message passing

One of the two techniques for communicating between parallel

processes (the other being shared memory).

A common use of message passing is for communication in a

parallel computer. A process running on one processor may

send a message to a process running on the same processor or

another. The actual transmission of the message is usually

handled by the run-time support of the language in which the

processes are written, or by the operating system.

Message passing scales better than shared memory, which is

generally used in computers with relatively few processors.

This is because the total communications bandwidth usually

increases with the number of processors.

A message passing system provides primitives for sending and

receiving messages. These primitives may by either

synchronous or asynchronous or both. A synchronous send

will not complete (will not allow the sender to proceed) until

the receiving process has received the message. This allows

the sender to know whether the message was received

successfully or not (like when you speak to someone on the

telephone). An asynchronous send simply queues the message

for transmission without waiting for it to be received (like

posting a letter). A synchronous receive primitive will wait

until there is a message to read whereas an asynchronous

receive will return immediately, either with a message or to

say that no message has arrived.

Messages may be sent to a named process or to a named

mailbox which may be readable by one or many processes.

Transmission involves determining the location of the

recipient and then choosing a route to reach that location.

The message may be transmitted in one go or may be split into

packets which are transmitted independently (e.g. using

wormhole routing) and reassembled at the receiver. The

message passing system must ensure that sufficient memory is

available to buffer the message at its destination and at

intermediate nodes.

Messages may be typed or untyped at the programming language

level. They may have a priority, allowing the receiver to

read the highest priority messages first.

Some message passing computers are the MIT J-Machine, the

Illinois Concert Project and

transputer-based systems.

Object-oriented programming uses message passing between

objects as a metaphor for procedure call.

(1994-11-11)