Lecture 09 Message Passing

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Lecture 09Message Passing
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Requirements to be satisfied

• Synchronization
• Process need to be synchronized to enforce mutual
  exclusion
• Communication
• Cooperating processes may need to exchange
  information
• Solutions
• Shared memory communication scheme
• Message passing communication scheme

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Operations Needed

• send(destination, message)
• A process send information in the form of a
  message to another process designated by a
  destination
• receive(source, message)
• A process receives information by executing the
  receive primitive, indicating the source of the
  sending process and the message

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Message Size

• Fixed size
• Physical implementation straightforward
• Difficult programming
• Variable size
• More complex physical implementation
• The programming task becomes simpler

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Message Buffering

• When a process sends a message to another
  process, is it copied into the receivers memory
  or in a buffer in between?
• No buffer
• The processes must be synchronized for a message
  transfer to take place (rendezvous)
• Bounded buffer
• The buffer has finite length N thus, at most N
  messages can reside in it
• Unbounded buffer
• Any number of messages can wait in the buffer.
  The sender is never delayed.

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Synchronization
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Direct Addressing

• The send primitive includes a specific identifier
  of the destination process
• send(P,message)
• The receive primitive
• The process explicitly designate a sending
  process
• receive(Q,message)
• Receive a message from any process
• receive(id-variable,message)

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Indirect Addressing

• The relationship between senders and receivers
  can be
• one to one
• many to one (useful for client/server
  interaction, the mailbox is referred as a port)
• one to many (useful for applications where a
  message is to be broadcast to a set of processes)
• many to many

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Bounded-buffer producer/consumer problem I

• const
• capacity / buffering capacity /
• null / empty message /
• var i integer
• ...
• create_mailbox(mayproduce)
• create_mailbox(mayconsume)
• for i 1 to capacity do send(mayproduce,null)
  
• parbegin producer consumer parend

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Bounded-buffer producer/consumer problem II

• procedure producer
• var pmsg message
• while true do
• receive(mayproduce,pmsg)
• pmsg produce
• send(mayconsume,pmsg)

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Bounded-buffer producer/consumer problem III

• procedure consumer
• var cmsg message
• while true do
• receive(mayproduce,cmsg)
• consume(cmsg)
• send(mayconsume,null)