A distributed shared database, or a distributed file system. This paper proposes a novel viewbased consistency model for distributed shared memory. A dsm system can be generally viewed as a set of nodes or clusters, each with its own memory connected by an interconnection network. 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. Shared variables in the critical section are made consistent when the release operation is per formed. Shared memory architecture may involve separating memory into shared parts distributed amongst nodes and main memory. Midway supports a novel memory consistency model called entry consistency ec. Two conventional memory models are utilized in many distributed shared memory systems. Implementation and consistency issues in distributed. A view is a set of ordinary, data objects that a processor has the right to access in a dataracefree program. The strict consistency model is the strongest form of memory coherence, having the most stringent consistency requirements.
The memory consistency model for a sharedmemory multiprocessor specifies the behavior of memory with respect to read and write operations from multiple processors. In our system, each node maintains a copy of each shared memory region at all times. Consistency issues in distributed shared memory systems. To ensure correct multiprocessor execution, memory models should be employed with care. Shared memory consistency models and the sequential. A consistency model is contract between a distributed data store and processes, in which the processes agree to obey certain rules in contrast the store promises to work correctly. A consistency model basically refers to the degree of consistency that. Several memory consistency models have been proposed in. We discuss recent hardware and compiler optimizations. Lazy release consistency for software distributed shared. There are many different models of consistency, some of which are included in the list below. With conventional smp systems, multiple processors execute. We present a softwareonly realization of distributed shared memory, implemented as a userlevel library. Relaxed consistency models we want an intuitive programming model such as sequential consistency and we want high performance we care about data races and reordering constraints for some parts of the program and not for others hence, we will relax some of the constraints for sequential.
In computer science, consistency models are used in distributed systems like distributed shared memory systems or distributed data stores the system is said to. Reconfigurable object consistency model for distributed. The consistency models are responsible for managing the state of shared data for the applications of a distributed shared memory dsm systems. The structure of the shared memory space of dsm system is very much dependent on the applications that the dsm system is intended to support 1. The viewbased consistency model requires that the data objects of a view are updated only before a. Pdf selectionbased weak sequential consistency models.
Memory consistency models for sharedmemory multiprocessors. A novel model that describes consistency in shared memory was developed, presented and discussed. Conference paper pdf available in lecture notes in computer science january 2005 with 40 reads how we measure reads. A data store may be physically distributed across multiple machines. Effectively, the consistency model places restrictions. What memory consistency model is the most attractive depends on the multiprocessing system architecture, targeted performance and costs. It says that if the software agrees to obey certain rules, the memory promises. Cox, and winy zwaenepoel department of computer science rice university abstract relaxed memory consistency models, such as release consisteracg, were introduced in order to reduce the impact of remote memory access latency in both software and hardware distributed shared memory dsm. The dspm model provides direct and shared memory accesses to a distributed set of pms and is an easy and efficient way for datacenter applications to. However, to avoid this may create problems in the way memory appears to behave. Sequential consistency memory model ensures that the view of the memory is consistent at all times from all the processors. Distributed shared memory rutgers university cs 417. The memory consistency model or memory model of a shared memorymultiprocessor system in. System design interview distributed cache duration.
The already proposed consistency models are inflexible and cannot adapt to the workload and environments. But there is another hard problem lurking amongst the tall weeds of computer science. Distributed operating systems, prentice hall, 1995, chapter 6 m. Distributed shared memory consistency objectbased model abdelfatah aref yahya and rana mohamad idrees bader faculty of science and information technology, alzaytoonah university of jordan p. Distributed shared memory consistency objectbased model. The memory consistency model for a shared memory multiprocessor specifies the behavior of memory with respect to read and write operations from multiple processors. A consistency model is the definition of when modifications to data may be seen at a given. Shared memory has been the standard for tightlycoupled systems multiprocessors, where the processors have uniform access to a single global memory. The memory consistency model of a sharedmemory multiprocessor provides a formal specification. With replica management, problem of consistency arises. We are the first to introduce the distributed shared persistent memory dspm model and among the first to build distributed pmbased systems. Essentially, a memory consistency model restricts the values that a read can return.
Specify the order by which shared memory access events of. Viewbased consistency for distributed shared memory. The paper proposes a novel view based consistency model for distributed shared memory. Consistency model is recognized as the model that can offer the maximum performance advantage among the weak sequential consistency models. A coherence protocol, chosen in accordance with a consistency model, maintains memory coherence. Shared memory systems consistency models distributed shared memory systems page based shared variable based reading.
Therefore mpsoc multiprocessor system on a chip or distributed mp system made from individual networked computers provide memory consistency differently. An implementation of the viewbased consistency model has been given. The consistency model is a guarantee that the state changes will behave in a certain way as long as the upper software layers follow a certain set of rules. Distributed shared memory consistency object based model. Reconfigurable object consistency model for distributed shared memory. Recent advances in memory consistency models for hardware. The data consistency model specifies a contract between programmer and system. The memory consistency model of a sharedmemory multiprocessor provides a formal speci. Memory consistency model although, shared memory systems allow multiple. Distributed shared memory rutgers cs rutgers university. Pdf a novel model that describes consistency in shared memory was developed, presented anddiscussed.
A shared memory system is said to support the strict consistency model if the value returned by a read operation on a memory. Ec guarantees that shared data become consistent at a processor when the processor acquires a synchronization object. Lots of consistency model defined by a wide variety of source including architecture system, application programmer etc. Singhal distributed computing distributed shared memory cup. Techniques for reducing consistencyrelated communication. Distributed shared memory dsm provides an abstraction of shared memory on a network of workstations. The system is said to support a given model if operations on memory. Problems with existing dsm systems are lack of portability due to compiler andor operating system modification requirements, and reduced performance due to significant synchronization and communication costs when. If replication or copy of shared data items is allowed in a dsm system, then maintaining shared data consistency also called memory coherence. Selectionbased weak sequential consistency models for. In this paper, we explore shared memory, memory consistency models and mechanisms for differentiating memory operations.
There are, of course, only two hard things in computer science. 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. Shared memory consistency models and the sequential consistency model introduction to parallel programming in openmp. Traditionally, consistency has been discussed in the context of read and write operations on shared data, available by means of distributed shared memory.
The memory consistency model for a sharedmemory multiprocessor specifies the behavior of memory. Pdf distributed shared memory consistency objectbased model. Designing memory consistency models for sharedmemory. Comparative study of various consistency models in. The memory consistency model of a shared memory multiprocessor provides a formal speci. The simplest and most intuitive model for programmers, sequential consistency, restricts the use of many performanceenhancingoptimizations exploited by uniprocessors. A consistency model is essentially a contract between the software and the memory. Distributed systems 1998, 2001 paul krzyzanowski 3 pingponging of that page between the two processors results.
Pdf objectbased consistency in a distributed shared memory. The already proposed consistency models are inflexible and cannot adapt to the workload and environments characteristics. The memory consistency model or memory model of a sharedmemory. 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. Distributed shared memory and memory consistency models. Dsm consistency model is a contract between dsm system and application programmer op1 response response invocation response response invocation invocation invocation process local memory manager op2 op3 opk a.
At times, some portions of shared memory may be inaccessible, due to coherence and consistency requirements. The motivation for dsm is that it allows a shared memory programming model to be employed, which has some advantages over messagebased models. Release consistency the synchronization access synchs in the weak consistency model can be re. The consistency model is responsible for managing the state of shared data for distributed shared memory systems.
Abstract in parallel and distributed applications there are two common programming models for interprocess communication. Journal of parallel and distributed computing, 154. 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. The memory consistency model of a shared memory multiprocessor for mally specifies how the memory system will appear to the programmer.
More generally, consistency model specifies what event orderings are legal when several processes are accessing a common set of locations. Consistency models exists in the context of the implementations of applications or services that manage distributed state. The current cilk mechanisms to support dagconsistent distributed shared memory on the connection machine cm5 are implemented in software. Memory consistency and event ordering in scalable shared. 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. However, in many commercial shared memory systems, the proces sors may observe an older value, causing unexpected behavior. The cause of, and solution to, all your multicore performance problems.
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