Abstract:
The present invention provides a test and set bypass mechanism which allows access to a semaphore while eliminating memory bandwidth degradation due to the traditional &#34;spin-locking&#34; problem. Generally, a storage and comparison structure in a processor, such as a content addressable memory (CAM), is used to store the address of the semaphore whenever it is requested. Thus, the process/processor, or other processors in a multiprocessor system, then need only check to see if the semaphore address is present in its respective storage and comparison structure. Consequently, there is no need to make multiple memory transactions for failed access of the semaphore, and hence, effective memory bandwidth is increased.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a system and method of utilizing semaphores to synchronize multiple processes, and more particularly to a system and method for providing a semaphore bypass mechanism which reduces the amount of memory bandwidth waste. 
     2. Discussion of Related Art 
     One of the major requirements of a shared-memory multiprocessor is being able to coordinate processes that are working on a common task. Typically, a programmer will use semaphores to synchronize the processes. As such, semaphores are widely used to prevent asynchronous accesses to resources such as devices or memory locations. A common semaphore implementation is by means of a test and set mechanism. In such a mechanism, a semaphore is a flag or label which is stored in an addressable memory location for controlling access to certain regions of memory or other addressable resources. 
     When a process wishes to access a region of memory, for example memory region xyz, with which a semaphore is associated, and wants to prevent other processes from accessing that region, the process writes all 1&#39;s into the semaphore associated with that particular memory region. If a different process should wish to access memory region xyz, it first checks the semaphore and if it finds that there are all 1&#39;s in the semaphore, the process knows that it is denied access. 
     Heretofore, the second process continually goes to memory and checks the semaphore to see if memory region xyz is available. This constant checking of the semaphore is called &#34;spin-locking&#34; and it wastes an enormous amount of memory bandwidth since the semaphore may not be available for a long time and each semaphore access means a memory transaction. For a more in-depth discussion on the above discussed synchronization mechanism, see Hennessy, et al., Computer Architecture a Quantitative Approach, Chapter 8, Morgan Kaufman Publishers (1990) which is hereby incorporated by reference herein in its entirety. 
     U.S. Pat. No. 5,050,072 to Earnshaw et al. (hereinafter the Earnshaw patent) describes a system for reducing common bus contention. It allows the semaphore test bit and set operations to be performed on each CPU&#39;s local bus. An identical copy of the semaphore lock bits are stored locally in SRAM on each CPU. SRAM&#39;s have a limited amount of storage space, and eventually the SRAM memory is filled with semaphores and the mechanism described in the Earnshaw patent will once again fall victim to spin locking. In addition, the Earnshaw patent caches semaphores. Caching of semaphores is expensive and difficult, and decreases system performance. Specifically, the SRAM used in the Earnshaw patent stores the identical semaphore lock bits of every other CPU&#39;s SRAM, which wastes a great amount of space. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, the present invention provides a test and set bypass mechanism and method which allows access to a semaphore while reducing memory bandwidth utilization due to the traditional &#34;spin-locking&#34; problem&#34; (i.e., providing greater effective memory bandwidth). Generally, a storage and comparison structure in a CPU, such as a content addressable memory (CAM), is used to store the address of the semaphore whenever it is requested. Since the semaphore address is stored in the storage compare device 120, there is no need to cache the semaphore. Rather, only the most recently failed semaphore addresses are stored in the storage and comparison structure. Thus, processes of the CPU, or other CPU&#39;s in a multiprocessor system, then need only to check to see if the semaphore address is present in its respective storage and comparison structure. Consequently, there is no need to make a memory transaction to access the semaphore, and hence, effective memory bandwidth is substantially increased. 
     The entry in the storage and comparison structure is cleared when any process/processor performs a write to a small region of memory enclosing the semaphore. The CPU checks (snoops) the shared memory bus to see if any CPU (including itself) is writing to the relevant semaphore. If any CPU writes to a semaphore that matches an entry in the storage and comparison device, that entry in the storage and comparison device is cleared. 
     When the owner of the locked resource (i.e., memory region or peripheral device), for example, clears the semaphore in memory, the semaphore address in the storage and comparison structure is also cleared. Then the resource is available to other processors/processes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features and advantages of the invention will be apparent from the following more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawings, in which: 
     FIG. 1 shows a general block diagram of a possible implementation of the datapath which incorporates the preferred embodiment of the present invention; 
     FIG. 2 shows a flowchart of a method of the present invention of accessing a memory location which is protected by a semaphore; and 
     FIG. 3 shows a flowchart of a method of the present invention of clearing an address from the storage compare device of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a test and set bypass circuit 100 of the present invention includes a basic memory requesting device 110 (such as a cache), a storage compare device 120, and a control block 130. A bus 105 interconnects the test and set bypass circuit 100 to a memory device. The bus 105 may also be connected to other such test and set bypass datapath mechanisms, for example, on a different CPU chip. 
     The storage compare device 120 is typically a content addressable memory (CAM). A valid bit is associated with each storage location in storage compare device 120. This safeguards against the possibility that a semaphore address that might randomly appear in storage compare device 120 (e.g., during a power up), will not be mistaken for a semaphore address which has been locked. Thus, when the valid bit is set, the semaphore address associated with that valid bit has been locked. The number of entries in the storage compare 120 is determined by the specific architectural constraints of the system utilizing the present invention. 
     As will be seen, the test and set bypass circuit 100 reduces the amount of memory bandwidth consumed by spin-locking for a semaphore. Referring to FIG. 2, a method of the present invention of accessing a memory location protected by a semaphore mechanism is now described. As shown in a block 210, a CPU (requesting device), or more precisely, a process in the processor, first requests access to a resource, with which a semaphore is associated, by issuing a test-and-set instruction (i.e., a predetermined instruction associated with a request to access a semaphore). Initially, the storage compare device 120 is checked to determine whether the semaphore is already locked, as shown in block 212. If it is not present in the storage compare device 120, block 220 stores the address of the semaphore in the storage compare device 120. Block 245 then performs a memory transaction to check the semaphore and determine whether it has been previously locked by another process/processor. 
     As shown in blocks 245 and 260, when the requesting device 110 is allowed to access a memory location (i.e., the semaphore associated with the memory location is not indicated, by storage compare device 120, to be locked), the requesting device will lock the semaphore and store the address of the semaphore in the storage compare device 120. Plural test-and-set instructions can result in plural entries being stored in the storage compare device 120. If, after checking the semaphore, the semaphore is locked (i.e., the semaphore contains all 1&#39;s ($FFFF&#39;s)), then all 1&#39;s are returned to the requesting device indicating that access has been denied. Moreover, the address of the semaphore is stored in storage compare device 120. 
     When another process/processor requests access to a semaphore, that process/processor checks its associated storage compare device 120, as shown in block 212. If the address of the requested semaphore is resident in storage compare device 120, the process/processor knows that the semaphore has not been released by the process/processor which locked it. The requesting device 110 receives all 1&#39;s (which means semaphore failed), as shown in block 250, and the semaphore is not requested from memory. Thus, memory bandwidth is not unnecessarily wasted. Subsequently, requesting device 110 may periodically repeat the test and set instruction, which checks the storage compare device 120, to determine whether the semaphore has been unlocked. 
     On the other hand, if the semaphore address is not in the storage compare unit 120, this means that the semaphore has not been previously requested or that it has been released or it has been replaced with another semaphore address. Consequently, a memory transaction occurs, and an access to the semaphore location is attempted by the requesting device 110. 
     Next, the system and method of the present invention for storing the semaphore address in the storage compare device 120 are described below. Reference is again made to FIG. 1. Initially, the storage compare device 120 is empty. When the requesting device 110 makes a request for a semaphore (i.e., a request for a portion of memory), the control block 130 looks up the storage compare device 120 to see if the semaphore address has already been stored. If the semaphore address is stored in this device, a HIT signal 122 is generated. This HIT signal 122 is received by the control block 130, which prevents the memory request from being sent out on the memory bus 105 by means of a control line 135. 
     Control block 130 also sends out a control signal 140. Control signal 140 is connected to a multiplexer 150. Multiplexer 150 selects, in accordance with the control line 140, either data from the memory bus 105 or data containing all 1&#39;s. If the HIT signal 122 is generated by the control block 130, the data containing all 1&#39;s is sent back to the requesting device. The data containing all 1&#39;s indicates that another process/processor currently has access to the particular memory location requested by the requesting device 110, and thus, the respective semaphore has been locked. 
     If no HIT signal 122 is generated by the storage compare device 120, the control block 130 knows that this semaphore has not been requested before by the instant requestor. The control block 130 then writes the semaphore address in the storage compare device 120 using a control line 137. In addition, control block 130 enables the control line 135, which in turn causes the memory request from the requesting device 110 to be sent out onto the memory bus 105. When the memory request has been completed, the data is returned to the requesting device by appropriately controlling the multiplexer 150 via control line 140. 
     Because of the large number of memory access requests, as well as the plurality of different memory locations that can be accessed at any given time, the storage compare device 120 can be filled with a large number of semaphore addresses. Eventually, the storage compare device 120 may become filled up. When there are no empty entries available in the storage compare device 120, prior entries can be replaced by one of the many replacement algorithms currently available, such as a Least Recently Used (LRU) algorithm. The new entry is then written over a previous entry. Thus, the bypass mechanism 100 avoids having to spine lock because of the current semaphore request. 
     Next, the method and system of the present invention for clearing an entry in the storage compare device 120 is described. Storage compare device 120 must be cleared if the semaphore is released. The storage compare device 120 is cleared if a write to any part of the smallest detectable memory block which encloses the semaphore is performed by any processor on the memory bus 105. In this way, the storage compare device 120 will never hold the address of a semaphore which has been cleared. In addition, the semaphore entry in the storage compare device 120 is cleared when any processor writes to the semaphore. 
     Referring to FIG. 3 in addition to FIG. 1, the present invention monitors (snoops) the memory bus 105 and determines if any memory transactions are occurring. If a memory request is sent on the memory bus 105 by any device, such as a CPU, the snoop mechanism will use the address signal detected to generate a corresponding snoop (request) address in the address generator 160. 
     As shown in a block 315, it is determined whether the memory request is a write request. If it is a write request, then the request is checked to determine whether it is a write from a test-and-set instruction, as shown in block 317. If it is a write from a test-and-set instruction, the entry in storage compare device 120 is not cleared. Otherwise, the generated snoop memory address is compared with the entries in the storage compare device 120, as shown in a block 320. If there is a HIT, that respective entry is cleared as shown in a block 325, since a write to the semaphore location means that the semaphore is released. 
     The snoop address generator 160 typically generates a cache sub-block address (high order bits) from the 11 bits of the memory row and column addresses appearing on the memory address bus 105. The snoop address generator 160 has control signals RAS, CAS, and WE, where RAS is the row address selector, CAS is the column address selector, and WE is the write enable. More address resolution is available from other control signals and timing information. 
     The present invention does not snoop the memory bus 105 for semaphore requests (other then writes) from other requesting devices. In other words, separate processes do not share information. Rather, each individual requesting device 110 is only concerned with the contents of its own storage compare device 120. Consequently, when a requesting device 110 requests access to a semaphore location that has previously been locked by another requesting device, only then does it proceed to store the semaphore address in its storage compare device 120. Although this might appear to be a wasted memory transaction, this process saves a large amount of memory space in the storage compare device 120. A considerably larger memory device (e.g., in the preferred embodiment a larger CAM 120) would be necessary if the address of every semaphore that every requesting device in the system locked was stored in local memory. Consequently, the present invention need only make one memory transaction prior to engagement of the semaphore bypass mechanism 100 of the present invention in order to save a substantial amount of memory space. 
     As discussed briefly above in connection with block 317 of FIG. 3, when you do a test and set operation to a semaphore in memory, essentially a read-modify-write operation is taking place. Thus, care must be taken so as to not clear the corresponding entry in any of the storage compare devices 120 during the write portion of the read-modify-write. The only time the storage compare device 120 is cleared is when there is a &#34;real&#34; write (i.e., a store). Thus, to avoid clearing the entry in the storage compare device, snooping is disabled in the preferred embodiment of the present invention. 
     Referring to FIG. 1, in a preferred embodiment of the present invention a share signal 190 indicates whether the system 100 is currently working on shared memory or not. The share signal 190 enables the snooping operation (i.e., cache coherency). When the share signal 190 is turned off the compare function described above is disabled at all of the processors. In other words, share signal 190 tells the snoop address generator 160 not to snoop during the write portion of the test and set instruction. 
     The present invention is not restricted to being used only in the traditional test and set mechanism. It could also be used in counting semaphores. Typically, a counting semaphore is implemented by providing a mechanism for storing particular values in the semaphore. Saving a different value in the semaphore has the effect of unlocking that memory location. The system and method of the present invention can be used without any additional limitations in a system and method that utilizes counting semaphores. 
     The memory bus 105 sends an encoded semaphore operation signal to all the snoop address generators 160 to indicate that a test-and-set instruction has executed. 
     The semaphore size should be the minimum distinct address that is either sent on the memory bus or can be derived from the memory bus controls. In the preferred embodiment of the present invention the semaphore size should be less than or equal to one memory word. In other words, if the smallest distinct memory address sent on the bus is on a word boundary, the size of the semaphore should preferably be on a word boundary. This ensures that entries are not unnecessarily cleared in storage compare device 120. 
     For example, if the requesting device first submits a test and set request with the memory address $0000 --  0001, and assuming that there are initially no entries in storage compare device 120, the memory address of $0000 --  0001 will be stored in storage compare device 120. When the requesting device 110 again submits another test and set request with memory address $0000 --  0002, this address when compared with the entries in the storage compare 120 will not create a HIT signal 122. 
     However, if the requesting device 110 now decides to release the first semaphore with memory address $0000 --  0001, and if the granularity of memory addresses on the memory bus 105 is a word size, the address seen on the memory bus is $0000 --  00xx (where xx are don&#39;t cares). When a comparison of this memory address is compared with the entries in storage compare 120, both memory address $0000 --  0001 and $0000 --  0002 will cause a HIT signal 122 and be cleared. This irregularity could be avoided if the semaphores have a size of a word or if only a single semaphore is held within a word. 
     In spite of this irregularity, there is no effect in terms of functionality. What this entails is that since $0000 --  0002 has been unnecessarily cleared, the next time when a process requests semaphore $0000 --  0002, an unnecessary memory request for the semaphore will be generated. 
     While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.