[W/K] :: 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://www.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)