Section 4 describes fundamental protocols and algorithms used to provide consistent shared data in a distributed system. Consistency models in distributed shared memory systems radhika gogia1, preeti chhabra2, rupa kumari3 dronacharya college of engineering, gurgaon, india gogia. A framework of memory consistency models springerlink. The second part discusses the issues that arise due to weakening memory consistency. Introduction in the early days of distributed computing, everyone implicitly assumed that programs on machine with no physically shared memory obviously ran in. The consistency models are responsible for managing the state of shared data for the applications of a distributed shared memory dsm systems.
Based on the understanding that the behavior of an execution is. Consistency models for distributed systems systutorials. Abstract a distributed shared memory dsm combines the advantage of. This article describes several models in an easy to understand way. 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. The most intuitive modelsequential consistencygreatly restricts. The paper provides an overview of recent advances in hardware optimizations. It is possible to run parts of a program in parallel, generally by using threads to specify. Singhal distributed computing distributed shared memory cup 2008 48 distributed computing. A tutorial introduction to the arm and power relaxed. The article describes how the memory consistency model affects many aspects of system design, including optimizations allowed in the compiler.
The purpose of this tutorial paper is to describe issues related to memory consistency models in a way that would be understandable to most computer professionals. The relationship of cache coherence protocols to memory consistency models. The order in which memory operations appear in program sequential consistency sc. A memory consistency model is a set of rules that governs how memory systems will process memory operations load store from multiple processors. A programming model for shared memory parallelism article pdf available december 2009 with 50 reads how we measure reads. Here, we aim to cover the memory models for the fragments of the instruction sets required for typical lowlevel concurrent algorithms in main memory, as they might appear in user or os kernel code. We focus on consistency models proposed for hardwarebased sharedmemory systems. In proceedings of the acm sigplan 2008 conference on. In this lecture, we will cover some of them in more detail. The memory consistency model of a sharedmemorymultiprocessor provides a formal speci.
A tutorial introduction to the arm and power relaxed memory models luc maranget inria susmit sarkar university of cambridge peter sewell university of cambridge october 10, 2012 revision. General interconnect to multiple memory modules means write arrival in memory is indeterminate. The cause of, and solution to, all your multicore performance problems. In this lesson, we discover the memory consistency model while focusing on issues relevant for programmers, especially when programming concurrent applications. Keywords memory consistency, performance, programming language, parallelism i. Shared memory consistency models specify a contract between programmer and system, wherein the system guarantees that if the programmer follows the rules, memory will be consistent and the results of memory operations will be predictable. Kourosh gharachorloo digital western research laboratory 250 university avenue palo alto.
Parallel systems that support the shared memory abstraction are becoming widely accepted in many areas of computing. Different threads trying to access the same memory location participate in a data race if at least one of the operations is a modification also known as store operation. The memory consistency model of a shared memory multiprocessor for mally specifies how the memory system will appear to the programmer. Operations of each proc appear in this sequence in order specified by. A transaction is a sequence of operations executed by a process that transforms.
Memory consistency models these models apply consistency constraints to all memory accesses accesses may require multiple messages and take signi. As such, the memory model influences many aspects of system design, including the design of programming languages, compilers, and the under. Nov 28, 2017 a memory consistency model is a set of rules that governs how memory systems will process memory operations load store from multiple processors. The memory consistency model for a system typically involves a tradeoff between performance and programmability. Shared memory concurrency in the real world working with. Shared memory consistency models and the sequential consistency model duration. Effectively, the consistency model places restrictions. S e p t e m b e r 1 9 9 5 wrl research report 957 shared memory consistency models. In computer science, consistency models are used in distributed systems like distributed shared. A tutorial introduction to the arm and power relaxed memory. Many of these models are originally specified with an emphasis on the system optimizations they allow.
Principles, algorithms, and systems sequential consistency using local writes. Write read write write read read, writesimple model for reasoning about parallel programsbut, intuitively reasonable reordering of memory operations in auniprocessor may violate sequential consistency modelmodern microprocessors reorder operations all the time to obtain. It was first defined as the property that requires that. Sequential consistency is one of the consistency models used in the domain of concurrent computing e. Writing correct and efficient programs for such systems requires a formal specification of memory semantics, called a memory consistency model. Operations of each proc appear in this sequence in order specified by its program. To avoid them one needs to prevent these threads from concurrently executing such conflicting operations. Answer is determined by the memory consistency model of the system determines the order in which sharedmemory accesses from different threads can appear to execute in other words. The memory consistency model for a sharedmemory multiprocessor specifies the behavior of memory with respect to read and write operations from multiple processors. Distributed shared memory and memory consistency models paul krzyzanowski introduction with conventional smp systems, multiple processors execute instructions in a single address space.
The memory consistency model of a sharedmemory sys tem determines the order in which. Essentially, a memory consistency model restricts the values that a read can return. A transaction is a sequence of operations executed by a process that transforms data from one consistent state to another. Compilers basically, what we want to do is develop models where one operation. The types of consistency models are datacentric and client centric consistency. Pai, student member, ieee, and parthasarathy ranganathan, student member, ieee invited paper the memory consistency model of a shared memory system determines the order in which memory operations will appear to execute to the. Local consistency is the weakest consistency model in shared memory systems. Recent advances in memory consistency models for hardware shared memory systems sarita v. Distributed shared memory is a service that manages memory across multiple nodes so that applications will have the illusion that they are running on a single shared memory machine. Designing memory consistency models for sharedmemory multiprocessors sarita v. Designing memory consistency models for sharedmemory. A consistency model basically refers to the degree of consistency that should be maintained for the shared memory data. 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. Lamport 16 relaxed memory consistency models rxm an rxm less restrictive than sc.
The already proposed consistency models are inflexible and cannot adapt to the workload and environments characteristics. Memory consistency model formal specification of mem system behavior to programmer program order the order in which memory operations appear in program sequential consistency sc. Specify the order by which shared memory access events of. Recent advances in memory consistency models for hardware. Atomic operations it is now possible to read and write to given memory location by the use of atomic load and atomic store operations.
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 of a sharedmemory multiprocessor provides a formal speci. Shared memory systems zuckerman overview of shared memory systems programming execution models memory consistency models a motivating example uniform memory consistency models strongest mcms weaker uniform mcms nonuniform memory consistency models hardwareoriented mcms software and programmeroriented mcms conclusion on. We focus on consistency models proposed for hardwarebased shared memory systems. Jul 21, 2014 in this lesson, we discover the memory consistency model while focusing on issues relevant for programmers, especially when programming concurrent applications.
N2 the memory consistency model of a system affects performance, programmability, and portability. Aug 11, 2017 shared and distributed memory architectures introduction to parallel programming in openmp. Many of these models are originally specified with an emphasis. The memory consistency model only affects the design of the hardware. 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. Pai, student member, ieee, and parthasarathy ranganathan, student member, ieee invited paper the memory consistency model of a shared memory system determines the order in which memory operations will appear to. Each node in the system owns some portion of the physical memory, and provides the operations reads and writes on that memory. However, in many commercial shared memory systems, the proces sors may observe an older value, causing unexpected behavior. Memory consistency models for sharedmemory multiprocessors. Dsm architecture each node of the system consist of one or more cpus and memory unit nodes are connected by high speed communication network simple message passing system for nodes to exchange information main memory of individual nodes is used to cache pieces of shared memory space 6.
There are, of course, only two hard things in computer science. Shared and distributed memory architectures youtube. Shared memory systems zuckerman overview of shared memory systems programming execution models memory consistency models a motivating example uniform memory consistency models strongest mcms weaker uniform mcms nonuniform memory consistency models hardwareoriented mcms software and programmeroriented mcms conclusion on mcms bibliography. This paper presents a framework of memory consistency models which describes the memory consistency model on the behavior level. Reconfigurable object consistency model for distributed. Memory consistency models david mosberger tr 9311 abstract this paper discusses memory consistency models and their in. Memory consistency models and advanced openmp comp 422lecture 8 31 january 2008. Consistency models for distributed systems tagged consistency model, distributed systems, programming, tutorial. Adve kourosh gharachorloo d i g i t a l western research laboratory 250 university avenue palo alto, california 94301 usa. But there is another hard problem lurking amongst the tall weeds of computer science. Jun 28, 2018 consistency models for distributed systems tagged consistency model, distributed systems, programming, tutorial. Memory consistency errors cant be understood purely in the context of javathe details of shared memory behavior on multicpu systems are highly architecturespecific, and to make it worse, x86 where most people coding today learned to code has pretty programmerfriendly semantics compared to architectures that were designed for multiprocessor. If a system supports the stronger consistency model, then the weaker consistency model is automatically supported but the converse is not true.
Shared memory consistency models and the sequential. We include memory reads and writes, registertoregister operations, branches, and the various kinds of dependency between instructions. Memory consistency and cache coherence carnegie mellon comp. The memory consistency model for a shared memory multiprocessor specifies the behavior of memory with respect to read and write operations from multiple processors. You can think of it as a contract between the processor architecture hardware and the software r. The memory consistency model of a system affects performance, programmability, and portability. Distributed shared memory dsm distributed shared memory is a service that manages memory across multiple nodes so that applications will have the illusion that they are running on a single sharedmemory machine.
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