Asynchronous Message Passing EE 524/CS 561 Wanliang Ma 03/08/2000

Mechanisms for Concurrent Programming Dynamic process creation Semaphores Asynchronous message passing Remote procedure call (RPC) Rendezvous

Call and Send Call : –Synchronous –two-way communication plus synchronization Send : –asynchronous –fork a new instance of a proc –one-way communication

Process Interaction Mechanisms

Asynchronous Message Passing Higher level and more powerful than semaphores Allow processes to communicate Allow processes to synchronize

Asynchronous Message Passing Implemented by having processes send messages to and receive messages from operations The sender continues after sending –asynchronous ( non-blocking) The receiver delays until receiving and removing a message –synchronous (blocking) –an abbreviation for more general mechanism

Outline of Chapter 9 Operations as message queues Semaphores revisited Data-containing semaphores Shared Operations

Operations as Message Queues A new use of operations as message queues The operation has no corresponding proc Receive statements service and remove the invocation of an operation from the message queue The executing process delays if no invocation is present

Operations as Message Queues How to invoke an operation : send and call Send invocation: –the invocation is appended to the message queue –the invoker continues after sending Call invocation : –synchronous form of message passing

Receive statement format receive op_id subscripts ( variable, variable, …)

Examples Stream merging Client-Serve models –one client process and one server process –multiple clients and one server process –multiple clients and multiple servers

Semaphores Revisited Semaphores:abbreviations for operations –a parameterless operation –with send restriction P statements:abbreviations for send statements V statements:abbreviations for receive statements

Semaphores Revisited

The two classes of mechanisms are equally efficient Both are more efficient than a general message passing semaphores make programs more concise and readable

Data-Containing Semaphores A generalization of standard semaphores contain data and a synchronization signal an unbounded buffer of messages that have been produced and not yet consumed Declared within a resource or a global Data is passed as a parameter of a operation

Data-Containing Semaphores Send statements to append data to the operation’s message queue receive statements to remove data from the operation’s message queue synchronization between processes accessing the queue is implicit through their use of send and receive statements

Data-Containing Semaphores Advantages –saves the programmer from having to write code that explicitly implements a list of free buffers and code that synchronizes access to the list. Disadvantages –less efficient

Shared Operations Declared at the top level within a resource or global can not be declared within processes If the operation is declared in a resource, only the processes located in the same resource can share the operation If the operation is declared in a global g, the processes located in g or in any resource or global that imports g can share the operation

Shared Operations applications –multiple instances of a process can service the same resource operation –server work queues

Shared Operations Example: –compute the integral of f(x) from 1 to r. –It employs a shared operation, bag, which contains a bag of tasks –Each task represents a sub-interval over which the integral of f is to be approximated