Similar to a uniprocessor, hll programming induces the two levels of memory consistency models depicted in figure1. In addition to digital equipments support, the author was partly supported by darpa contract n00039. Designing memory consistency models for sharedmemory. Performance evaluation of memory consistency models for sharedmemory multiprocessors. A memory model for a shared memory multiprocessor commonly and often implicitly assumed by programmers is that of sequential consistency, which guarantees that all memory accesses will appear to. Essentially, a memory consistency model restricts the values that a read can return. Shared memory multiprocessors and cache coherence kai shen 222011 csc 258458 spring 2011 1 shared memory multiprocessors limitation of instruct ionlevel parallelism dddependences complexity to support highdeg ree instructionlevel parallelism multiple processors sharing memory processor processor 222011 csc 258458 spring 2011 2 memory. Memory consistency models for shared memory multiprocessors. Abstract performance evaluation of memory consistency. Shared memory multiprocessors mem cis 371 martinroth. In computer science, consistency models are used in distributed systems like distributed shared memory systems or distributed data stores such as a filesystems, databases, optimistic replication systems or web caching. In addition, memory accesses are cached, buffered, and pipelined to bridge the. In proceedings of the 1991 international conference on.
Apr 26, 2020 shared memory consistency models electronics and communication engineering ece notes edurev is made by best teachers of electronics and communication engineering ece. Memory consistency models a memory consistency model defines the permitted reorderings of memory operations during execution a contract between hardware and software. Adve computer sciences department university of wisconsinmadison. Previous descriptions of memory consistency models in sharedmemory multiprocessor systems are mainly expressed as constraints on the memory access event ordering and hence are hardwarecentric. Designing and implementing memory consistency models for. The memory consistency model or memory model of a shared memorymultiprocessor system in. Pdf performance evaluation of memory consistency models. The memory consistency model supported by a multiprocessor architecture determines the amount of buffering and pipelining that may be used to hide or reduce the latency of memory accesses. Memory consistency and event ordering in scalable sharedmemory multiprocessors kourosh gharachorloo, daniel lenoski, james laudon, phillip gibbons, anoop gupta, and john hennessy. Designing memory consistency models for sharedmemory multiprocessors sarita v. A system exhibits processor consistency if the order in which other processors see the writes from any individual processor is the same as the order they were issued.
The most commonly assumed memory model is sequential consistency sc. Memory models are an issue not only at the hardware level, but also at the programmer level. The memory consistency model or memory model of a sharedmemory multiprocessor system influences the performance and the programmability of the system. The simplest and most intuitive model for programmers, sequential consistency, restricts the use of many performanceenhancingoptimizations exploited by uniprocessors. Memory consistency models for sharedmemory multiprocessors. A framework of memory consistency models springerlink. Abstract performance evaluation of memory consistency models for sharedmemory multiprocessors. Adve, and tracy harton, proceedings of the 7th international. The most commonly assumed memory consistency model for sharedmemory multiprocessors is sequential consistency sc, which is a straightforward extension of the sequential programming model, and is therefore simple to reason with. Model of a shared memory multiprocessor angel vassilev nikolov, national university of lesotho, 180, roma summary we develop an analytical model of multiprocessor with private caches and shared memory and obtain the steadystate probabilities of the system. The model of memory presented to the user can have a signi cant impact on the amount of e ort required to produce a correct and e cient parallel program. Virtual sharedmemory multiprocessors a thesis submitted to the school of computer science and engineering at the university of new south wales in ful.
This paper introduces a new model of memory consistency. In addition, memory accesses are cached, buffered, and pipelined to bridge the gap between slow shared memory and fast processors. The memory consistency model or memory model of a sharedmemorymultiprocessor system in. Kourosh gharachorloo digital western research laboratory. The memory consistency model, or memory model, supported by a shared memory multiprocessor directly affects its performance. The model affectsprogrammabilitybecause programmers must use it to reason a bout. We focus on consistency models proposed for hardwarebased sharedmemory systems. Proceedings of the fourth international conference on architectural support for programming languages and operating systems performance evaluation of memory consistency models for shared memory multiprocessors. Unless carefully controlled, such architectural optimizations can cause memory accesses to be executed in an order different from what the programmer expects. The hardwarecentric nature of these models, however. Scalable shared memory multiprocessors distribute memory among the processors and use scalable interconnection networks to provide high bandwidth and low latency communication. Recent advances in memory consistency models for hardware.
Pdf performance evaluation of memory consistency models for. Multiprocessors should support simple memory consistency. Behavior in equilibrium can be studied and analyzed. Phd thesis, computer system laboratory, stanford university, december 1995. A memory model for a sharedmemory multiprocessor commonly and often implicitly assumed by programmers is that of sequential consistency, which guarantees that all memory accesses will appear to. The system is said to support a given model if operations on memory follow specific rules. Memory consistency model the memory consistency model of a shared memory multiprocessor provides a formal specification of how the memory system will appear to the programmer, eliminating the gap between the behavior expected by the programmer and the actual behavior supported by a system. Shared memory multiprocessors obtained by connecting full processors together processors have their own connection to memory processors are capable of independent execution and control thus, by this definition, gpu is not a multiprocessor as the gpu cores are not. This paper presents a framework of memory consistency models which describes the memory consistency model on the behavior level.
Parallel processing important for future sharedmemory is desirable model challenge to build sharedmemory systems that give high performance are easy to program memory consistency model. The memory consistency model, or memory model, supported by a sharedmemory multiprocessor directly affects its performance. The interface for memory in a shared memory multiprocessor is called amemory consistency model. Consistency models are all about ordering constraints on. Designing memory consistency models for shared memory multiprocessors sarita v. However, in many commercial shared memory systems, the proces sors may observe an older value, causing unexpected behavior. This document is highly rated by electronics and communication engineering ece students and has been viewed 295 times. The data consistency model specifies a contract between programmer and system. The most commonly assumed memory consistency model for shared memory multiprocessors is sequential consistency sc, which is a straightforward extension of the sequential programming model, and is therefore simple to reason with. The memory consistency model of a sharedmemory multiprocessor provides a formal speci. Owing to this architecture, these systems are also called symmetric sharedmemory multiprocessors smp hennessy. Memory consistency and event ordering in scalable shared. Many of these models are originally specified with an emphasis. Scalable sharedmemory multiprocessors distribute memory among the processors and use scalable interconnection networks to provide high bandwidth and low latency communication.
The set of allowable memory access orderings forms the memory consistency model or event ordering model for an architecture. The most intuitive model for programmers, sequential consistency, restricts many performanceenhancing optimizations. The memory consistency model of a shared memory multiprocessor for mally specifies how the memory system will appear to the programmer. Memory consistency model memory model formal specification of how shared memory will appear to programmers consistency restricts values that can be returned by a read during execution why memory consistency models.
We aim to describe memory consistency models in a way that most computer professionals would understand. An evaluation of memory consistency models for shared. Consistency models are not about dependent memory operations in a single processors instruction stream these are respected even by processors that reorder instructions. Memory consistency models for shared memory multiprocessors and dsm syst ems electrotechnical conference, 1996. Oct, 2015 apr 26, 2020 shared memory consistency models electronics and communication engineering ece notes edurev is made by best teachers of electronics and communication engineering ece. The memory consistency model or memory model of a shared memory multiprocessor system influences the performance and the programmability of the system. Memory consistency models cpu io system software app app app cpu cpu cpu cpu cpu readings. For higher performance, several alternative models have been proposed.
Designing and implementing memory consistency models for sharedmemory multiprocessors. Recently, use of some form of relaxed memory consistency model has been proposed as a means to improve performance of cachebased shared memory multiprocessors 27, 1. Previous descriptions of memory consistency models in shared memory multiprocessor systems are mainly expressed as constraints on the memory access event ordering and hence are hardwarecentric. The memory consistency model or memory model of a sharedmemory multiprocessor system in. Citeseerx document details isaac councill, lee giles, pradeep teregowda. An evaluation of memory consistency models for sharedmemory systems with ilp processors, vijay s. Sequential consistencylamport a multiprocessor is sequentially consistent if the result of any execution is the same as if the operations of all the processors. Several memory consistency models have been proposed in. The basic premise is that most shared memory applications. Abstract performance evaluation of memory consistency models. More generally, the consistency model specifies what event orderings are legal when several processes are accessing a common set of locations. These range from sequential consistency on one end, allowing very limited buffering, to release consistency on the other end, allowing extensive buffering and pipelining. However, sc places severe restrictions on the use of highperformance hardware and compiler optimizations.
Two techniques to enhance the performance of memory consistency models. In addition, memory accesses are cached, buffered, and pipelined to bridge the gap between the slow shared memory and the fast processors. Performance evaluation of memory consistency models for. The memory consistency model or memory model of a shared memory multiprocessor system in. Architectural mechanisms for explicit communication in. Several different consistency models have been proposed. Based on the understanding that the behavior of an execution is. A memory consistency model determines the order in which memory references can be executed by the system and greatly affects the implementation and performance of a system 614. Transactional memory model is the combination of cache coherency and memory consistency models as a communication model for shared memory systems supported by software or hardware. Shared memory consistency models university of arizona.
Eliminate gap between expected behavior behavior supported by a system. Multiprocessors should support simple memory consistency models. Processor consistency is one of the consistency models used in the domain of concurrent computing e. As the interface between the programmer and the system, the effect of the memory consistency model is pervasive in a shared memory system. It is part of the interface between a sharedmemory program and any hardware or software that may transform that program it both specifies the possible behaviors of the memory accesses for the programmer and constrains the legal transformations and executions. A transaction is a sequence of operations executed by a process that transforms. In the context of sharedmemory systems, the memory model specifies the values that a sharedmemory read in a program may return. Consistency models consistency models are not about memory operations from different processors. Based on the understanding that the behavior of an. The memory consistency model of a system affects performance, programmability, and portability. Performance evaluation of memory consistency models for shared memory multiprocessors.
Pdf the use of systems with multiple processors that support shared memory programming paradigm is rapidly increasing nowadays. I the author argues that multiprocessors should support sequential consistency. This is important if the performanceenhancing features being incorporated by system designers are to be correctly and widely used by programmers. Memory consistency models for sharedmemory multiprocessors kourosh gharachorloo december 1995 also published as stanford university technical report csltr95685.