CT213 Inter Process Communication

1. CT213 – Inter Process Communication/Synchronization Petronel Bigioi

2. Content Messages Messages design issues Synchronization Addressing Message transmission Format Queuing discipline Readers/Writers problem Unix synchronization and communication Windows 2000 synchronization and communication

3. Inter Process Communication Inter process data exchange, execution state report, results collection is part of inter process communication; it can be done using some shared memory zones, therefore synchronization is required Semaphores are primitive (yet powerful) tools to enforce mutual exclusion and for process coordination; still, it may be difficult to produce a correct program using semaphores The difficulty is caused by the fact that wait and signal operations may be scattered throughout a program and is not easy to see an overall effect

4. Messages When a process interacts with another, two main fundamental requirements must be satisfied: communication and synchronization Message passing provides both of those functions Message-passing systems come in many forms; this section will provide a general view of typical features found in such systems The message-passing function is normally provided in the form of primitives: Send (destination, message) Receive (source, message)

5. Messages design issues Synchronization Blocking vs. Non-blocking Addressing Direct Indirect Message transmission Through value Through reference Format Content Length Fixed Variable Queuing discipline FIFO Priority

6. Synchronization Send gets executed in a process Sending process is blocked until the receiver gets the message Sending process continues its execution in a non blocking fashion Receive gets executed in a process If a message has been previously sent, the message is received and execution continues If there is no waiting message then Process can be blocked until a message arrives The process continues to execute, abandoning the attempt to receive So both the sender and receiver can be blocking or not

7. Synchronization … There are three typical combinations of systems Blocking send, blocking receive Both the receiver and sender are blocked until the message is delivered (provides tight sync between processes) Non-blocking send, blocking receive The sender can continue the execution after sending a message, the receiver is blocked until message arrives (it is probably the most useful combination) Non-blocking send, non-blocking receive Neither party waits Typically only one or two combinations are implemented

8. Addressing Direct addressing Send primitive include the identifier of the receiving process Receive can be handled in two ways Receiving process explicitly designates the sending process (effective for cooperating processes) Receiving process is not specifying the sending process (known as implicit addressing); in this case, the source parameter of receive primitive has a value returned when the receive operation has been completed Indirect addressing The messages are not sent directly from sender to receiver, but rather they are sent to a shared data structure consisting of queues that temporarily can hold messages; those are referred to as mailboxes. Two communicating process: One process sends a message to a mail box Receiving process picks the message from the mailbox

9. Indirect process communication Indirect addressing decuples the sender form the receiver allowing for greater flexibility. Relationship between sender and receiver: One to one Private communications link to be set up between two processes Many to one Useful for client server interaction; one process provides services to other processes; in this case, the mailbox is known as port One to many Message or information is broadcasted across a number of processes Many to many