Abstract:
Methods, systems, and articles of manufacture consistent with the present invention coordinate distribution of shared memory to threads of control executing in a program by using a cooperative synchronization protocol. The protocol serializes access to memory by competing threads requesting assignment of memory space, while allowing competing threads that have already been assigned memory space, to share access to the memory. A designated area of memory assigns the memory to requesting threads. The protocol is an application level entity and therefore does access the operating system to serialize the memory allocation process.

Description:
This is a continuation of application Ser. No. 09/244,135, filed Feb. 4, 1999, now U.S. Pat. No. 6,341,338 which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   This invention relates generally to multi-threaded applications, and more particularly to methods for coordinating the distribution of shared memory to threads of control executing in a data processing system. 
   BACKGROUND OF THE INVENTION 
   Multi-threading is the partitioning of a computer program that performs a particular process into logically independent “threads” of control that can execute in parallel. Each thread includes a sequence of instructions and data used by the instructions to carry out a particular program task, such as a computation or input/output function. 
   Although a single processor can execute instructions of only one thread at a time, the processor can execute multiple threads in parallel by, for example, executing instructions corresponding to one thread until reaching a selected instruction, suspending execution of that thread, and executing instructions corresponding to another thread, until all threads have completed. In this scheme, as long as the processor has begun executing instructions for more than one thread during a given time interval all executing threads are said to be “running” during that time interval, and multi-processing is achieved. 
   A process includes multiple threads operating out of a common address space. Processes may be configured such that portions of their address space are shared, that is, are mapped to a common physical location, or a memory. Multiple threads within a process share portions of that memory address space. It is possible for more than one thread to believe it has sole access to a portion of memory and for it to perform a series of reads and writes to an area of memory while another thread is performing reads and writes to the same area of memory. Such concurrent operation by competing threads may confuse operation of a process. Therefore, in a shared memory environment the shared memory must be allocated to each thread based on a coordination technique ensuring that only one thread has access to an area of shared memory at a time. 
   Current memory allocation techniques assign address space to threads in one of two ways: distribution or mutual exclusion. Distribution techniques partition shared memory into equal-sized blocks and assign the memory blocks to threads as requested. Each memory block must be large enough to meet the memory requirements of the thread requiring the most memory. Total memory requirements are dictated by the maximum demand for access to shared memory of a given thread multiplied by the total number of threads involved. This method is particularly wasteful when individual threads do not require similar amounts of shared memory. Even slight differences in the threads&#39; memory requirements can have a significant impact on the total memory requirement. 
   Mutual exclusion techniques, which may be implemented in hardware or software, coordinate use of shared memory among threads by reading and writing shared variables and pointers referring to the shared addresses. Implementation of these techniques requires accessing the operating system, which is both slow and expensive. 
   In a mutual exclusion implementation the operating system only permits one thread at a time to access shared memory. Shared memory is assigned to a thread by exchanging signals between threads, such that a thread can be forced to stop at a specified point during execution until it has received a specific signal. More specifically, the operating system may allow a thread to access shared memory by locking the shared memory, allocating the memory for exclusive use of the thread, and unlocking the memory once the thread has completed the memory operation. 
   While the memory area is locked other threads are prevented from accessing it until the lock has been removed. This method is an acceptable way of allocating memory for one-time or infrequent operations, for example, the one-time startup required by a process. However, computing generally requires execution of operations that occur hundreds or even thousands of times per second, for example, when many threads are modifying a common memory area by performing a “write” operation. For such operations this method of locking and unlocking shared memory creates bottlenecks in system performance. As the number of threads making demands on system performance increases, the likelihood of a system bottleneck increases. This of course negatively impacts system performance. 
   SUMMARY OF THE INVENTION 
   Methods, systems, and articles of manufacture consistent with the present invention allocate blocks of a shared memory to threads during execution in a data processing system. Methods, systems, and articles of manufacture consistent with the present invention, as embodied and broadly described herein, involve operations that include receiving a memory allocation request, determining whether access to the shared memory is permitted for purposes of the allocation request, identifying a next available memory block in response to the memory allocation request, and designating the next available memory block in response to the request. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an implementation of the invention and, together with the description, serve to explain the advantages and principles of the invention. In the drawings, 
       FIG. 1  depicts a block diagram of a data processing system suitable for practicing methods and systems consistent with the present the invention; 
       FIG. 2  depicts a logical view of multiple threads accessing a shared memory; 
       FIG. 3  depicts a block diagram used to explain the operation of a shared memory system operating in accordance with a shared memory protocol consistent with the present invention; 
       FIG. 4  depicts a physical representation of threads being allocated blocks of a shared memory in a manner consistent with the present invention; and 
       FIG. 5  depicts a flow chart illustrating operations performed by a shared memory allocation and coordination process consistent with an implementation of the present invention. 
   

   DETAILED DESCRIPTION 
   Reference will now be made in detail to an implementation consistent with the present invention as illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings and the following description to refer to the same or like parts. 
   Overview 
   In accordance with methods and systems consistent with the present invention, a programmer writing a multi-threaded application program specifies in the program a total amount of memory space to be shared by the program threads during execution. The programmer also specifies a “block size,” that is, the size of each partition of the total memory space to be allocated on an as-needed basis to the threads during execution. Blocks of the shared memory space are allocated to the threads serially, meaning that when a thread requires memory (for example, the computational task being performed by the thread exceeds the thread&#39;s current memory allocation), the thread determines whether its request can be processed and, if so, the thread is allocated the next available block in the shared memory space. Each thread may have a unique identifier and all thread identifiers may be provided at the beginning of program execution. Memory blocks may be assigned to the threads using the identifiers. Information may also be maintained in a designated area of the shared memory to identify each thread by its identifier as well as the specific block(s) of shared memory assigned to each thread. 
   A next available block indicator located in a designated area of the shared memory maintains information identifying the location (i.e., beginning address) in the shared memory for the next available block. Since all blocks are the same size, the indicator is simply updated by a fixed value based on the block size at the conclusion of each memory allocation cycle. 
   The programmer also specifies a shared memory protocol for the threads. This protocol, which may be included as part of the program itself or separate from the program, consists of a set of rules governing the way in which threads cooperate to access the shared memory for purposes of performing the memory allocation function. According to this protocol, when a thread requires memory, it first determines whether another thread is currently accessing the shared memory space for memory allocation purposes. This may be done by providing a “token” in a designated area of the shared memory space. The needy thread determines whether the token is available, meaning that no other thread is accessing the shared memory space to be allocated a memory block. If the token is available, the needy thread “takes” the token (i.e., indicates that the token is not currently available) and proceeds to be allocated the next available block. 
   If, however, the token is “taken,” meaning that another thread is accessing the shared memory space to be allocated a memory block, the needy thread does not proceed. Rather, the needy thread monitors the token, waiting for it to become available, at which time the thread can be allocated a next available memory block. 
   Once a thread completes the allocation process, it releases the token, thus making the token available for other memory allocation cycles. At the completion of an allocation cycle the next available block indicator is updated so that the next time a thread seeks to be allocated a memory block the indicator reflects the next available block. 
   Although access to the shared memory space is exclusive to a single thread for purposes of the allocation process, all of the threads operating in parallel can access their designated block(s), even when the allocation process for a particular thread is ongoing. The only restriction in this scheme relates to memory block allocation, only one thread at a time can be allocated a block. Additionally, all of the threads potentially have access to all of the shared memory space. In other words, no thread has exclusive control over a particular block or set of blocks specified at the beginning of program execution. 
   Computer Architecture 
     FIG. 1  depicts an exemplary data processing system  100  suitable for practicing methods and implementing systems consistent with the present invention. Data processing system  100  includes a computer system  105  connected to a network  190 , such as a Local Area Network, Wide Area Network, or the Internet. 
   Computer system  105  contains a main memory  125 , a secondary storage device  130 , a processor  140 , an input device  150 , and a video display  160 . All of these components operate in a manner commonly understood by those skilled in the art. For example, processor  140  executes an operating system and application programs. Input device  150  receives user input and video display  160  displays output, for example, from an application program being executed by processor  140 . 
   Main memory  125  and secondary storage device  130  may be used to store all of portions of the operating system and applications programs when they are not being executed by processor  140 . For purposes of this description, however, main memory  125  is shown in  FIG. 1  as including both an operating system  120  and application program  110 . Operating system  120  represents any operating system, although an operating system that specifically facilitates multi-threaded program execution may be preferred, for example, the Solaris® operating system from Sun Microsystems, Inc. 
   Application program  110  is a multi-threaded program that performs a particular process using independent, logical threads of control that execute in parallel on processor  140 . Although a single processor is shown, one skilled in the art will appreciate multiple processors may be used to implement methods and practice systems consistent with the present invention. 
   During execution of application program  110 , its threads share access to main memory  125  and secondary storage  130 . Specific implementations of memory in a data processing system may use a virtual memory to store some of the data associated with a process, allowing main memory to store data associated with more processes. In that type of system threads also share access to the virtual memory area. 
   Application program  110  includes a shared memory protocol  115 . Shared memory protocol  115  provides a set of guidelines for threads to follow when accessing memory. Shared memory protocol  115  interacts with the threads of an application running in program  110  and with memory  125 . One skilled in the art will appreciate that other implementations consistent with the present invention may implement shared memory protocol  115  separate from program  110  in either software or hardware. 
   Although aspects of this implementation are depicted as being stored in memory  125 , one skilled in the art will appreciate that all or part of system and methods consistent with the present invention may be stored on or read form other computer-readable media, such as secondary storage devices, like hard disks, floppy disks, and CD-ROM; a digital signal received from a network such as the Internet; or other forms of ROM or RAM. 
     FIG. 2  depicts a logical representation of multiple threads sharing access to memory. When multiple threads share access to memory, each of the threads is able to access any portion of the memory.  FIG. 2  shows multiple threads  210   a – 210   n  sharing access to memory  200 . Because each of the threads  210   a – 210   n  may simultaneously access all of memory, it is possible for more than one thread use a portion of memory being used by another thread, confusing an operation. Therefore, threads operating in a shared memory environment compete for use of memory. Methods and systems consistent with the present invention are directed to coordinating the distribution of access to shared memory by competing threads. 
   Shared-Memory Allocation and Distribution System 
     FIG. 3  is a logical representation of a shared memory configuration consistent with the present invention. Threads  304 ,  306 ,  308 , and  310  may correspond to one or more processes that are being executed concurrently by processor  320 . Again, although one processor  320  is shown in  FIG. 3 , methods and systems consistent with the present invention may operate in multi-processor environments as well. 
   Memory  340  includes a file allocation table (“FAT”)  345 . To facilitate parallel execution of multiple threads, memory  340  is logically partitioned into segments of equal size, as designated by a user of a shared memory system operating in accordance with the principles of the shared memory protocol  330 . The partition size is used to assign a block of memory, corresponding to address space, to a thread. 
   When an application program begins execution and spawns its threads, each thread agrees to access memory  340  according to the rules of shared memory protocol  330  Shared memory protocol  330  describes the format of the FAT  345  to each thread. According to protocol  330 , each thread agrees to view an indicator indicating whether another thread is accessing memory, to be allocated a block, before securing memory for itself The protocol  330  allows threads that have already secured access to a block of memory to use that block for the duration of the application&#39;s execution. Each time a thread needs to secure access to a new block of memory, it must follow the rules of the shared memory protocol  330 . 
   For example, if thread  304  has secured access to memory block  350 , it may access that block of memory for the duration of a program&#39;s execution. However, while thread  306  secures access to memory block  355 , thread  304  may not attempt to secure access to another block of memory  340 . While thread  306  secures access to a block of memory  340 , thread  304  may only access the block(s) of memory it has already secured. Once the indicator in the FAT  345  indicates that thread  306  has completed its secure operation, thread  304  may attempt to secure an additional block of memory. 
   The FAT  345  is used to assign a block of memory to a thread. Included in the FAT  345  are tables corresponding to current assignments of blocks of memory to threads using shared memory  340 , an indicator reflecting whether a thread is currently attempting to secure access to a block of memory, and a pointer indicating a next available memory block. A thread secures access to a block of memory by receiving an assignment of a block of memory from the FAT. When a thread secures a block of memory, it increments a pointer in the FAT to the next available memory block, shown in  FIG. 4 . 
     FIG. 4  is a physical representation of how a shared memory  400  may be allocated to threads and maintained by methods and systems consistent with the present invention. Before a program executes, a user specifies both the size of a shared memory  400  and the size of individual memory blocks  415 ,  420 ,  425 , and  430 . Each memory block corresponds to a set of addresses. Once the size of the shared memory  400  and the size of its blocks  415 ,  420 ,  425 , and  430  have been assigned, they remain constant throughout a program&#39;s execution. 
   Each thread that accesses memory  400  in accordance with the principles of the present invention is uniquely identifiable. Upon a first thread in a process beginning to execute, the process is assigned a process identification code. When a thread receives assignment of a memory block it is assigned a thread identification code and a memory location code. Each thread of a program may be uniquely identified by a global thread identification code, a combination of a process identification code for the process corresponding to the thread, and a thread identification code. This identification feature enables methods and systems consistent with the present invention to handle any parallel processing application, including multi-threaded applications as well as multi-processor applications running processes containing multiple threads. 
   Further details on operation of a shared memory system operating in accordance with the principles of the shared memory protocol will now be explained with reference to the flow diagram of  FIG. 5 . Shared memory protocol begins with user initialization. During initialization, the user&#39;s program specifications are set for the size of both the shared memory and of the individual memory blocks to be assigned to threads accessing memory (step  505 ). The size of the shared memory area and the size of the memory blocks do not affect the operation of a shared memory allocation or distribution process consistent with the present invention. Therefore, the user may specify the sizes by using any number of criteria, for example, the memory requirements of a majority of the threads running. 
   Before attempting to secure access to a block of memory a thread determines whether another thread is securing access to a block of memory. If a thread determines that a “token” to secure access to memory is available (step  530 ), the thread secures a block of memory for itself by accessing the FAT (step  535 ). 
   Upon securing access to a block of memory, the FAT is used to assign the thread a memory location code representing the memory location of the memory block and a thread identification code, used to identify the thread within a process (step  535 ). Because threads are automatically assigned a next available memory block, threads of different processes may be assigned consecutive memory blocks. Each thread receives a memory block corresponding to the size of the memory blocks set by the user during initialization  505 . If a thread requires more memory than that contained in an individual memory block, the thread will be assigned enough memory blocks to meet its request. 
   Once all of the location and identification assignments have been made, a memory assignment counter, maintained in the FAT region of the shared memory, is incremented (step  540 ). Incrementing the counter initiates setting a pointer to a next available memory location. Once a thread has secured access to a block of memory, the thread releases the “token” so that another thread may secure access to a block of memory (step  545 ). 
   A thread may place as many memory requests as needed. A shared memory system operating in accordance with the principles of the shared memory protocol ensures that all executing threads needing memory have an opportunity to secure access to memory by repeating the memory assignment process until all threads have completed execution (step  550 ). 
   CONCLUSION 
   Methods and systems consistent with the present invention coordinate distribution of shared memory among threads of control executing in a parallel computing environment. Distributing shared memory resources in a manner consistent with the principles of the present invention does not require accessing the operating system, a costly and time consuming event, to distribute shared memory among threads. Nor does a system consistent with the principles of the present invention require prohibiting competing threads from accessing a shared memory area while another thread accesses the memory. 
   Systems consistent with the present invention are applicable to all multi-threaded programs written in all computer programming languages, including Fortran 77, Java, C, and C++. 
   Although the foregoing description has been described with reference to a specific implementation, those skilled in the art will know of various changes in form and detail which may be made without departing from the spirit and scope of the present invention as defined in the appended claims and the full scope of their equivalents.