Patent Document

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to computing systems, and more particularly, to increasing the throughput of a processor during cache misses. 
     2. Description of the Relevant Art 
     Pipelining is used to increase the throughput of instructions per clock cycle (IPC) of processor cores, or processors. However, the throughput may still be reduced due to certain events such as pipeline stalls. Stalls may be caused by a branch misprediction, a cache miss, data dependency, or other, wherein no useful work may be performed for a particular instruction during a clock cycle. 
     Different techniques are used to fill these unproductive cycles in a pipeline with useful work. Some examples include loop unrolling of instructions by a compiler, branch prediction mechanisms within a core and out-of-order execution within a core. An operating system may divide a software application into processes and further divide processes into threads. A thread, or strand, is a sequence of instructions with no control flow instructions that may share memory and other resources with other threads and may execute in parallel with other threads. A processor core may be constructed to execute more than one thread per clock cycle in order to increase efficient use of the hardware resources and reduce the effect of stalls on overall throughput. A microprocessor may include multiple processor cores to further increase parallel execution of multiple instructions per clock cycle.  
     The above techniques may hide some of the unproductive clock cycles due to cache misses by overlapping them with useful work of other instructions. If the latencies of L1 and L2 cache misses are great, some unproductive cycles may still occur in the pipeline and the IPC may still decrease. Some techniques to decrease the stall cycles due to cache misses include using larger sized caches, using higher associativity in the caches, speculatively prefetching instructions and data, use non-blocking caches, using early restart or critical word first, using compiler optimizations, or other. 
     Some scientific applications are memory intensive such as high performance computing (HPC) software applications. A few application examples include climate simulations of the world&#39;s oceans, complex fluid dynamic (CFD) problems such as a tunnel model of an aircraft wing using Navier/Stokes equations, computational chemistry, and an air quality model used by the U.S. environment protection agency (EPA). These scientific applications are memory intensive with ratios of memory instructions per single floating-point instruction as high as 400 to 1,400. Also, the codes tend to be loop-intensive and benefit from architectures that offer single-instruction-multiple-data (SIMD) operations. The loops are able to operate on multiple data elements in a data set with a single operation. 
     Therefore, a critical performance bottleneck for a processor executing code as described above, is a processor&#39;s forwarding-store buffer and the cache design. The stall cycles from cache misses need to be reduced in order to efficiently supply data to the operations. A non-blocking cache may be used in order to perform hits-under-misses and increase the IPC. 
     A problem may arise with scientific applications that do not have data locality and therefore have high data dependency such as computational chemistry. A  non-blocking cache may not help, since the data from hits-under-miss may not be used until the data from the cache miss is returned. A blocking cache may ensure in-order supply of the data, but the latencies from the cache miss and subsequent cache hits accumulate and reduce the IPC. 
     In view of the above, efficient methods and mechanisms for increasing the throughput of processors are desired.  
     SUMMARY OF THE INVENTION 
     Systems and methods for increasing the throughput of a processor during cache misses are contemplated. In one embodiment, a computer system is provided comprising a processor, a cache subsystem, and a memory. A bypass retry device may be included in a first-level cache. In an alternative embodiment, the bypass retry device may be provided outside the other components, but coupled to the cache and processor. The processor may be executing scientific applications that are memory intensive and have data dependency between instructions. The processor may generate memory requests to the cache in order to retrieve data for a software application. The processor may not be able to execute a block of instructions out-of-order due to a data dependency. The cache may not contain the data required for a memory request. In this case, the processor needs to send the request to the next level in the cache subsystem and possibly to memory if the data is not located in the next level. 
     While the cache miss is being serviced, the processor may continue to generate memory requests to the cache. The memory line data from subsequent cache hits may not be returned to the processor, since the corresponding instructions may not be able to proceed until the cache miss data is retrieved. The bypass retry device may store the address and memory line data of subsequent cache hits. Also, it may store the cache miss address and memory line data when they are retrieved and send them to the cache for a cache line replacement. 
     Selection logic within the bypass retry device may determine the priority of sending data to the processor. For example, the selection logic may determine a first priority memory line to send to the processor is a memory line returned from a cache miss. Then the selection logic may determine a second priority memory line to send to the processor is a stored memory line after the first priority  memory line has been sent. Next, it may determine a third priority memory line to send to the processor is data from a cache hit after all stored memory lines have been sent. Therefore, the data for memory requests are provided to the processor in the same order as they were generated from the processor without delaying subsequent memory requests after a cache miss. 
     In another aspect of the invention, a method is provided to supply a processor with requested data in the order they were generated from the processor. Subsequent memory requests after a cache miss are allowed to proceed in order to reduce the memory access latencies. Data for a software application may be stored by the method and a subset of the data may also be stored in a cache. As the application is executed, memory requests for the data may be generated and sent to the cache. If the data is not found in the cache, a miss signal is generated and the data is retrieved from memory. During the retrieval of the miss data, subsequent memory requests are generated and allowed to proceed. The data for the subsequent cache hits are stored by the method. Also, the cache miss address and memory line data may be stored when they are retrieved and they may be sent them to the cache for a cache line replacement. 
     The method may determine the priority of sending data to the processor. It may determine a first priority memory line to send to the processor is a memory line returned from a cache miss. Then the method may determine a second priority memory line to send to the processor is a stored memory line after the first priority memory line has been sent. Next, it may determine a third priority memory line to send to the processor is data from a cache hit after all stored memory lines have been sent.  
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a generalized block diagram illustrating one embodiment of a computing system. 
         FIG. 2  is a generalized block diagram illustrating one embodiment of a load operation bypass retry circuit. 
         FIG. 3  is a flow diagram of one embodiment of a method for increasing the throughput of a processor during a load operation cache miss. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments are shown by way of example in the drawings and are herein described in detail. It should be understood, however, that drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.  
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , one embodiment of a computing system  100  is shown. One or more processors  102   a - 102   d  may be included in system  100 . As used herein, elements referred to by a reference numeral followed by a letter may be collectively referred to by the numeral alone. For example, processors  102   a - 102   d  may be collectively referred to as processors  102 . Each processor  102  may include one or more processor cores. Each core within processor  102  generally includes circuitry to execute instructions of a predefined instruction set architecture (ISA). Each core may include a superscalar microarchitecture with a multi-stage pipeline. In some embodiments, a multi-stage pipeline may perform out-of-order execution of the instructions of the ISA. Also, each core may be configured to simultaneously execute instructions for one or more threads of a software application. Various embodiments may be chosen for the implementation of processor  102  and its cores. 
     In one embodiment, a memory subsystem accompanying processors  102  may include several levels. The highest level may include the registers within processors  102 . The next level may include a cache  106 . Cache  106  may be on the same semiconductor die as processor  102  or it may be located off-die, but near to processor  102 . Each processor  102  may have its own cache  106 . A translation lookaside buffer (TLB) may be included for each cache  106  and subsequent levels of caches for address matching of the requested memory line. Processors  102  may perform speculative prefetching of both instructions from an i-cache and data from a d-cache. 
     A lower level of the memory subsystem may include cache  130 , which may be shared by processors  102  and caches  106 . Below the cache hierarchy may be a memory controller  140  to interface with lower-level memory that may comprise  other levels of cache on the die outside the microprocessor, dynamic random access memory (DRAM), dual in-line memory modules (dimms) in order to bank the DRAM, a hard disk, or a combination of these alternatives. 
     Interconnect  120  may be configured to convey memory requests from processors  102  to cache  130  or to memory controller  140  and the lower levels of the memory subsystem. Also, interconnect  120  may convey received memory lines and control signals from lower-level memory via memory controller  140  to processors  102  and caches  106  and  130 . Interconnect bus implementations between interconnect  120 , memory controller  140 , interface  150 , processors  102 , and caches  106  and  130  may comprise any suitable technology. Interface  150  generally provides an interface for I/O devices off the microprocessor to the memory subsystem and processors  102 . I/O devices may include peripheral network devices such as printers, keyboards, monitors, cameras, card readers, hard disk drives or other. 
     In one embodiment, as will be discussed in further detail below, bypass retry  104  is configured to maintain in-order memory requests to processor  102  without blocking subsequent memory requests after a cache miss. When a cache miss occurs with cache  106 , a non-blocking version of cache  106  allows subsequent memory requests from processor  102  to proceed. However, without bypass retry  104 , the memory lines returned to processor  102  are out-of-order. A blocking version of cache  106  does not allow subsequent memory requests to proceed after a cache miss. Memory lines are returned to processor  102  in-order. However, the latency is greater than the latency of a non-blocking version. This greater latency may reduce system performance. 
     After a cache miss, bypass retry  104  allows subsequent memory requests from processor  102  to cache  106  to proceed. However, memory lines are returned to  processor  102  in-order. Therefore, cache  106  may be non-blocking, and processor  102  receives the memory lines in-order. Processor  102  may be executing code for scientific applications that have a high data dependency between instructions of a loop. Table 1 shows the differences between the different implementations of handling a cache miss. For illustrative purposes, this example assumes a one clock cycle latency for a cache hit and a four clock cycle latency for a cache miss. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Timing Flow of Different Cache Implementations. 
               
             
          
           
               
                 Clock 
                 Blocking First- 
                 Non-blocking 
                   
               
               
                 Cycle 
                 Level Cache 
                 First-Level Cache 
                 Bypass Retry 
               
               
                   
               
               
                 1 
                 1 st  level cache 
                 1 st  level cache miss - 
                 1 st  level cache 
               
               
                   
                 miss - Load A. 
                 Load A. 
                 miss - Load A. 
               
               
                 2 
                 Processor waits 
                 1 st  level cache hit - 
                 1 st  level cache hit - 
               
               
                   
                 for data - Load A. 
                 Load B. 
                 Load B. 
               
               
                 3 
                 Processor waits 
                 1 st  level cache hit - 
                 1 st  level cache hit - 
               
               
                   
                 for data - Load A. 
                 Load C. Processor 
                 Load C. 
               
               
                   
                   
                 receives data - 
               
               
                   
                   
                 Load B. 
               
               
                 4 
                 Processor receives 
                 Cache miss data 
                 Fill Buffer filled - 
               
               
                   
                 data - Load A. 
                 returns and is 
                 Load A. Processor 
               
               
                   
                   
                 buffered- Load A. 
                 receives data - 
               
               
                   
                   
                 Processor receives 
                 Load A. 
               
               
                   
                   
                 data - Load C. 
               
               
                 5 
                 1 st  level cache hit - 
                 1 st  level cache hit - 
                 1 st  level cache hit - 
               
               
                   
                 Load B. 
                 Load D. Processor 
                 Load D. 
               
               
                   
                   
                 receives data - 
                 Processor receives 
               
               
                   
                   
                 Load A. 
                 data - Load B. 
               
               
                 6 
                 1 st  level cache hit - 
                 Processor receives 
                 Processor receives 
               
               
                   
                 Load C. 
                 data - Load D. 
                 data - Load C. 
               
               
                   
                 Processor receives 
               
               
                   
                 data - Load B. 
               
               
                 7 
                 1 st  level cache hit - 
                   
                 Processor receives 
               
               
                   
                 Load D. 
                   
                 data - Load D. 
               
               
                   
                 Processor receives 
               
               
                   
                 data - Load C. 
               
               
                 8 
                 Processor receives 
               
               
                   
                 data - Load D. 
               
               
                   
               
             
          
         
       
     
       FIG. 2  illustrates one embodiment of a memory datapath  200  within computing system  100 . Processor  102  is coupled to cache  106  in order to send memory requests. Whether there is a hit or a miss in cache  106 , control signals are sent to control  250  within bypass retry  104 . Also control  250  receives input from load buffer  210  and fill buffer  230 . Circuitry in control  250  determines during each clock cycle the source of data to send to processor  102 . In one embodiment, there may be three choices for the source of data. These three choices may be sent to inputs  0 - 2  of a selection device such as mux gate  240 . 
     One choice for the source of data, which may be sent to input  0  of mux gate  240 , may be the data from a lower-level memory such as a level-2 (L2) cache or DRAM. This memory line data may be servicing a prior miss in cache  106 . Fill address  220  and fill buffer  230  may store the address and data of this memory line respectively. Also, status bits may be stored in each entry of these buffers to denote the information is available. If only one outstanding cache miss may be serviced, there may be only one entry  222  in fill address  220  and one entry  232  in fill buffer  230 . In alternative embodiments, multiple outstanding cache misses may be serviced. Fill address  220  and fill buffer  230  need as many entries as the number of possible outstanding cache misses. Also fill address  220  and fill buffer  230  may be implemented as first-in-first-out (FIFO) buffers. The control logic and queues within processor  102  becomes more complex with each outstanding cache miss and this complexity may set the limit on the number of outstanding cache misses. 
     A second choice for the source of data, which may be sent to input  2  of mux gate  240 , may be queued cache hit data stored in load buffer  210 . This data may be  queued as hit-under-miss data. A prior miss in cache  106  may still be in the process of being serviced, but subsequent memory requests are allowed to proceed. However, the data of the subsequent cache hits may not be allowed to be sent to processor  102  ahead of the prior cache miss data. Therefore, the subsequent cache hit data may be queued in a FIFO buffer, such as load buffer  210 , until the prior cache miss data arrives from a lower-level memory. After the prior cache miss data is sent to processor  102 , then the queued data in load buffer  210  may be sent to processor  102 . The queued data is stored in an entry  212 , which may hold the memory line address, data, and status bits, such as a valid bit to denote the data is ready. The number of entries in load buffer  210  may be determined by the difference between the latency of a cache hit and a cache miss. This number is used for hit-under-miss cache accesses. If miss-under-miss cache accesses are permitted, the number of entries will increase. 
     A third choice for the source of data, which may be sent to input  1  of mux gate  240 , may be cache hit data. If no cache miss is being serviced and the load buffer is empty, then the cache hit data may be forwarded to processor  102 . 
     Referring to  FIG. 3 , a method  300  of one embodiment for increasing the throughput of a processor during a load operation cache miss is shown. In block  302 , a processor is executing instructions of a software application, such as a scientific application having a high data dependency between instructions within a loop. The processor may need to make a memory request during execution (decision block  304 ). If a previous cache miss is being serviced (decision block  306 ) and the data has been returned from lower-level memory such as a L2 cache or DRAM (decision block  308 ), then this returned data is sent to the processor in block  310 . Whether or not the prior cache miss data returned, if the current first-level (L1) cache memory access is a hit (decision block  312 ), then the L1 cache hit data is placed in the load buffer, a FIFO queue, in block  314 . If the current  L1 cache memory access is a miss (decision block  312 ), then the address of the current memory request is sent to the L2 cache in block  316 . 
     If a previous cache miss is not being serviced (decision block  306 ) and the load buffer is not empty (decision block  318 ), then the memory line data at the head of the load buffer, a FIFO queue, is sent to the processor in block  324 . Flow then continues to decision block  312  as described above. If the load buffer is empty (decision block  318 ) and the current first-level (L1) cache memory access is a hit (decision block  320 ), then the L1 cache hit data is sent to the processor in block  322 . If the L1 cache memory access is a miss (decision block  320 ), then the address of the current memory request is sent to the L2 cache in block  316 . 
     Although the embodiments above have been described in considerable detail, numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.

Technology Category: g