i.e, shared versus private, in-line versus lookaside) employed in data processing system 100 may vary.

[0029] L2 caches 108a-1081 and L3 caches 110a-1101 are connected to system memory 114 via system bus 112. Also connected to system bus 112 may be a memory mapped device 116, such as a graphics adapter providing a connection for a display (not shown), and input/output (I/O) bus bridge 118. I/O bus bridge 118 couples system bus 112 to I/O bus 120, which may provide connections for I/O devices 122, such as a keyboard and mouse, and nonvolatile storage 124, such as a hard disk drive. System bus 112, I/O bus bridge 118, and I/O bus 120 thus form an interconnect coupling the attached devices, for which alternative implementations are known in the art.

[0030] Non-volatile storage 124 stores an operating system and other software controlling operation of system 100, which are loaded into system memory 114 in response to system 100 being powered on. Those skilled in the art will recognize that data processing system 100 may include many additional components not shown in FIG. 1, such as serial and parallel ports, connections to networks or attached devices, a memory controller regulating access to system memory 114, etc. Such modifications and variations are within the spirit and scope of the present invention.

[0031] Referring to FIG. 2, a timing diagram for a combined data access and related cast out/deallocate operation in accordance with a preferred embodiment of the present invention is illustrated. In the exemplary embodiment, the combined operation is initiated on system bus 112 by an L2 cache 108a-1081 in response to a cache miss for a data access operation within the L2 cache. However, the combined operation may be employed for transmission on any bus by any storage device requiring related data access and cast out/deallocate operations. Similarly, the data access operation is a READ in the exemplary embodiment, but may be any data access operation (e.g., WRITE, etc.).

[0032] When a cache miss occurs within the L2 cache for a data access operation, the cache controller for the L2 cache should be able to determine from the coherency state of the selected victim whether a cast out or deallocate operation will be required, either to preserve data within the cache location selected to be updated by the replacement policy or to update inclusivity information. Moreover, an indexed cache organization is employed for caches within the preferred embodiment. Cache lines are stored within the cache in congruence classes, sets of cache lines identified by a common index field within the system addresses for the cache lines in a congruence class.

[0033] Within an indexed cache organization, a portion of the system address for a cache line are treated as a tag, another portion of the system address is treated as the index, and the remaining bits are an intra-cache line address. The index field of the address is employed by the cache directory and the cache memory to locate congruence classes. The cache directory stores tags for cache lines contained within cache memory within the congruence class identified by the index, and compares the tag of a target address to the tags within the congruence class. If a match is identified, the corresponding cache line within cache memory is the target data.

[0034] In the prior art, the address for a data access operation and the address for a related cast out or deallocate operation are transmitted in separate system bus operations. However, within an indexed cache organization of the type described, the target data of a data access operation and the victim selected by the replacement policy are members of the same congruence class. Therefore the index field will be identical for both the data access and the cast out or deallocate operations. In the present invention, the index for the congruence class containing the target cache lines for both the data access and the cast out/deallocate (“Index”) is combined with the tags for the cache line targeted by the data access (“Tag RD”) and the cache line targeted by the cast out or deallocate (“Tag CO/DA”).

[0035] The index and the two tags are then transmitted on the system bus in a single bus operation, which may require multiple bus cycles as shown. Fewer total bus cycles are required to transmit the combined index and tags, however, since the index need only be transmitted once. As illustrated in the example of FIG. 2, the directory state (“CO/DA State”) of the cast out victim cache line—i.e., coherency state and/or LRU state—may also be appended to the address in the combined or merged bus operation. The combined response—to both the data access and the cast out operations—is driven two cycles after transmission of the cast out/deallocate victim state is complete as described in further detail below.

[0036] The combined operation of the present invention may be employed whenever the need to preserve some unique aspect of data arises, requiring a cast out operation for a related data access. Under the basic MESI coherency protocol, which includes the modified (M), exclusive (E), shared (S), and invalid (I) coherency states, a modified cache segment should be written to lower level storage when selected to be replaced. The modified state indicates that cache data has been modified with respect to corresponding data in system memory without also modifying the system memory data, such that the only valid copy of the data is within the cache entry storing the modified cache line or segment.

[0037] For exclusive, shared, or invalid cache segments, the cache segment selected for replacement need not be written to lower level storage since either (1) a valid copy already exists elsewhere in storage, or (2) the contents of the cache segment are invalid. The exclusive state indicates that the cache entry is consistent with system memory but is only found, within all caches at that level of the storage hierarchy, in the subject cache. The shared state indicates that the cache entry may be found in the subject cache as well as other caches at the same level in the storage hierarchy, with all copies of the data being consistent with the corresponding data in system memory. Finally, the invalid state indicates that a cache entry—both the data and the address tag—within a given cache entry is no longer coherent with either system memory or other caches in the storage hierarchy. However, in these instances a deallocate operation may be required as described below.

[0038] Coherency states implemented as extensions to the basic MESI protocol may also require a cast out, or elect to perform a cast out, and therefore benefit from the present invention. For example, the recent (R) state, essentially a variant of the shared state, indicates that the cache entry may be found in both the subject cache and as well as other caches at the same level in the storage hierarchy, and that all copies of the data in the subject cache and other caches are consistent with the corresponding data in system memory, but also indicates that the subject cache, of all caches containing the shared data, most recently received the data in a system bus transaction such as a read from system memory. While a cast out is not necessary to preserve data integrity in such a case, a cast out operation may be useful to accurately maintain the recent state, and the combined address bus transaction of the present invention may be utilized for that purpose.

[0039] The combined operation of the present invention may also be employed whenever the need to communicate a deallocation is required; for instance, to permit a lower level, in-line and inclusive cache to update inclusivity information. Thus, when a cast out is not required for coherency when selecting a victim to make room for the target of a data access operation, a deallocate operation will frequently be required to update inclusivity information. The combined operation—with the same index and address tags but different opcodes—may be employed for both situation.

[0040] The combined operation of the present invention will save bus cycles over the dual operation scheme of the known art. If each index or tag requires a full bus cycle to completely transmit, the combined address of the present invention may be transmitted in three bus cycles (neglecting the optional state information), rather than four bus cycles as would be required for separate data access and cast out operations. The additional bus cycle is saved because the index field need only be transmitted once for both operations.

[0041] The resulting system bus transaction condenses, within a single address, the information required for both the data access operation and the related cast out. The combined index and tags may be transmitted in any predefined order, and may be transmitted on a single bus cycle as shown in FIG. 2 or over multiple consecutive bus cycles. If the combined address is transmitted over multiple bus cycles, the index should be transmitted first to allow the receiving devices to begin a directory lookup at the earliest possible time. The tags may be transmitted during subsequent cycles and still be timely for the comparators employed to compared directory tags to the target tag(s). See commonly assigned, copending U.S. patent application Ser. No. 09/345,302 (Docket No. AT9-98-812) entitled “INDEX BASED SYSTEM ADDRESS BUS,” incorporated herein by reference.

[0042] With reference now to FIG. 3, a diagram of a storage device which snoops and responds to the combined operation for related data access and cast out/deallocate operations in accordance with a preferred embodiment of the present invention is depicted. The elements depicted are employed in L2 caches 108a-1081 and in L3 caches 110a-1101. A cache controller 302 receives and transmits operations relating to data within cache memory 304 from upstream and downstream buses through bus interface units (“BIU”) 306aand 306b. A directory lookup 308 is employed to locate cache lines within cache memory 304 and an LRU unit 310 implements the replacement policy for updating cache lines within cache memory 304. Snoop logic 312 detects operations initiated by a horizontal storage device (i.e., another L2 cache for L2 caches 108a-1081, and another L3 cache for L3 caches 110a-1101). Snoop logic 312 also controls the snoop response driven by the respective L2 cache in response to snooped operations.

[0043] The logical organization of data within the cache is in tables containing cache directory entries 314 and a corresponding data array 316. The cache directory entries 314 contain the address tag for the corresponding cache lines within data array 316, as well as the coherency state, the LRU status, and an inclusivity (“I”) state for the respective cache line. The coherency state indicates the cache line consistency with other copies of the cache line in other storage devices within the system. The LRU status indicates the LRU position for the cache line within a congruence class. The inclusivity state indicates whether the cache line is stored within a logically in-line, higher level cache.

[0044] When a data access operation is received from a processor or higher level storage device, cache controller 302 may trigger the LRU 310 to select a victim, then look up the selected victim to determine if a cast out would be required to update the corresponding cache line and, if so, retrieve the tag for the current contents of the potential victim. This may be performed concurrently with the directory lookup and tag comparison employed to determine whether the received data access operation generates a cache hit or miss.

[0045] On the system bus side of the respective cache, when a combined operation for related data access and cast out/deallocate operations is detected by cache controller 302 on a lower level bus coupling the cache and horizontal caches to lower levels of the storage hierarchy, snoop logic 312 may access cache directory entries 314 for both the data access target and the cast out/deallocate victim to determine whether the target or victim are contained within data array 316 and, if so, to ascertain the coherency state and LRU position for the target and victim within the respective cache. Snoop logic 312 then drives a snoop response for the data access and cast out or deallocate operations to the combined response logic.

[0046] Referring to FIGS. 4A through 4C, diagrams of a mechanism for upgrading a cache coherency state as a result of a combined data access and cast out/deallocate operation in accordance with a preferred embodiment of the present invention are illustrated. The example selected for the purposes of describing the invention relates to L2 caches 108a-1081 and system bus 112 depicted in FIG. 1.

[0047] In the exemplary embodiment shown for the present invention, an L2 cache receives a data access operation from an in-line processor which misses. A cast out or deallocate is required within L2 cache for replacement of an existing cache segment by the data access target. The bus interface logic 402 of the L2 cache therefore initiates (acting as a “bus master” after requesting and being granted the system bus) a combined data access and cast out/deallocate operation described above on the system bus. The combined data access operation requests a read of the cache line with the address A (“RD A”) and a cast out or deallocate of the cache line with the address B (“CO/DA B”).

[0048] The combined operation is detected by snoop logic 404, 406 and 408 within the remaining L2 caches coupled to the system bus, and is also received by combined response logic 410 (typically a part of the bus controller or the memory controller). Snoop logic 404, 406 and 408 checks the state of both the data access target and the cast out/deallocate victim within the respective L2 cache. For both the data access target and the cast out/deallocate victim, snoop logic 404, 406 and 408 determines whether the subject cache line is contained within the respective L2 cache and, if so, what coherency state and LRU position are associated with the subject cache line in the respective L2 cache. Of particular interest, as will be shown below, are the coherency state and LRU position of the cast out/deallocate victim. Snoop logic 404, 406 and 408 may also determine whether the respective L2 cache contains an invalid entry in the congruence class for the data access target and the cast out/deallocate victim.

[0049] Based on the presence or absence of the subject cache line and the coherency state of the subject cache line, snoop logic 404, 406 and 408 selects appropriate responses to the data access and cast out/deallocate operations. The responses selected may include a null response to either the data access or the cast out/deallocate, a retry for the data access or the cast out/deallocate, or an intervention for the data access.

[0050] The selected responses are “merged” by snoop logic 404, 406 and 408 by selecting a single response code representing both selected responses to the data access and cast out/deallocate operations. Snoop logic 404, 406 and 408 also appends the cache state for the cast out/deallocate victim—i.e., the coherency state, the LRU position, or both—to the merged response. It should be noted that the “merged response” is the joint response of a single storage device to two related (and, within the prior art, formerly discrete) data access and cast out/deallocate operations, and differs from the “combined response” of all storage devices coupled to a bus on which the operations are initiated. Snoop logic 404, 406 and 408 then drives merged response with any appended information to the combined response logic 410.

[0051] Combined response logic 410 receives the merged responses of snoop logic 404, 406 and 408 and generates a combined response to be driven on system bus 112. Appended to the combined response is the coherency state of the victim within each snooper coupled to the system bus (i.e., the L2 caches corresponding to snoop logic 404, 406 and 408). The combined response is received by the bus master 402 and snoop logic 404, 406 and 408. Snoop logic 404, 406 and 408, upon detecting the combined response and the appended coherency information, determines whether an upgrade of the coherency state within the corresponding L2 cache is possible.

[0052] In some instances, snoop logic 404, 406 or 408 may identify an opportunity to upgrade a coherency state from the combined data access and cast out/deallocate operation or from the combined response to the combined operation, and may exploit the identified upgrade opportunity. In other cases, however, it may be necessary or preferable for combined response logic 410 to identify and exploit upgrade opportunities. For example, the L2 caches corresponding to snoop logic 404, 406, and 408 may each have an apparently equal opportunity to upgrade the coherency state of the victim. Additionally, combined response logic 410 may have additional information not available to snoop logic 404, 406, and 408 (e.g., from in-line L3 caches). Alternatively, it may just be simpler for combined response logic 410 to append an upgrade directive to the combined response than to append the coherency state and LRU position information for each snooper responding to the combined operation.

[0053] FIG. 4B illustrates an example of circumstances when coherency upgrade opportunities are better identified and exploited by combined response logic 410. In the example shown, bus master 402 initiates a cast out/deallocate of the victim B, which is in a recent (R) coherency state within the corresponding L2 cache. The R coherency state is a variant of the shared coherency state indicating that the respective storage device was the last within a horizontal set of storage devices to access the corresponding cache line, and therefore has shared intervention responsibility. The cast out of a cache line in the R coherency state provides an upgrade opportunity for any cache containing the victim in a shared coherency state.

[0054] Snoop logic 404, 406 and 408 each drive snoop responses indicating that victim B is in a shared coherency state within corresponding L2 caches. Thus, each snooper has an apparently equal opportunity to upgrade to the recent coherency state. Combined response logic 410 therefore selects a snooper to exploit the upgrade opportunity, and appends the upgrade directive to the combined response.

[0055] The selection of a snooper by combined response logic 410 to exploit the upgrade opportunity may be random. However, in the example, shown, combined response logic 410 selects a snooper to exploit the upgrade opportunity based on the LRU position of the victim B within the respective snoopers. Combined response logic 410 receives the LRU position in the snoop responses. In the example shown, victim B has an LRU position of MRU-3 (“most recently used minus three”) in the L2 cache corresponding to snoop logic 404, an LRU position of MRU-1 in the L2 cache corresponding to snoop logic 406, and an LRU position of MRU-6 in the L2 cache corresponding to snoop logic 408. Combined response logic 410 selects the L2 cache containing the victim in the highest LRU position (MRU-1 in the L2 cache corresponding to snoop logic 406) to exploit the upgrade opportunity, extending the tenure of the cache line within the L2 level of storage as long as possible. Combined response logic 410 appends an upgrade directive to that effect to the combined response, and need not append the coherency states or LRU position.

[0056] FIG. 4C illustrates another example of a situation in which combined response logic 410 is better situated that individual snoopers to identify and exploit an upgrade opportunity. In this example, the victim B has a tagged (T) coherency state within the L2 cache corresponding to bus master 402. The tagged coherency state is a variant of the modified coherency state identifying the cache within a given storage level which has writeback responsibility for the associated cache line. A cast out of a cache line in the T coherency state provides an upgrade opportunity for any cache within the same storage level having the victim in a shared coherency state.

[0057] Each remaining L2 cache in the example shown has the victim B in the shared coherency state. Therefore, combined response logic 410 selects one—either randomly or, as an optimization, based on respective LRU position—to assume the T coherency state.

[0058] With reference now to FIGS. 5A and 5B, high level flow charts for a process of upgrading a cache coherency state utilizing a combined response to a combined data access and cast out/deallocate operation in accordance with a preferred embodiment of the present invention are depicted. FIG. 5A depicts the process of selecting a snooper for upgrading and transmitting the upgrade directive, which is executed within combined response logic for storage devices snooping the combined data access and cast out/deallocate operation.

[0059] The process begins at step 502, which depicts detection by snoop logic of the combined operation, with appended coherency state information, on a bus coupling the storage device initiating the operation to horizontal storage devices (storage at the same level in the storage hierarchy) and the combined response logic. The process next passes to step 504, which illustrates receiving the responses from snoopers and lower level storage devices to the combined operation and formulating the combined response. The process then passes to step 506, which depicts a determination of whether a coherency upgrade for a snooper is possible for the victim of the cast out/deallocate portion of the combined operation.

[0060] If a coherency upgrade is possible, the process then passes to step 508, which illustrates selecting a snooper storage device to upgrade the coherency of a cache line corresponding to the victim. The selection of the snooper to upgrade the coherency state may be made based on LRU position or other criteria. The process passes next to step 510, which depicts appending the upgrade directive to the combined response.

[0061] The process then passes to step 512, which illustrates transmitting the combined response with the appended upgrade directive, if any, on the bus coupling the combined response logic to the storage device initiating the combined operation. The process passes next to step 514, which depicts the process becoming idle until another combined operations is received.

[0062] FIG. 5B depicts the process of handling an upgrade directive, which is executed within a storage device snooping a combined data access and cast out/deallocate operation from a horizontal storage device. The process begins at step 516, which illustrates receiving the combined response on the bus coupling the snoopers to the combined response logic. The process next passes to step 518, which depicts a determination of whether an upgrade directive is appended to the combined response. If so, the process proceeds to step 520, which depicts a determination of whether the appended upgrade directive is directed to the subject storage device (the snooper processing the received combined response).

[0063] If the subject storage device is identified within the upgrade directive, the process passes to step 522, which illustrates upgrading the coherency state of the cache line corresponding to the victim as directed. The process then passes to step 524, which depicts the process becoming idle until another combined response is received.

[0064] 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.