1.2 Message Passing

In message passing, a substantial distance exists between the programming model and the actual hardware primitives, with user communication performed through operating systems or library calls that perform the low-level actions including the actual communication operation. The most common user-level communication operations on message passing are variants of the send and receive. In its simplest form send specifies a local data buffer that is to be transmitted and a receiving process(typically on a remote processor).Receive specifies a sending process and a local data buffer into which the transmitted data is to be placed.together a matching send and receive causes a data transfer from one processor to another.In most message passing systems, the send process also allows an identifier or tag to be attached to the message, and the receiving operation specifies a matching rule( such as a specific tag from a specific processor)

The combination of a send and a matching receive accomplishes a memory to memory copy, where each end specifies its local data address, and a pair wise synchronization event. There are several possible variants of this synchronization event, depending upon whether the send completes when the receive has been executed, when the send buffer is available for reuse, or when the request has been accepted. Similarly, the receive can potentially wait until a matching send occurs or simply post the receive. Each of these variants have somewhat different semantics and different implementation requirements. Message passing has long been used as a means of communication and synchronization among arbitrary collections of cooperating sequential processes, even on a single processor. Important examples include programming languages, such as CSP and Occam, and common operating systems functions, such as sockets. Parallel programs using message passing are typically quite structured, like their shared-memory counter parts. Most often, all nodes execute identical copies of a program, with the same code and private variables. Usually, processes can name each other using a simple linear ordering of the processes comprising a program.