Message-passing
Introduction
Processes can communicate exchanging messages.
A general communication mechanism, not requiring explicit shared memory, that includes both communication and synchronization.
Valid for uniprocessor and multiprocessor systems.
Two operations are required:
send and receive.
A communication link is required.
That can be categorized in different ways:
Direct or indirect communication.
Synchronous or asynchronous communication.
Type of buffering.
Direct and indirect communication
Symmetric direct communication.
A process that wants to communicate must explicitly name the receiver or sender.
send(P, msg)– send message msg to process P
receive(P, msg) – receive message msg from process P
A communication link in this scheme has the following properties:
it is established automatically between a pair of communicating processes.
it is associated with exactly two processes.
between a pair of communicating processes there exist exactly one link.
Asymetric direct communication.
Only the sender must explicitly name the receiver.
send(P, msg)– send message msg to process P
receive(id, msg) – receive message msg from process P
Indirect communication
The messages are sent to and received from mailboxes, or ports.
send(M, msg)– send message msg to mailbox M
receive(M, msg) – receive message msg from mailbox M
A communication link in this scheme has the following properties:
it is only established if the pair of communicating processes has a shared mailbox.
it may be associated with more than two processes.
between a pair of processes there may exist more than one link (a mailbox per each).
The problem of two or more processes trying to receive a message from the same mailbox.
Is it allowed?
If allowed, which one will succeed?
Synchronization
From a synchronization point of view, there are different design options for implementing send and receive.
Blocking send – the sending process blocks until the message is received by the receiving process or by the mailbox.
Nonblocking send – the sending process sends the message and resumes operation.
Blocking receive – the receiver blocks until a message is available.
Nonblocking receive – the receiver retrieves either a valid message or the indication that no one exits.
Different combinations of send and receive are possible.
Buffering
There are different design options for implementing the link supporting the communication.
Zero capacity – there is no queue.
the sender must block until the recipient receives the message.
Bounded capacity – the queue has finite length.
if the queue is full, the sender must block until space is available.
Unbounded capacity – the queue has (potentially) infinite length.
Bounded-buffer problem
There is no need to deal with mutual exclusion and synchronization explicitly.
the send and receive primitives take care of it.
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