Abstract:
A technique for performing a plurality of operations in a shared memory system having a plurality of addresses is disclosed. The technique includes entering into a speculative mode, speculatively performing each of the plurality of operations on addresses in the shared memory system, marking addresses in the shared memory system that have been operated on speculatively as being in a speculative state, and exiting the speculative mode, wherein exiting the speculative mode includes marking the addresses in the shared memory system that have been operated on as being in a non-speculative state.

Description:
CROSS REFERENCE TO OTHER APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 60/610,028 entitled VIRTUAL MACHINE filed Sep. 14, 2004. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to computer memory. More specifically, speculative multiaddress atomicity (SMA) is disclosed. 
     BACKGROUND OF THE INVENTION 
     A shared memory system includes a plurality of addresses that are accessed by multiple agents. For example, a shared memory system could include an L 1 /L 2  cache and a common memory store. In software, there are many instances in which two or more threads share a block of data. There are times where a thread must do a set of operations on one or more addresses without interference from another thread (e.g. read a value, increment it, store a new value). Typically, software will use locks to protect a set of addresses from being accessed by other threads while one thread is accessing those addresses. This locking creates a serialization point for software. However, many times this locking is done at a much coarser grain or more strictly than is actually needed. It would be desirable to eliminate the need for software locks in such cases and improve the efficiency of the shared memory system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of the invention are disclosed in the following detailed description and the accompanying drawings. 
         FIG. 1  is a block diagram illustrating a shared memory system. 
         FIG. 2  is a block diagram illustrating a cache. 
         FIG. 3  is a state diagram illustrating various states of a cache. 
         FIG. 4A  is a state diagram illustrating various states of a line in a cache and the state transitions resulting from a load or a store under speculation. 
         FIG. 4B  is a state diagram illustrating various states of a line in a cache state transitions resulting from committing or aborting. 
         FIG. 5A  is a flowchart illustrating an abort process. 
         FIG. 5B  is a flowchart illustrating a commit process. 
     
    
    
     DETAILED DESCRIPTION 
     The invention can be implemented in numerous ways, including as a process, an apparatus, a system, a composition of matter, a computer readable medium such as a computer readable storage medium or a computer network wherein program instructions are sent over optical or electronic communication links. In this specification, these implementations, or any other form that the invention may take, may be referred to as techniques. In general, the order of the steps of disclosed processes may be altered within the scope of the invention. 
     A detailed description of one or more embodiments of the invention is provided below along with accompanying figures that illustrate the principles of the invention. The invention is described in connection with such embodiments, but the invention is not limited to any embodiment. The scope of the invention is limited only by the claims and the invention encompasses numerous alternatives, modifications and equivalents. Numerous specific details are set forth in the following description in order to provide a thorough understanding of the invention. These details are provided for the purpose of example and the invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in detail so that the invention is not unnecessarily obscured. 
       FIG. 1  is a block diagram illustrating a shared memory system  100 . In this example, shared memory system  100  is shown to include cache domain  104 , cache domain  106 , coherency network  108 , and memory  110 . Cache domain  104  is shown to include a CPU and an L 1  cache. Cache domain  106  is shown to include a CPU and an L 1  cache. Cache domains  104  and  106  and memory  110  communicate over coherency network  108 . Cache domains  104  and  106  share memory  110 . Every element connected to coherency network  108  is coherent. 
     There are numerous ways in which memory coherency can be maintained. In some embodiments, the shared memory system follows a cache coherency protocol that includes a modify, share, and/or invalid state, such as MSI or MESI. The coherency network may use snoops, directory-based, broadcast, or other protocols. Each cache domain could include multiple cache levels. For example, cache domain  104  could include an L 1 /L 2  cache. Shared memory system  100  could include any number of processors, threads, and memory, and any number of cache domains. Memory  110  could be a global memory and system  100  could include any type of local memory. 
     In this example, a cache line in the modify state can return the invalid state in response to a coherency request. In addition, each cache has an ability to write back to memory or save data when a store is performed. The motivation for these attributes is more fully described below. 
       FIG. 2  is a block diagram illustrating a cache. In this example, cache  200  is shown to include 512 lines each having one of six states: invalid, share speculative read, share commit, modify speculative write, modify speculative read, and modify commit. In this example, each line is marked as being in a state using the shadow memory and tag value. The tag value indicates whether the line is in a modify, share, or invalid state. The tag value indicates an observable (or non-speculative) state. In some embodiments, in response to a coherency request, the tag value of that line is returned. The shadow value indicates whether the line is in a speculative write, speculative read, or invalid state. The shadow value indicates a speculative (or nonobservable) state. In some embodiments, a set of shadow values is stored in a register file so that multiple values can be read or modified in one cycle. 
     In some embodiments, there are two speculative bits in the shadow memory. In some embodiments, a three bit value is used to store the state. In this example, each line is shown to have a state. This example could also apply to an address where each address has a state. Although a line(s) may be described, any of the examples described herein could also apply to an address(es). 
     In some embodiments, a speculative state is applied to a plurality of caches. 
       FIG. 3  is a state diagram illustrating various states of a cache. For example, this state diagram could apply to the cache illustrated in  FIG. 2 . In this example, state diagram  400  is shown to include observable mode  404 , speculative (or SMA) mode  406 , committing state  408 , and aborting state  410 . In some embodiments, a set of software instructions are provided, which include “speculate”, “commit”, and “abort”. 
     A cache in observable mode  404  transitions to speculative mode  406  when a “speculate” command is received. When in speculative mode  406 , speculative operations may be performed on one or more lines or addresses in the cache. For example, an operation could include load (read) or store (write). Speculative operations are operations performed speculatively on the cache, but are not necessarily later committed. For example, during speculative mode, the cache may be disturbed, in which case the line may be invalidated, and the original value of the line retrieved from memory. In some embodiments, the operations performed on the cache when it is in speculative mode  406  form an atomic transaction. An atomic transaction satisfies all ordering and visibility constraints of the shared memory system. The cache transitions to committing state  408  when a “commit” command is received. When in committing state  408 , speculative stores made to the cache are committed. When the committing process is complete, the cache returns to observable mode  404 . 
     When the cache is in speculative mode  406 , the cache enters aborting mode  410  when a speculative line in the cache is disturbed or an “abort” command is received. In some embodiments, a line is disturbed when a coherency request or eviction is received. For example, if another processor loads data to an address to which data was speculatively loaded or stored, a coherency request is made, and that address is disturbed. If another processor stores or loads data to an address to which data was speculatively stored, that address is disturbed. Also, if another processor stores data to an address to which data was speculatively loaded, that address is disturbed. An eviction could occur when a cache runs out of space. When in aborting state  410 , speculative stores made to the cache are aborted. For example, lines or addresses to which data was speculatively stored may be invalidated. In some embodiments, the original data in those lines or addresses can be retrieved from memory. The original data is the last committed data in the line, prior to the speculative stores. 
       FIG. 4A  is a state diagram illustrating various states of a line in a cache and the state transitions resulting from a load or a store under speculation. For example, the state diagram may apply to a line (or address) in cache  200 . In this example, state diagram  300  is shown to include three observable states and three speculative states. When the cache is in observable mode, each line is in an observable state. When the cache is in speculative mode, each line can be in a speculative state. The three observable states are indicated by a double circle, and include invalid state  304 , share state  306   b , and modify state  308   c . The subscript “C” on states  306   b  and  308   c  indicates that the line has been committed, and thus is not in a speculative state. 
     In some embodiments, the software is aware of observable states, but not of nonobservable states (i.e., speculative states). In some embodiments, the speculative states are the states that occur during an atomic transaction, where the transaction includes a set of one or more operations that are speculatively performed. 
     The three speculative states include share state  306   a , modify state  308   a , and modify state  308   b . The subscript “SR” on share state  306   a  indicates that the line is in a speculative read state. The subscript “SW” on state  308   a  indicates that the line is in a speculative write state. The subscript “SR” on state  308   b  indicates that the line is in a speculative read state. For example, referring to  FIG. 2 , a line in modify speculative write state  308   a  would have a tag value indicating that it is in a modify state and a shadow value indicating that it is in a speculative write state. 
     In this example, a standard MSI coherency state diagram is modified to include SMA speculative state transitions. Any cache coherency mechanism can be similarly modified in other embodiments. In this example, the state diagram transitions are described as follows: 
     A line in invalid state  304  transitions to share speculative read state  306   a  when a load is received for that line. When a store is received, the line transitions to modify speculative write state  308   a.    
     A line in share speculative read state  306   a  remains in the same state when a load is received. When a store is received, the line transitions to modify speculative write state  308   a.    
     A line in modify speculative write state  308   a  remains in the same state when a load or a store is received. 
     A line in share commit state  306   b  transitions to share speculative read state  306   a  when a load is received. When a store is received, the line transitions to modify speculative write state  308   a.    
     A line in modify commit state  308   c  transitions to modify speculative read state  308   b  when a load is received. When a store is received, the line transitions to modify speculative write state  308   a  and the (original) modified data is saved, as more fully described below. In some embodiments, the modified data is written back to memory. 
     A line in modify speculative read state  308   b  remains in the same state when a load is received. When a store is received, the line transitions to modify speculative write state  308   a  and the (original) modified data is saved, as more fully described below. In some embodiments, the modified data is written back to memory. 
     The (original) modified data is saved when there may be a need to retain the modified data. For example, when a store is performed on a line in modify commit state  308   c , the line transitions to modify speculative write state  308   a . The modified data stored in the line is saved before the store is performed. In the event that an abort occurs, the line transitions to invalid state  304  and the modified data can be retrieved from memory. In some embodiments, the modified data is saved to a lower level of the cache hierarchy rather than memory. In some embodiments, the modified data is stored in a local shadow location. 
     In some embodiments, transitioning the state of the line includes marking the address (or line) in the cache as being in the new state. 
       FIG. 4B  is a state diagram illustrating various states of a line in a cache and the state transitions resulting from committing or aborting. For example, the state diagram may apply to a line (or address) in cache  200 . In this example, the cache is in speculative mode and the line is in one of three speculative states: share speculative read state  306   a , modify speculative write state  308   a , or modify speculative read state  308   b . When committing or aborting, the line transitions to one of three observable states: invalid state  304 , share commit state  306   b , or modify commit state  308   c.    
     When committing, share speculative read state  306   a  transitions to share commit state  306   b . Modify speculative write state  308   a  and modify speculative read state  308   b  transition to modify commit state  308   c . When aborting, share speculative read state  306   a  transitions to share commit state  306   b . Modify speculative write state  308   a  transitions to invalid state  304 . Modify speculative read state  308   b  transitions to modify commit state  308   c.    
     If the cache is in observable mode  404 , each line is in one of observable states  304 ,  306   b , and  308   c  and the state of each line can be observed. If the cache is in speculative mode  406 , if a speculative line is disturbed, an abort occurs, and the cache returns to observable mode  404 . If a non-speculative line is disturbed while in speculative mode, an abort will not necessarily occur. In some embodiments, a cache line can only be in a speculative state while in speculative mode. Each line in a speculative state returns to one of states  304 ,  306   b , and  308   c , depending on which speculative state  306   a ,  308   a , and  308   b , the line was in prior to the abort. 
       FIG. 5A  is a flowchart illustrating an abort process. In some embodiments, this process is performed on cache  200  when it is in aborting state  410 . In this example, the state of each line in the cache is stored as shown in  FIG. 2 . When a coherency request is made of a line, the tag value of that line is returned. 
     In some embodiments, a response mechanism is set to respond “invalid” for all modify speculative write lines ( 502 ). As shown in  FIG. 4B , a line that is in modify speculative write state  308   a  transitions to invalid state  304  when an abort occurs (during aborting state  410 ). In some embodiments, the abort process is atomic. The processor waits until the lines are transitioned (e.g., the tag values are changed) to invalid before responding to coherency requests. In some embodiments, rather than wait, a response mechanism is set to respond “invalid” for those lines ( 502 ). 
     The state of each share speculative read line is changed to share commit ( 504 ). The state of each modify speculative write line is changed to invalid ( 506 ). The state of each modify speculative read line is changed to modify commit ( 508 ). In ( 504 ) and ( 508 ), the tag value of the line does not change, so setting a response mechanism is not useful. The state transitions for ( 504 )-( 508 ) are shown in  FIG. 4B . The software is notified ( 510 ) of the abort. For example, a trap, flag, interrupt, or exception could be generated. In some embodiments, the cache is now in observable mode  404 . 
     In this example, when a coherency request is made of a line, the tag value of that line is returned. In other embodiments, the state of each line may be stored in other ways and other response mechanism(s) may be set. 
     There are various alternative ways to designate a set of observable and speculative states. For example, rather than designating state  308   a  as modify speculative write, state  308   a  could be designated as invalid speculative write. In this case, ( 502 ) could include setting the response mechanism to respond “modify” for all invalid speculative write lines. 
       FIG. 5B  is a flowchart illustrating a commit process. In some embodiments, this process is performed on cache  200  when it is in committing state  408 . In this example, the state of each line in the cache is stored as shown in  FIG. 2 . When a coherency request is made of a line, the tag value of that line is returned. 
     In this example, a response mechanism is set to observable mode ( 520 ). ( 520 ) is a non-operation, to contrast ( 502 ) in  FIG. 5A , in which a response mechanism is set. The state of each share speculative read line is changed to share commit ( 522 ). The state of each modify speculative write line is changed to modify commit ( 524 ). The state of each modify speculative read line is changed to modify commit ( 526 ). The state transitions for ( 522 )-( 526 ) are shown in  FIG. 4B . In some embodiments, the cache is not in observable mode  404 . 
     In this example, when a coherency request is made of a line, the tag value of that line is returned. In other embodiments, the state of each line may be stored in other ways and one or more response mechanisms may be set. In some embodiments, the commit process is atomic. 
     Although the foregoing embodiments have been described in some detail for purposes of clarity of understanding, the invention is not limited to the details provided. There are many alternative ways of implementing the invention. The disclosed embodiments are illustrative and not restrictive.