Shared memory model without threads in this programming model, processestasks share a common address space, which they read and write to asynchronously. Cpsc662 distributed computing dsm 1 1 distributed shared memory shared memory systems consistency models distributed shared memory systems page based sharedvariable based reading. The memory consistency model of a shared memory multiprocessor for mally specifies how the memory system will appear to the programmer. Shared and distributed memory architectures introduction to parallel programming in openmp. Adsm system logically implements the sharedmemory model on a physically distributedmemory system. 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.
Memory consistency models distributed systems uni ulm. The already proposed consistency models are inflexible and cannot adapt to the workload and environments characteristics. A relatively new conceptdistributed shared mem ry,combines the advantages of the two approaches. Distributed shared memory dsm simulates a logical shared memory address space over a set of physically distributed local memory systems. Sharedmemory system multiprocessor distributedmemory system multicomputercommunication costs more of an issue. Shared memory and distributed memory are lowlevel programming abstractions that are used with certain types of parallel programming. A dsm system can be generally viewed as a set of nodes or clusters, each with its own memory connected by an interconnection network. Performance modeling and measurement of parallelized code. 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. The data consistency model specifies a contract between programmer and system. In distributed shared memory systems, processes share data across node boundaries transparently. Shared memory consistency models and the sequential consistency model. Implementation and consistency issues in distributed. Distributed shared memory dsm is a resource management component of a distributed operating system that implements the shared memory model in distributed systems, which have no physically shared memory.
Design issues in implementation of distributed shared. Various mechanisms such as locks semaphores may be used to control access to the shared memory. Such systems employ a variety of protocols to maintain a consistent view of data across all local memories. Shared memory model without threads high performance. Software distributed shared memory virtual shared memory the virtual memory management mechanism is used for shared memory management ivy u. In the dsm model, each process has its own local memory, and each shared variable is physically located in the local memoryofone. Distributed shared memory tamu computer science people pages.
Distributed shared memory is an implementation of the shared memory concept in distributed systems no physically shared memory. Although parallel processing systems, particularly those based on message passing or distributed memory model, have been researched for decades leading to. In computer science, distributed shared memory dsm is a form of memory architecture where. A distributed shared memory is a mechanism allowing endusers processes to access shared data without using interprocess communications. Comparative study of various consistency models in. Distributed operating systems, prentice hall, 1995, chapter 6. Mechanisms for distributed shared memory index of university of. The consistency models are responsible for managing the state of shared data for the applications of a distributed shared memory dsm systems. Shared and distributed memory architectures youtube. The motivation for dsm is that it allows a shared memory programming model to be employed, which has some advantages over messagebased models. The global arrays toolkit attempts to offer the best features of the shared and distributed memory models. Conference paper pdf available in lecture notes in computer science january 2005 with 42 reads how we measure reads. Essentially, a memory consistency model restricts the values that a read can return. An implementation of a dsm can be categorized as pagebased, sharedvariable based or object based depending on the granularity of the.
Memory consistency models for sharedmemory multiprocessors. However, in many commercial shared memory systems, the proces sors may observe an older value, causing unexpected behavior. Memory coherence and access synchronization in a dsm system that allows replication of shared data item, copies of shared data item may simultaneously be available in the main memories of a number of nodes. Shared memory consistency models and the sequential. Historically, these systems 15,19,45,47 performed poorly, largely due to limited internode bandwidth, high internode latency, and the design decision of piggybacking on the virtual memory system for seamless global memory accesses. When assisted by hardware, it can also provide a lowoverhead. A distributed shared memory system implements the shared memory model on a. Distributed shared memory distributed shared memory dsm allows applications running on separate computers to share data or address ranges without the programmer having to deal with message passing insteadtheunderlyingtechnologyhwormwwillinstead the underlying technology hw or mw will send the messages to keep the dsm consistent or. Transformations of mutual exclusion algorithms from the. Distributed shared memory ajay kshemkalyani and mukesh singhal distributed computing. Distributed shared object memory microsoft research. Dsm provides direct system support of the shared memory programming model. A typical configuration is a cluster of tens of highperformance workstations and sharedmemory multiprocessors of two or three different architectures, each with a processing power. Principles, algorithms, and systems cambridge university press a.
In 1986, li proposed a different scheme, known as distributed shared memory. Then an example of a simple program is given that can be implemented in dsm environment using. Singhal distributed computing distributed shared memory cup 2008 1 48. There are two major shared memory models for distributed systems with local memory. The main point of dsm is that it spares the programmer the concerns of message passing when writing applications that might otherwise have to use it.
A dsm system logically implements the sharedmemory model on a physically distributedmemory system. This chapter describes one basic model of associative memory, called the sparse distributed memory, and relates it to other models and circuits. The second part discusses the issues that arise due to weakening memory consistency. Reconfigurable object consistency model for distributed. Distributed shared memory dsm systems provide the abstraction of a common virtual address space across a network of processors. As the interface between the programmer and the system, the effect of the memory consistency model is pervasive in a shared memory system. The basic idea behind the weak consistency model is enforcing consistency on a group of memory reference operations rather than individual operations. My research on memory consistency models has been enriched throughcollaborationswith phil gibbons and sarita adve. Both hardware and software implementations have been proposed in the literature.
In computer science, distributed shared memory dsm is a form of memory architecture where physically separated memories can be addressed as one logically shared address space. Cache coherence in shared memory access multi processor environment. The shared memory model provides a virtual address space that is shared among all computers in a distributed system. The synchronization access synchs in the weak consistency model can be refined as a pair of acquire. Distributed shared memory dsm systems simplify the task of writing. Distributed shared memory dsm is an attempt to combine the scalability and inexpensiveness of message passing with the convenience of the shared memory programming model li and hudak, 1989. This paper describes the goals, programming model and design of disom, a software based distributed shared memory system for a multicomputer composed of heterogeneous nodes connected by a highspeed network.
A distributed shared memory system that supports the weak consistency model uses a special variable called a synchronization variable which is used to synchronize memory. Shared memory consistency models and the sequential consistency model duration. In a distributed shared memory dsm system that implements an entry consistency model, shared data must be explicitly associated to a synchronization variable, and it can be accessed only in. Distributed systems 19982009 paul krzyzanowski 1 lectures on distributed systems distributed shared memory and memory consistency models paul krzyzanowski introduction with conventional smp systems, multiple processors execute instructions in a. I have also enjoyed working with andreas nowatzyk on the sparc v9 rmo model, and with jim horning, jim saxe, and yuan yu on the digital alpha memory model. In this paper, we explore shared memory, memory consistency models and mechanisms for differentiating memory operations. Consistency and replication distributed software systems. Adve y and kourosh gharachorloo z y department of electrical and computer engineering rice university houston, texas 772511892 z western research laboratory digital equipment corporation palo alto, california 943011616. Shared memory vs message passing programming model. Various mechanisms such as locks semaphores are used to control access to the shared memory, resolve contentions and to prevent race conditions and deadlocks.
Multiplewriter protocol widely used in simple numas, noras or pcclusters without. The system is said to support a given model if operations on memory follow specific rules. Sys tem designers can implement the specific mechanism for achieving the sharedmemory abstraction in hardware or software in a variety of. An advantage of this model from the programmers point of view is that the notion of data ownership. All nodes in the distributed shared memory system perceive the same illusion of a single address space figure 1. A dsm is essentially a way of seamlessly sharing the physical memories of loosely connected systems. The model affectsprogrammabilitybecause programmers must use it to reason a bout. To solve the memory coherence problem that deal with the consistency of a piece of shared data lying in the main memories of two or more. A taxonomybased comparison of several distributed shared. Reconfigurable object consistency model for distributed shared memory. Under our implementation, each node on a network can host a single thread that operates inside a larger pool of threads within a shared memory program.
Shared memory and distributed shared memory systems. In addition to digital equipments support, the author was partly supported by darpa contract n00039. Shared memory systems cover a broad spectrum, from systems that maintain consistency entirely in hardware to those that do it entirely in software. Abstract distributed shared memory dsm 1 has become a very popular paradigm in distributed systems.
Memory consistency models for sharedmemory multiprocessors kourosh gharachorloo december 1995 also published as stanford university technical report csltr95685. Software distributed shared memory dsm systems provide shared memory abstractions for clusters. Distributed shared memory dsm systems aim to unify parallel processing systems that rely on message passing with the shared memory systems. This paper presents a new programming model for heterogeneous computing, called asymmetric distributed shared memory adsm, that maintains a shared logical memory space for cpus to access objects in the accelerator physical memory but not vice versa. Tempest combines finegrain coherence support, an active message model, and. Distributed shared memory dsm system is a new and attractive area of research which combines the advantages of both sharedmemory parallel processors multiprocessors and distributed systems multicomputers. Modelling and model checking a distributed shared memory.
Here, the term shared does not mean that there is a single centralized memory, but that the address space is shared same physical address on two processors refers to the same location in memory. It implements a sharedmemory programming model in which data locality is managed explicitly by the programmer. Li and hudak proposed several of the pioneering protocols for dsm. In other words, the goal of a dsm system is to make interprocess communications transparent to endusers. Relaxed consistency models and software distributed memory. Lots of consistency model defined by a wide variety of source including architecture system, application programmer etc. Distributed shared memory dsm combines the two concepts. Memory consistency models 1 david mosberger tr 9311 abstract this paper discusses memory consistency models and their in. Shared memory allows multiple processing elements to share the same location in memory that is to see each others reads and writes without any other special directives, while distributed memory requires explicit commands to transfer data from one.
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