Use of a cache ownership mechanism to synchronize multiple dayclocks

Abstract

A method of and apparatus for improving the efficiency of a data processing system employing multiple dayclocks using the facilities which maintain coherency of the system's level cache memories. These efficiencies result from dedicating a separate individual dayclock to each of the multiple instruction processors within the data processing system thereby decreasing access time and user queuing. These individual dayclocks are each incremented at one microsecond intervals. However, these individual dayclocks require periodic synchronization to avoid system level time-tagging problems. This synchronization occurs at 20 microsecond intervals using the cache coherency maintenance hardware of the system.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to data processing systems employing multiple instruction processors and more particularly relates to multiprocessor data processing systems employing multiple dayclocks in conjunction with multiple levels of cache memory.

2. Description of the Prior Art

It is known in the art that the use of multiple instruction processors operating out of common memory can produce problems associated with the processing of obsolete memory data by a first processor after that memory data has been updated by a second processor. The first attempts at solving this problem tended to use logic to lock processors out of memory spaces being updated. Though this is appropriate for rudimentary applications, as systems become more complex, the additional hardware and/or operating time required for the setting and releasing of locks can not be justified, except for security purposes. Furthermore, reliance on such locks directly prohibits certain types of applications such as parallel processing.

The use of hierarchical memory systems tends to further compound the problem of data obsolescence. U.S. Pat. No. 4,056,844 issued to Izumi shows a rather early approach to a solution. The system of Izumi utilizes a buffer memory dedicated to each of the processors in the system. Each processor accesses a buffer address array to determine if a particular data element is present in its buffer memory. An additional bit is added to the buffer address array to indicate invalidity of the corresponding data stored in the buffer memory. A set invalidity bit indicates that the main storage has been altered at that location since loading of the buffer memory. The validity bits are set in accordance with the memory store cycle of each processor.

U.S. Pat. No. 4,349,871 issued to Lary describes a bussed architecture having multiple processing elements, each having a dedicated cache memory. According to the Lary design, each processing unit manages its own cache by monitoring the memory bus. Any invalidation of locally stored data is tagged to prevent use of obsolete data. The overhead associated with this approach is partially mitigated by the use of special purpose hardware and through interleaving the validity determination with memory accesses within the pipeline. interleaving of invalidity determination is also employed in U.S. Pat. No. 4,525,777 issued to Webster et al.

Similar bussed approaches are shown in U.S. Pat. No. 4,843,542 issued to Dashiell et al, and in U.S. Pat. No. 4,755,930 issued to Wilson, Jr. et al. In employing each of these techniques, the individual processor has primary responsibility for monitoring the memory bus to maintain currency of its own cache data. U.S. Pat. No. 4,860,192 issued to Sachs et al, also employs a bussed architecture but partitions the local cache memory into instruction and operand modules.

U.S. Pat. No. 5,025,365 issued to Mathur et al, provides a much enhanced architecture for the basic bussed approach. In Mathur et al, as with the other bussed systems, each processing element has a dedicated cache resource. Similarly, the cache resource is responsible for monitoring the system bus for any collateral memory accesses which would invalidate local data. Mathur et al, provide a special snooping protocol which improves system throughput by updating local directories at times not necessarily coincident with cache accesses. Coherency is assured by the timing and protocol of the bus in conjunction with timing of the operation of the processing element.

An approach to the design of an integrated cache chip is shown in U.S. Pat. No. 5,025,366 issued to Baror. This device provides the cache memory and the control circuitry in a single package. The technique lends itself primarily to bussed architectures. U.S. Pat. No. 4,794,521 issued to Ziegler et al, shows a similar approach on a larger scale. The Ziegler et al, design permits an individual cache to interleave requests from multiple processors. This design resolves the data obsolescence issue by not dedicating cache memory to individual processors. Unfortunately, this provides a performance penalty in many applications because it tends to produce queuing of requests at a given cache module.

The use of a hierarchical memory system in a multiprocessor environment is also shown in U.S. Pat. No. 4,442,487 issued to Fletcher et al. In this approach, each processor has dedicated and shared caches at both the L1 or level closest to the processor and at the L2 or intermediate level. Memory is managed by permitting more than one processor to operate upon a single data block only when that data block is placed in shared cache. Data blocks in dedicated or private cache are essentially locked out until placed within a shared memory element. System level memory management is accomplished by a storage control element through which all requests to shared main memory (i.e. L3 level) are routed. An apparent improvement to this approach is shown in U.S. Pat. No. 4,807,110 issued to Pomerene et al. This improvement provides prefetching of data through the use of a shadow directory.

A further improvement to Fletcher et al, is seen in U.S. Pat. No. 5,023,776 issued to Gregor. In this system, performance can be enhanced through the use of store around L1 caches used along with special write buffers at the L2 intermediate level. This approach appears to require substantial additional hardware and entails yet more functions for the system storage controller.

Inherent in architectures which employ cache memory, is that the storage capacity is substantially less than the memory located at lower levels in the hierarchy. As a result, memory locations within the cache memory must often be cleared for use by other data quantities more recently needed by the instruction processor. For store-in cache memories, this means that those quantities modified by the instruction processor must first be rewritten to system memory before the corresponding location is available to store newly requested data. This “flushing” process tends to delay the availability of the newly requested data.

A problem very similar to the maintenance of cache memory coherency is the synchronization of dayclock information. In real time systems, an accurate dayclock is required to enable time tagging of data and events. Such time keeping is also necessary to provide certain priority determinations when these rest, at least in part, on relative timing. The use of a single clock by multiple processors tends to be slow and cumbersome. However, providing multiple dayclocks necessitates synchronization to ensure consistency to time tagging amongst processor.

SUMMARY OF THE INVENTION

The present invention overcomes the problems found in the prior art by providing a method of and apparatus for synchronizing multiple dayclocks within a multiple processor system having hierarchical memory. This synchronization process is basically provided by the cache memory coherency maintenance facilities.

The preferred mode of the present invention includes up to four main memory storage units. Each is coupled directly to each of up to four “pod”s. Each pod contains a level three cache memory coupled to each of the main memory storage units. Each pod may also accommodate up to two input/output modules.

Each pod may contain up to two sub-pods, wherein each sub-pod may contain up to two instruction processors. Each instruction processor has two separate level one cache memories (one for instructions and one for operands) coupled through a dedicated system controller, having a second level cache memory, to the level three cache memory of the pod.

Each instruction processor has a dedicated system controller associated therewith. A separate dayclock is located within each system controller.

Unlike many prior art systems, both level one and level two cache memories are dedicated to an instruction processor within the preferred mode of the present invention. The level one cache memories are of two types. Each instruction processor has an instruction cache memory and an operand cache memory. The instruction cache memory is a read-only cache memory primarily having sequential access. The level one operand cache memory has read/write capability. In the read mode, it functions much as the level one instruction cache memory. In the write mode, it is a semi-store-in cache memory, because the level two cache memory is also dedicated to the instruction processor.

In accordance with the present invention, the level two cache memory is of the store-in type. Therefore, the most current value of an operand which is modified by the corresponding instruction processor is first located within the level two cache memory. When the replacement algorithm for the level two cache memory determines that the location of that operand must be made available for newly requested data, that operand must be “flushed” into the lower level memory to avoid a loss of the most current value.

Waiting for flushing of the old data before requesting the new data induces unacceptable latency. Therefore, according to the present invention, a flush buffer is provided for temporary storage of the old data during the flushing process. Though this temporary storage appears at first to be a mere extension to the level two storage capacity, it greatly enhances efficiency because the flush process really does not need to utilize the level two cache memory.

The old data is moved from the level two cache memory to the flush buffer as soon as the replacement algorithm has determined which data to move, and the newly requested data is requested from the lower level memory. The flush process subsequently occurs from the flush buffer to the lower level of memory without further reference to the level two cache. Furthermore, locations within the level two cache memory are made available for the newly requested data well before that data has been made available from the lower level memory.

In accordance with the preferred mode of the present invention, each storage controller has a dedicated dayclock. With more than one storage controller in the system, the dayclocks run in a master/slave relationship. Each dayclock has a register within the corresponding system controller which is readily accessible by the associated instruction processor.

Each system controller increments its own dayclock register every microsecond. This incrementation is generated by the bus logic, thereby synchronizing it within the system controller. An individual dayclock may be have its contents invalidated and updated from the master in a manner similar to a dedicated cache memory location.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects of the present invention and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof and wherein:

FIG. 1 is an overall block diagram of a fully populated system in accordance with the present invention;

FIG. 2 is a schematic block diagram of one pod;

FIG. 3 is a schematic block diagram of one instruction processor along with its dedicated system controller;

FIG. 4 is a detailed diagram of the flush process of the present invention;

FIG. 5 is a detailed diagram showing the flush buffers of the present invention; and

FIG. 6 is a detailed timing diagram of the dayclock synchronization process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is an overall block diagram of fully populated data processing system according to the preferred mode of the present invention. This corresponds to the architecture of a commercial system of Unisys Corporation termed “Voyager”.

The main memory of the system consists of up to four memory storage units, MSU 10 , MSU 12 , MSU 14 , and MSU 16 . Being fully modular, each of these four memory storage units is “stand-alone” and independent of one another. Each has a separate point-to-point dedicated bi-directional interface with up to four “pods”, POD 18 , POD 20 , POD 22 , POD 24 . Again, each of the up to four pods is separate and independent of one another.

The contents of POD 20 are shown by way of example. For the fully populated system, POD 18 , POD 22 , and POD 24 are identical to POD 20 . The interface between POD 20 and each of the four memory storage units (i.e., MSU 10 , MSU 12 , MSU 14 , and MSU 16 ), is via a third level cache memory designated cached interface, CI 26 , in this view. CI 26 couples with two input/output controllers, I/O Module 44 and I/O Module 46 , and two sub-pods, SUB 28 and SUB 30 . A more detailed explanation of the POD 20 is provided below.

The above described components are the major data handling elements of the system. In the fully populated system shown, there are sufficient components of each type, such that no single hardware failure will render the complete system inoperative. The software employed within the preferred mode of the present system utilizes these multiple components to provide enhanced reliability for long term operation.

The remaining system components are utilitarian rather than data handling. System Oscillator 32 is the primary system time and clocking standard. Management System 34 controls system testing, maintenance, and configuration. Power Controller 36 provides the required electrical power. System Oscillator 38 , Management System 40 , and Power Controller 42 provide completely redundant backup capability.

FIG. 2 is a more detailed block diagram of POD 20 . The level three cache memory interfaces directly with the memory storage units via TLC Controller 26 (see also FIG. 1 ). The actual storage for the level three cache memory is TLC SRAMS 48 . As indicated this static random access memory consists of eight 16 byte memory chips.

Subpod 28 and subpod 30 each contain up to two individual instruction processors. These are designated Voyager IP 50 , Voyager IP 52 , Voyager IP 54 , and Voyager IP 56 . As explained in detail below, each contains its own system controller. In accordance with the preferred mode of the present invention, these instruction processors need not all contain an identical software architecture. FIG. 3 is a more detailed block diagram of Voyager IP 50 , located within Subpod 28 , located within POD 20 (see also FIGS. 1 and 2 ). As explained above, each instruction processor has a dedicated system controller having a dedicated level two cache memory. Instruction processor 64 has two dedicated level one cache memories (not shown in this view). One level one cache memory is a read-only memory for program instruction storage. Instruction processor 64 executes its instructions from this level one cache memory. The other level one cache memory (also not shown in this view) is a read/write memory for operand storage.

Instruction processor 64 is coupled via its two level one cache memories and dedicated system controller 58 to the remainder of the system. System controller 58 contains input logic 74 to interface with instruction processor 64 . In addition, data path logic 70 controls movement of the data through system controller 58 .

The individual dayclock dedicated to instruction processor 64 is a register which is located within Locks, Dayclocks, and UPI 62 of Voyager System Controller 58 . It is automatically incremented at one microsecond intervals from an internal source. As is readily apparent, maintaining an individual, dedicated dayclock for use by instruction processor 64 greatly decreases the access time and prevents queuing by multiple users. However, the deployment of multiple dayclocks within the system necessitates a synchronization process, which is described in greater detail below.

The remaining elements of system controller 58 provide the level two cache memory functions. SLC data ram 66 is the data actual storage facility. Control logic 70 provides the cache management function. SLC tags 72 are the tags associated with the level two cache memory. FLC-IC Dup. Tags 76 provides the duplicate tags for the level one instruction cache memory of instruction processor 64 . Similarly, FLC-OC Dup. Tags 78 provides the duplicate tags for the level one operand cache memory of instruction processor 64 . For a more complete discusses of this duplicate tag approach, reference may be made with the above identified co-pending and incorporated U.S. Patent Applications.

FIG. 4 is a detailed functional diagram showing the flushing process of the preferred mode of the present invention. Following a level one cache memory miss, a data request is made from level one operand cache memory 114 of instruction processor 110 (see also FIG. 3 ). In accordance with the present invention, the data request is made on memory bus 118 .

If the requested data is found within second level cache memory 122 (i.e., a cache hit), the data access occurs. However, if a cache miss occurs within second level cache memory 122 (i.e., the data is not present), a level three cache memory request is made via path 178 and memory bus 130 . As soon as the data is available, it is transferred from memory bus 130 via path 180 .

To provide a place to store the newly requested data, cache memory 122 may need to flush some older data, if all locations are full. The selection of which location(s) to flush is in accordance with a least recently used algorithm as modified in accordance with the above identified and incorporated co-pending patent applications. The data to be flushed is transferred to flush buffer 186 from which the data is rewritten to level three memory via bus 130 . Because this data is flushed from level two cache memory 122 to flush buffer 186 before the rewrite can be accomplished, space becomes quickly available within level two cache memory 122 for accommodating the newly requested data as soon as available.

FIG. 5 is detailed diagram showing the data flow in accordance with the preferred architecture. Upon being notified of a level two cache miss, priority logic 188 determines which locations are to be flushed. This selection is made in the manner discussed above. The location(s) to be flushed is communicated to tag RAM 190 and data RAM 196 via addressing path 192 .

Access of tag RAM 190 provides a determination whether there has been any modification to the data within level two cache memory. If there has been no modification as noted within tag RAM 190 , no further write operation to level three memory is required. If the data has been modified, however, path 194 notifies priority logic 188 that the modified data to be flushed must be rewritten to level three memory.

Assuming that a rewrite is necessary, the data is accessed from data RAM 196 and transferred via path 200 to temp register 198 . Further latency is reduced by employing two flush buffers (i.e., flush buffer 0 132 and flush buffer 1 134 ) as shown. Temp register 198 routes the data to be rewritten to either flush buffer 0 132 via path 202 or to flush buffer 1 134 as each becomes available.

The data to be flushed is stored within the selected flush buffer while the rewriting process is accomplished. The data to transferred to level three memory via path 136 and bus 130 .

FIG. 6 is a detailed timing diagram showing how the cache maintenance facilities are utilized to synchronize the individual, dedicated dayclocks. As explained above, each individual, dedicated dayclock is separately incremented at one microsecond intervals (see FIG. 3 ).

All individual, dedicated dayclocks are synchronized every 20 microseconds. When an instruction processor issues a Load Dayclock function, the system controller updates the dayclock with the new value and continues incrementing the dayclock until the next 20 microsecond synchronization. At the first 20 microsecond interval, the system controller stops the dayclock incrementation to permit other instruction processors to update their respective dayclocks.

In addition, an invalidate request is sent to the other processors, using the cache coherency facilities, to invalidate their respective dayclocks. Once the other system controllers realize their clock is invalid, each issues a request to update its own dayclock. At the next 20 microsecond interval, each dayclock is synchronized and resumes incrementation according to their bus logic. At his same time, a request is sent to update the I/O dayclock value. After an acknowledgment is sent back to the instruction processor, normal operation is resumed.

Having thus described the preferred embodiments in sufficient detail for those of skill in the art to make and use the present invention, those of skill in the art will be readily able to apply the teachings found herein to yet other embodiments within the scope of the claims hereto attached.

Claims

1. In a data processing system having a plurality of processors, wherein each processor has a dedicated cache memory responsively coupled to a lower level memory and wherein said data processing system employs a cache maintenance facility, the improvement comprising:a. A plurality of individual dayclocks wherein each dayclock is dedicated to a different one of said plurality of processors; and b. A system controller which synchronizes each of said plurality of individual dayclocks via said cache maintenance facility.

2. A data processing system according to claim 1 wherein each of said plurality of dayclocks has a dedicated circuit which periodically increments the corresponding one of said plurality of dayclocks.

3. A data processing system according to claim 2 wherein said system controller periodically synchronizes each of said plurality of individual dayclocks.

4. A data processing system according to claim 3 wherein said periodic synchronization occurs at an interval greater than said period incrementation.

5. A data processing system according to claim 4 wherein said periodic synchronization is at 20 microseconds and said periodic incrementation is at one microsecond.

6. A data processing system comprising:a. A plurality of processors; b. A plurality of cache memories responsively coupled to said plurality of processors; c. second level memory responsively coupled to each of said plurality of cache memories having a cache maintenance facility; d. A plurality of individual dayclocks with each of said individual dayclocks dedicated to a different one of said plurality of processors; and e. A system controller which synchronizes said plurality of dayclocks via said cache maintenance facility.

7. A data processing system according to claim 6 wherein each of said plurality of dayclocks has an internal incrementation circuit.

8. A data processing system according to claim 7 wherein each of said individual incrementation circuits increments at a first periodic interval.

9. A data processing system according to claim 8 said dayclock synchronization circuit synchronizes said plurality of dayclocks at a second periodic interval.

10. A data processing system according to claim 9 wherein said second periodic interval is longer than said first periodic interval.

11. A method of operating a plurality of dayclocks in a system having a cache maintenance facility comprising:a. Incrementing each of said plurality of dayclocks via a different one of a plurality of incrementation circuits; and b. synchronizing said plurality of dayclocks via said cache maintenance facility.

12. A method according to claim 11 wherein said incrementing step occurs at a first periodic interval.

13. A method according to claim 12 wherein said synchronizing step occurs at a second periodic interval.

14. A method according to claim 13 wherein said second interval is greater than said first interval.

15. A method according to claim 14 wherein said first interval is one microsecond and said second interval is 20 microseconds.

16. An apparatus comprising:a. Plurality of means for executing program instructions each having a cache memory; b. Plurality of means for providing time data wherein each of said plurality of providing means is responsively coupled to a different one of said plurality of executing means; c. Means responsively coupled to each of said cache memories for maintaining coherency of said cache memories; and d. Means responsively coupled to said plurality of providing means for synchronizing said providing means via said maintaining means.

17. An apparatus according to claim 16 wherein each of said plurality of providing means further comprises an incrementing means.

18. An apparatus according to claim 17 wherein each of said incrementing means operates at a first periodic interval.

19. An apparatus according to claim 18 wherein said synchronizing means operates at a second periodic interval.

20. An apparatus according to claim 19 wherein said second periodic interval is larger than said first periodic interval.