# 6.1. Introduction

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Remote Memory Access ( RMA) extends the communication mechanisms of MPI by allowing one process to specify all communication parameters, both for the sending side and for the receiving side. This mode of communication facilitates the coding of some applications with dynamically changing data access patterns where the data distribution is fixed or slowly changing. In such a case, each process can compute what data it needs to access or update at other processes. However, processes may not know which data in their own memory need to be accessed or updated by remote processes, and may not even know the identity of these processes. Thus, the transfer parameters are all available only on one side. Regular send/receive communication requires matching operations by sender and receiver. In order to issue the matching operations, an application needs to distribute the transfer parameters. This may require all processes to participate in a time consuming global computation, or to periodically poll for potential communication requests to receive and act upon. The use of RMA communication mechanisms avoids the need for global computations or explicit polling. A generic example of this nature is the execution of an assignment of the form `A = B(map)`, where `map` is a permutation vector, and `A, B` and `map` are distributed in the same manner.

Message-passing communication achieves two effects: communication of data from sender to receiver; and synchronization of sender with receiver. The RMA design separates these two functions. Three communication calls are provided: MPI_PUT (remote write), MPI_GET (remote read) and MPI_ACCUMULATE (remote update). A larger number of synchronization calls are provided that support different synchronization styles. The design is similar to that of weakly coherent memory systems: correct ordering of memory accesses has to be imposed by the user, using synchronization calls; the implementation can delay communication operations until the synchronization calls occur, for efficiency.

The design of the RMA functions allows implementors to take advantage, in many cases, of fast communication mechanisms provided by various platforms, such as coherent or noncoherent shared memory, DMA engines, hardware-supported put/get operations, communication coprocessors, etc. The most frequently used RMA communication mechanisms can be layered on top of message passing. However, support for asynchronous communication agents (handlers, threads, etc.) is needed, for certain RMA functions, in a distributed memory environment.

We shall denote by origin the process that performs the call, and by target the process in which the memory is accessed. Thus, in a put operation, source=origin and destination=target; in a get operation, source=target and destination=origin.

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