# Patent

## Title
Proximity communication-based off-chip cache memory architectures

## Abstract

A proximity interconnect module includes a plurality of off-chip cache memories. Either disposed external to the proximity interconnect module or on the proximity interconnect module are a plurality of processors that are dependent on the plurality of off-chip cache memories for servicing requests for data. The plurality of off-chip cache memories are operatively connected to either one another or to one or more of the plurality of processors by proximity communication. Each of the plurality of off-chip cache memories may cache certain portions of the physical address space.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application contains subject matter that may be related to the subject matter in the following U.S. applications filed on Jun. 17, 2005 and assigned to the assignee of the present application: “Cache Preloading in a Proximity Communication-Based Off-Chip Cache Architecture” Ser. No. 11/155,103; “Speculative Memory Accesses in a Proximity Communication-Based Off-Chip Cache Memory Architecture” Ser. No. 11/155,374; and “Cache Protocol Enhancements in a Proximity Communication-Based Off-Chip Cache Memory Architecture” Ser. No. 11/156,332.
BACKGROUND
As shown in FIG. 1, a computer system 10 includes at least a processor 12 and a main memory 14. The processor 12 has a central processing unit (CPU) 13 that is used to perform most, if not all, of the data processing and computing operations of the computer system 10. Data needed by or generated from the processor 12 may be stored in the main memory 14. However, because the speed of the processor 12 is typically significantly faster than that of the main memory 14, memory of smaller size and faster speed (referred to and known as “cache” memory) is often implemented to allow the processor 12 to access frequently and/or recently requested data faster than it would otherwise take to obtain such data from the main memory 14.
Still referring to FIG. 1, the processor 12 may also have an “on-chip” (i.e., on the same semiconductor die as the processor 12) cache memory 16 and an “off-chip” (i.e., not on the same semiconductor die as the processor 12) cache memory 18. When the processor 12 requests data, a cache controller (not shown) causes the “on-chip” cache memory 16 to be searched for the requested data. If the search in the “on-chip” cache memory 16 does not result in a “hit” (i.e., a cache “miss” occurs), the “off-chip” cache memory 18 is searched for the requested data. If the requested data is not found in the “off-chip” cache memory 18, the requested data is retrieved from the relatively slow main memory 14. In the case where data is retrieved from the main memory 14, either or both of the “on-chip” cache memory 16 and the “off-chip” cache memory 18 may be updated with that retrieved data.
Those skilled in the art will recognize that a processor may have any number of cache memory levels, which are typically referred to by number in order of decreasing proximity to the processor. Further, those skilled in the art will recognize that any number of cache memories may be “on-chip” and any number of cache memories may be “off-chip.”
SUMMARY
According to one aspect of one or more embodiments of the present invention, a system comprises: a plurality of processors; and a plurality of off-chip cache memories, at least one of which is operatively connected by proximity communication to at least one of at least one of the plurality of processors and at least another of the plurality of off-chip cache memories.
According to another aspect of one or more embodiments of the present invention, a method of performing computer system operations comprises: executing an operation using a processor; and issuing a request for data needed for the executing, wherein the request is forwarded to an off-chip cache memory by proximity communication.
According to another aspect of one or more embodiments of the present invention, a system comprises: a proximity interconnect module operatively connected to main memory and system I/O interconnect; a plurality of off-chip cache memories disposed on the proximity interconnect module; and a plurality of processors operatively connected to the plurality of off-chip cache memories, where requests for data by one of the plurality of processors is serviced by at least one of the plurality of off-chip cache memories using proximity communication.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 shows a portion of a computer system.
FIG. 2 shows a proximity interconnect module.
FIGS. 3A and 3B show integrated circuits configured for proximity communication.
FIG. 4 shows a system architecture in accordance with an embodiment of the present invention.
FIG. 5 shows a system architecture in accordance with an embodiment of the present invention.
FIG. 6 shows a system architecture in accordance with an embodiment of the present invention.
FIG. 7 shows a system architecture in accordance with an embodiment of the present invention.



DETAILED DESCRIPTION
While a computer system such as that shown in FIG. 1 may be formed of a single processor, other computer systems may be formed of a plurality of computing modules, each having a plurality of processors. For example, as shown in FIG. 2, a computing module 30 may have processors 32, 34, 36, 38, 40, 42, 44, 46 and may be connected to an external main memory 48 and other computing modules (not shown). The processors 32, 34, 36, 38, 40, 42, 44, 46 are operatively connected to one another by a technique known and referred to as “proximity communication” (regions for proximity communication indicated by solid boxes shown at the corners of the processors 32, 34, 36, 38, 40, 42, 44, 46). A computing module having a proximity interconnect “switching fabric” may be referred to as a “proximity interconnect” module. In other words, a proximity interconnect module is a substrate upon which proximity communication is implemented for a collection of integrated circuits disposed on the substrate.
Proximity communication (also referred to and known as “proximity I/O communication”) involves wireless data communication between closely-positioned integrated circuits. Instead of depending on relatively low-bandwidth interconnect (e.g., wires) for communicating data between integrated circuits, a proximity communication-enabled integrated circuit has embedded transmitting and receiving circuits that are “capacitively coupled” to respective receiving and transmitting circuits in another proximity communication-enabled integrated circuit. When capacitively coupled, a voltage change on a transmitting circuit induces a voltage change on a corresponding receiving circuit of the facing integrated circuit. This makes it possible to transmit signals directly between the integrated circuits without having to route the signal through intervening signal lines within, for example, a printed circuit board (PCB) or a ceramic multi-chip module (MCM). Further, those skilled in the art will note that proximity communication allows for higher bandwidth data communication than is typically possible with conventional interconnect (e.g., wires or PCB traces).
FIGS. 3A and 3B show integrated circuits configured for proximity communication. In FIGS. 3A and 3B, a first integrated circuit 90 is positioned for proximity communication with a second integrated circuit 92. Using, for example, capacitive coupling as described above, data is capable of being transferred (i) from transmitting circuits 94 of the first integrated circuit 90 to receiving circuits 96 of the second integrated circuit 92 and (ii) from transmitting circuits 98 of the second integrated circuit 92 to receiving circuits 100 of the first integrated circuit 90.
While the processors 32, 34, 36, 38, 40, 42, 44, 46 shown in FIG. 2 share main memory 48, in one or more embodiments of the present invention, off-chip cache memories are interspersed within a proximity communication fabric of a given proximity interconnect module. In other words, embodiments of the present invention generally relate to techniques and designs for connecting off-chip cache memories with processors in a proximity communication-based fabric. In essence, those skilled in the art will note that such interspersed off-chip cache memories may be viewed as a unitary off-chip cache memory.
FIG. 4 shows an exemplary system architecture in accordance with an embodiment of the present invention. In FIG. 4, processors 50, 52, 54, 56, 58, 60, 62, 64, 66 are interspersed with off-chip cache memories 110, 112, 114, 116, 118, 120, 122, 124, 126, 128, 130, 132, 134, 136, 138, 140 such that each processor 50, 52, 54, 56, 58, 60, 62, 64, 66 is connected by proximity communication to a different off-chip cache memory at each of its four corners. Similarly, each off-chip cache memory 110, 112, 114, 116, 118, 120, 122, 124, 126, 128, 130, 132, 134, 136, 138, 140 (except those at the edges of proximity interconnect module 49) is connected by proximity communication to a different processor at each of its four corners.
In one or more embodiments of the present invention, each off-chip cache memory 110, 112, 114, 116, 118, 120, 122, 124, 126, 128, 130, 132, 134, 136, 138, 140 caches data for all four processors to which it is attached. Physical address space may be divided into four quadrants, where a particular off-chip cache memory caches data from a particular one of the four quadrants. The quadrants may be assigned to the off-chip cache memories 110, 112, 114, 116, 118, 120, 122, 124, 126, 128, 130, 132, 134, 136, 138, 140 such that each processor is connected by proximity communication to an off-chip cache memory from each quadrant. In such a manner, because any given processor 50, 52, 54, 56, 58, 60, 62, 64, 66 is “locally” connected to four off-chip cache memories that each hold cached data belonging to a different quadrant of the physical address space, each processor 50, 52, 54, 56, 58, 60, 62, 64, 66 may be characterized as having its own complete off-chip cache memory.
Those skilled in the art will note that because there is more than one off-chip cache memory for each quadrant in FIG. 4, in one or more embodiments of the present invention, a cache coherence protocol to maintain consistence among the off-chip cache memories may be implemented and supported.
Those skilled in the art will note that at the edges of proximity interconnect module 49, off-module interconnect (e.g., cables, backplane buses) may be used to access off-module memory controllers, input/output (“I/O”) interfaces, and module-to-module interconnect.
Those skilled in the art will note that in one or more embodiments of the present invention, each off-chip cache memory quadrant may have end-around connections so as to reduce a “hop” count incident with traversing the proximity interconnect module.
Those skilled in the art will note that in one or more other embodiments of the present invention, the physical address space may be divided into any number of portions. For example, the physical address space may be divided into two portions instead of four.
FIG. 5 shows an exemplary system architecture in accordance with an embodiment of the present invention. In FIG. 5, processors 152, 154, 156, 158, 160, 162, 164, 166, 168, 170, 172, 174, 176, 178, 180, 182, 184, 186, 188, 190, 192, 194, 196, 198, 200, 202, 204, 206, 208, 210, 212, 214 are connected to a proximity interconnect module 150 having an array of off-chip cache memories 216, 218, 220, 222, 226, 228, 230, 232, 234, 236, 238, 240, 242, 244, 246, 248, 250, 252, 254, 256, 258, 260, 262, 264, 266.
Within the proximity interconnect module 150, an array of similar off-chip cache memories 216, 218, 220, 222, 226, 228, 230, 232, 234, 236, 238, 240, 242, 244, 246, 248, 250, 252, 254, 256, 258, 260, 262, 264, 266 is interconnected such that a small group of off-chip cache memories forms the off-chip cache memory for a given processor. The off-chip cache memories in these groups cache addresses in distinct, statically defined regions of the physical address space. As shown in FIG. 5, each off-chip cache memory 216, 218, 220, 222, 226, 228, 230, 232, 234, 236, 238, 240, 242, 244, 246, 248, 250, 252, 254, 256, 258, 260, 262, 264, 266 is designated as being one of four quadrants into which the physical address space of the total off-chip cache is divided. Further, those skilled in the art will note that in one or more embodiments of the present invention, a particular off-chip cache memory may be shared by a relatively small number of processors.
In one or more embodiments of the present invention, each off-chip cache memory 216, 218, 220, 222, 226, 228, 230, 232, 234, 236, 238, 240, 242, 244, 246, 248, 250, 252, 254, 256, 258, 260, 262, 264, 266 may have several switches that have connections to neighboring off-chip cache memories using proximity communication at the corners of its package. The switches implement a fabric that may carry cache-coherence traffic among multiple off-chip cache memories caching the same address region (e.g., off-chip cache memories of the same quadrant in FIG. 5). This fabric may also carry cache-coherence traffic between the off-chip cache memories 216, 218, 220, 222, 226, 228, 230, 232, 234, 236, 238, 240, 242, 244, 246, 248, 250, 252, 254, 256, 258, 260, 262, 264, 266 and the processors 152, 154, 156, 158, 160, 162, 164, 166, 168, 170, 172, 174, 176, 178, 180, 182, 184, 186, 188, 190, 192, 194, 196, 198, 200, 202, 204, 206, 208, 210, 212, 214.
Those skilled in the art will note that in one or more embodiments of the present invention, because proximity communication may be sufficiently dense enough to allow the architecture shown in FIG. 5 to support a low latency snooping-based cache-coherence protocol by confining high bandwidth snoop traffic to the proximity interconnect module 150. The off-chip cache memories 216, 218, 220, 222, 226, 228, 230, 232, 234, 236, 238, 240, 242, 244, 246, 248, 250, 252, 254, 256, 258, 260, 262, 264, 266 may filter out irrelevant snoop traffic that would otherwise saturate one or more of the processors 152, 154, 156, 158, 160, 162, 164, 166, 168, 170, 172, 174, 176, 178, 180, 182, 184, 186, 188, 190, 192, 194, 196, 198, 200, 202, 204, 206, 208, 210, 212, 214. Those skilled in the art will note that such filtering is particularly important when dealing with relatively narrow processor interfaces (e.g., HyperTransport).
Those skilled in the art will note that in one or more embodiments of the present invention, one or more of processors 152, 154, 156, 158, 160, 162, 164, 166, 168, 170, 172, 174, 176, 178, 180, 182, 184, 186, 188, 190, 192, 194, 196, 198, 200, 202, 204, 206, 208, 210, 212, 214 may be connected to its own memory (not shown).
Those skilled in the art will note that in one or more embodiments of the present invention, each off-chip cache memory quadrant may have end-around connections so as to reduce a “hop” count incident with traversing the proximity interconnect module.
FIG. 6 shows an exemplary system architecture in accordance with an embodiment of the present invention. In FIG. 6, off-chip cache memories 270, 272, 274, 276 having cache memory and controller logic are distributed along with processors 278, 280, 282, 284 in a proximity communication fabric such that any processor 278, 280, 282, 284 may retrieve data from any off-chip cache memory 270, 272, 274, 276.
Those skilled in the art will note that in one or more embodiments of the present invention, the off-chip cache memories 270, 272, 274, 276 may constitute a relatively large, address-interleaved, multi-bank, set-associative, shared off-chip cache memory.
FIG. 7 shows an exemplary system architecture in accordance with an embodiment of the present invention. In FIG. 7, off-chip cache memories 290, 292, 294, 296 having cache memory and controller logic are distributed along with processors 300, 302, 304, 306, 308 in a fully-connected crossbar switch fabric formed of proximity interconnect and hard-wired paths (shown by arrows in FIG. 7) such that any processor may retrieve data from any off-chip cache memory. In one or more embodiments of the present invention, the processors 300, 302, 304, 306, 308 and the off-chip cache memories 290, 292, 294, 296 may be interconnected using dedicated point-to-point wiring connections enabled by the large number of available conductors in the proximity interconnect grid.
Those skilled in the art will note that in one or more embodiments of the present invention, full connection routing may require dedicated physical paths for each combination of endpoints.
Advantages of the present invention may include one or more of the following. In one or more embodiments of the present invention, the use of proximity communication in a cache architecture allows for increased bandwidth, thereby potentially leading to improved performance.
In one or more embodiments of the present invention, the use of proximity communication in a cache architecture allows for the implementation and use of off-chip cache memories, thereby reducing memory access times by reducing accesses to main memory.
In one or more embodiments of the present invention, the use of proximity communication in a cache architecture having off-chip cache memories allows for lower cache access times as opposed to that incurred with conventional interconnect.
In one or more embodiments of the present invention, because the use of proximity communication in a cache architecture allows for increased connectivity, more off-chip cache memories may be implemented, thereby improving cache performance.
In one or more embodiments of the present invention, because computer system throughput is in part dependent on and limited by the size, latency, and “sharedness” of an off-chip cache memory, embodiments of the present invention may enable substantial improvements in these aspects, thereby improving system performance.
In one or more embodiments of the present invention, within a proximity interconnect module, fast and wide off-chip cache memory interfaces may be integrated with wide fast connections provided by proximity interconnect at the four corners of each chip. Because the feasible off-chip signal count of proximity interconnect is markedly greater than that of conventional interconnect, a higher performance system may be possible.
In one or more embodiments of the present invention, because the high signal count of proximity interconnect allows for multiple wide, fast connections between off-chip cache memories on a proximity interconnect module, an off-chip cache memory may be constructed using several smaller chips of moderate cost rather than a single chip of high cost.
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.

## Claims

1. A system, comprising:
a first processor;
a first cache overlapping a first corner of the first processor;
a second cache overlapping a second corner of the first processor;
a third cache overlapping a third corner of the first processor;
a fourth cache overlapping a fourth corner of the first processor, wherein the first cache, the second cache, the third cache, and the fourth cache belong to a plurality of off-chip cache memories and are operatively connected to the first processor by proximity communication; and
a physical memory comprising a physical address space,
wherein the first cache is configured to cache data for the first processor only from a first portion of the physical address space,
wherein the second cache is configured to cache data for the first processor only from a second portion of the physical address space,
wherein the third cache is configured to cache data for the first processor only from a third portion of the physical address space, and
wherein the fourth cache is configured to cache data for the first processor only from a fourth portion of the physical address space.

2. The system of claim 1, further comprising:
a proximity interconnect module, wherein the first processor and the plurality of off-chip cache memories are disposed on the proximity interconnect module.

3. The system of claim 1, further comprising:
a proximity interconnect module, wherein the first processor is disposed external to the proximity interconnect module, wherein the proximity interconnect module includes functionality to cause the first processor to communicate with at least one of the plurality of off-chip cache memories.

4. The system of claim 3, wherein at least one of the plurality of off-chip cache memories is disposed on the proximity interconnect module.

5. The system of claim 1, wherein the proximity communication comprises at least one of capacitive coupling, inductive coupling, magnetic coupling, and optical coupling.

6. The system of claim 1, wherein the first portion, the second portion, the third portion, and the fourth portion are each one-fourth of the physical address space.

7. The system of claim 1, further comprising:
a second processor;
a fifth cache overlapping a first corner of the second processor; and
a sixth cache overlapping a second corner of the second processor,
wherein the fifth cache and the sixth cache belong to the plurality of off-chip cache memories and are operatively connected by proximity communication to the second processor,
wherein the third cache further overlaps a third corner of the second processor,
wherein the fourth cache further overlaps a fourth corner of the second processor,
wherein the third cache and the fourth cache are further operatively connected by proximity communication to the second processor,
wherein the fifth cache is configured to cache data for the second processor only from the first portion of the physical address space,
wherein the sixth cache is configured to cache data for the second processor only from the second portion of the physical address space,
wherein the third cache is further configured to cache data for the second processor only from the third portion of the physical address space, and
wherein the fourth cache is further configured to cache data for the second processor only from the fourth portion of the physical address space.

8. A system, comprising:
a first processor;
a proximity interconnect module operatively connected to main memory, wherein the main memory comprises a physical address space;
a first cache overlapping a first corner of the first processor;
a second cache overlapping a second corner of the first processor;
a third cache overlapping a third corner of the first processor; and
a fourth cache overlapping a fourth corner of the first processor,
wherein the first cache, the second cache, the third cache, and the fourth cache belong to a plurality of off-chip cache memories disposed on the proximity interconnect module,
wherein the first cache is configured to cache data for the first processor only from a first portion of the physical address space,
wherein the second cache is configured to cache data for the first processor only from a second portion of the physical address space,
wherein the third cache is configured to cache data for the first processor only from a third portion of the physical address space, and
wherein the fourth cache is configured to cache data for the first processor only from a fourth portion of the physical address space.

9. The system of claim 8, wherein the first processor is disposed on the proximity interconnect module.

10. The system of claim 8, wherein the proximity communication comprises at least one of capacitive coupling, inductive coupling, magnetic coupling, and optical coupling.

11. The system of claim 8, wherein the first portion, the second portion, the third portion, and the fourth portion are each one-fourth of the physical address space.

12. The system of claim 8, further comprising
a second processor;
a fifth cache overlapping a first corner of the second processor; and
a sixth cache overlapping a second corner of the second processor,
wherein the fifth cache and the sixth cache belong to the plurality of off-chip cache memories and are operatively connected by proximity communication to the second processor,
wherein the third cache further overlaps a third corner of the second processor,
wherein the fourth cache further overlaps a fourth corner of the second processor,
wherein the third cache and the fourth cache are further operatively connected by proximity communication to the second processor,
wherein the fifth cache is configured to cache data for the second processor only from the first portion of the physical address space,
wherein the sixth cache is configured to cache data for the second processor only from the second portion of the physical address space,
wherein the third cache is further configured to cache data for the second processor only from the third portion of the physical address space, and
wherein the fourth cache is further configured to cache data for the second processor only from the fourth portion of the physical address space.