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Distributed shared memory using barcode drawer for .net vs 2010 control to generate, create qr barcode image in .net vs 2010 applications. Oracle Reports Service Figure 12.7 Exam QR Code ISO/IEC18004 for .NET ples to illustrate definitions of PRAM consistency and slow memory.

The initial values of variables are zero.. P1 P2 W(x, 2) W( y, 4) R( y, 4). W(x, 7) R(x, 0) R(x, 0) R(x, 7). (a) Slow memory qrcode for .NET but not PRAM consistent P1 P2 W(x, 2) W( y, 4) R( y, 4) W(x, 7) R(x, 7) R(x, 0) R(x, 2). (b) Violation of slow memory consistency Implementations Slow memory can be implemented using a broadcast primitive that is weaker than even the FIFO broadcast. What is required is a FIFO broadcast per variable in the system, i.e.

, the FIFO property should be satisfied only for updates to the same variable. The implementation details are left as Exercise 12.7.

. 12.2.6 Hierarchy of consistency models Based on the def initions of the memory consistency models seen so far, there exists a hierarchy among the models, as depicted in Figure 12.8..

12.2.7 Other models based on synchronization instructions We have seen sev eral popular consistency models. Based on the consistency model, the behavior of the DSM differs, and the programmer s logic therefore depends on the underlying consistency model. It is also possible that newer consistency models may arise in the future.

The consistency models seen so far apply to all the instructions in the distributed program. We now briefly mention some other consistency models that are based on a different principle, namely that the consistency conditions apply only to a set of distinguished synchronization or coordination instructions. These synchronization instructions are typically from some run-time library.

A common example of such a statement is the barrier synchronization. Only. Figure 12.8 A strict hierarchy of the memory consistency models. No consistency m odel Pipelined RAM (PRAM) Sequential consistency Linearizability/ atomic consistency/ strict consistency Causal consistency Slow memory. 12.2 Memory consistency models the synchronizat ion statements across the various processors must satisfy the consistency conditions; other program statements between synchronization statements may be executed by the different processors without any conditions. Examples of consistency models based on this principle are: entry consistency, weak consistency, and release consistency. The synchronization statements are inserted in the program based on the semantics of the types of accesses.

For example, accesses may be conflicting (to the same variable) or non-conflicting (to different variables), conflicting accesses may be competing (a Read and a Write, or two Writes) or non-conflicting (two Reads), and so on. We outline the definitions of these consistency models but skip further implementation details of such models..

Weak consistency [11]. Some application s do not require even seeing all Writes, let alone seeing them in some order. Consider the case of a process executing a CS, repeatedly reading and writing some variables in a loop. Other processes are not supposed to read or write these variables until the first process has exited its CS.

However, if the memory has no way of knowing when a process is in a CS and when it is not, the DSM has to propagate all Writes to all memories in the usual way. But by using synchronization variables, processes can deduce whether the CS is occupied. A synchronization variable in this model has the following semantics: it is used to propagate all writes to other processors, and to perform local updates with regard to changes to global data that occurred elsewhere in the distributed system.

When synchronization occurs, all Writes are propagated to other processes, and all Writes done by others are brought locally. In an implementation specifically for the CS problem, updates can be propagated in the system only when the synchronization variable is accessed (indicating an entry or exit into the CS). Weak consistency (defined by [11]) has the following three properties which guarantee that memory is consistent at the synchronization points: Accesses to synchronization variables are sequentially consistent.

No access to a synchronization variable is allowed to be performed until all previous writes have completed everywhere. No data access (either Read or Write) is allowed to be performed until all previous accesses to synchronization variables have been performed. An access to the synchronization variable forces Write operations to complete, and effectively flushes the pipelines.

Before reading shared data, a process can perform synchronization to ensure it accesses the most recent data.. Release consistency [12]. The drawback of weak consistency is that when a synchronization variable is accessed, the memory does not know whether this is being done because the.
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