• Mechanism for processes to communicate and to synchronize
their actions.
• Message system – processes communicate with each other
without resorting to shared variables.
• IPC facility provides two operations:
– send(message) – message size fixed or variable
– receive(message)
• If P and Q wish to communicate, they need to:
– establish a communication link between them
– exchange messages via send/receive
– establish a communication link between them
– exchange messages via send/receive
• Implementation of communication link
– physical (e.g., shared memory, hardware bus)
– logical (e.g., logical properties)
– physical (e.g., shared memory, hardware bus)
– logical (e.g., logical properties)
DIRECT COMMUNICATION
• Processes must name each other explicitly:
– Symmetric Addressing
• Send (P, message) – send to process P
• Receive(Q, message) – receive from Q
– Asymmetric Addressing
• Send (P, message) – send to process P
• Receive(Q, message) – receive from Q
– Asymmetric Addressing
•send (P, message) – send to process P
•receive(id, message) – rx from any; system sets id = sender
•receive(id, message) – rx from any; system sets id = sender
•Primitives:
–send(A, message) – send a message to mailbox A
–receive(A, message) – receive a message from mailbox A
•Properties of Communication Link
–Links established automatically between pairs
–processes must know each others ID
–Exactly one link per pair of communicating processes
–processes must know each others ID
–Exactly one link per pair of communicating processes
•Disadvantage: a process must know the name or ID of the process(es) it wishes to communicate with
INDIRECT COMMUNICATION
• Messages are directed and received from mailboxes(also referred to as ports).
- Each mailbox has a unique id.
- Processes can communicate only if they share a mailbox.
• Properties of communication link
- Link established only if processes share a common mailbox
- A link may be associated with many processes.
- Each pair of processes may share several communicationlinks.
- Link may be unidirectional or bi-directional.
• Operations
- create a new mailbox
- send and receive messages through mailbox
- destroy a mailbox
• Primitives are defined as:
– send a message to mailbox Areceive(A, message)
– receive a message from mailbox A
• Mailbox sharingF P1, P2, and P3 share mailbox A.
- P1, sends; P2 and P3 receive.
- Who gets the message?
• Solutions
- Allow a link to be associated with at most two processes.
- Allow only one process at a time to execute a receiveoperation.
-Allow the system to select arbitrarily the receiver. Sender is notified who the receiver was.
SYNCHRONIZATION
• Message passing may be either blocking or non-blocking.
–Blocking Send: sender blocked until message received by mailbox or process
–Nonblocking Send: sender resumes operation immediately after sending
–Blocking Receive: receiver blocks until a message is available
–Nonblocking Receive: receiver returns immediately with either a valid or null message.
–Blocking Send: sender blocked until message received by mailbox or process
–Nonblocking Send: sender resumes operation immediately after sending
–Blocking Receive: receiver blocks until a message is available
–Nonblocking Receive: receiver returns immediately with either a valid or null message.
• Blocking is considered synchronous
• Non-blocking is considered asynchronous
• Send and receive primitives may be either blocking or non-blocking.
BUFFERING
• All messaging system require framework to temporarily buffer messages. These queues are implemented in one of three ways:
1.)Zero Capacity – 0 messages
Sender must wait for receiver (rendezvous).
2.) Bounded Capacity – finite length of n messages
Sender must wait if link full.
3.) Unbounded Capacity – infinite lengthSender never waits.
PRODUCER-CONSUMER EXAMPLE
• One process generates data – the producer
• The other process uses it – the consumer
• If directly connected – time coordination
- How would they coordinate the time ??
• One process generates data – the producer
• The other process uses it – the consumer
• If not directly connected – have a buffer
- Buffer must be accessible to both
- Finite Capacity N – Number in use - K
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