message passing

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
   (http://ai.mit.edu/projects/cva/cva_j_machine.html), the
   Illinois Concert Project
   (http://www-csag.cs.uiuc.edu/projects/concert.html) and
   {transputer}-based systems.

   {Object-oriented programming} uses message passing between
   {objects} as a metaphor for procedure call.

   (1994-11-11)