Stacey Levine Chapter 4.1 Message Passing Communication

Introduction Basic Message passing

Communication Models Support for processes to communicate among themselves. Traditional (centralized) OS’s: –Provide local (within single machine) communication support. –Distributed OS’s: must provide support for communication across machine boundaries. Over LAN or WAN.

Message Passing Basic communication primitives: –Send message. –Receive message. Modes of communication: –Synchronous versus asynchronous.... Send Sending Q... Receiving Q

Synchronization Stages Sendersource networkdestinationreceiver kernel Message ack Non blocking send is 1 st step. Everything else is blocking

Communication may take place using either message passing or shared memory. Communication Models

Synchronous Communication Blocking send –Blocks until message is transmitted –Blocks until message acknowledged Blocking receive –Waits for message to be received Process synchronization.

Asynchronous Communication Non-blocking send: sending process continues as soon as message is queued. Blocking or non-blocking receive: –Blocking: Timeout. Threads. –Non-blocking: proceeds while waiting for message. Message is queued upon arrival. Process needs to poll or be interrupted.

Direct Communication

Indirect Communication Indirect communications : –messages sent to and received from mailboxes (or ports) mailboxes can be viewed as objects into which messages placed by processes and from which messages can be removed by other processes –each mailbox has a unique ID –two processes can communicate only if they have a shared mailbox

The Producer Consumer Problem The producer-consumer problem illustrates the need for synchronization in systems where many processes share a resource. In the problem, two processes share a fixed-size buffer. One process produces information and puts it in the buffer, while the other process consumes information from the buffer. These processes do not take turns accessing the buffer, they both work concurrently. Herein lies the problem. What happens if the producer tries to put an item into a full buffer? What happens if the consumer tries to take an item from an empty buffer?

Producer

Consumer

Sleep and Wakeup Waiting is wasteful Processes waiting to enter their critical sections waste processor time checking to see if they can proceed –when a process is not permitted to access its critical section, it uses a system call known as Sleep, which causes that process to block –. The process will not be scheduled to run again, until another process uses the Wakeup system call

Semaphores In the Producer-Consumer problem, semaphores are used for two purposes: mutual exclusion and synchronization.

Sleep and Wakeup using Monitors

Message Passing Sempahores and Monitors use shared memory. Not effective on Distributed systems

Sleep and Wakeup using Message Passing Two primitives, SEND and RECEIVE are used in the message passing scheme. SEND primitive sends a message to a destination process RECEIVE primitive receives a message from a specified source process. Message Passing works on distributed systems because these messages can be sent from machine to machine through a network. Typically, each process has a mailbox; a buffer which receives all the messages which are sent to that process. The destination of the SEND and RECEIVE system calls is a process' mailbox, not the process itself.

Message Passing Interface MPI is a message-passing standard Process = A regular sequential language + calls to MPI functions for sending and receiving messages MPI has many implementations, MPI contains > 130 functions MPI 2.1 is the current version

How is Mutual Exclusion implemented with Message Passing This pictures shows an interaction between a process that wants to access a shared resource, and a resource control process. Three messages are sent here. Message (1) is a request for access to the resource. Process P1 then blocks, waiting for a reply from the resource control process. Message (2) is the reply sent by the control process. This message wakes up P1, and P1 now uses the resource. Message (3) informs the resource control process that P1 is done using the resource.

Examples …. SSL Local system events (mouse movements..) HTTP RPC …

References Operating System Concepts, Silberschatz,Galvin and Gange 2002 Message passing information from The University of Edinburgh MPI-2: standards beyond the message-passing model Lusk, E.; Massively Parallel Programming Models, Proceedings. Third Working Conference on Nov Page(s): Digital Object Identifier /MPPM Massively Parallel Programming Models, Proceedings. Third Working Conference on A multithreaded message-passing system for high performance distributed computing applications Park, S.-Y.; Lee, J.; Hariri, S.; Distributed Computing Systems, Proceedings. 18th International Conference on May 1998 Page(s): Digital Object Identifier /ICDCS Distributed Computing Systems, Proceedings. 18th International Conference on Lessons for massively parallel applications on message passing computers Fox, G.C.; Compcon Spring '92. Thirty-Seventh IEEE Computer Society International Conference, Digest of Papers Feb Page(s): Digital Object Identifier /CMPCON Compcon Spring '92. Thirty-Seventh IEEE Computer Society International Conference, Digest of Papers. An analysis of message passing systems for distributed memory computers Clematis, A.; Tavani, O.; Parallel and Distributed Processing, Proceedings. Euromicro Workshop on Jan Page(s): Digital Object Identifier /EMPDP Parallel and Distributed Processing, Proceedings. Euromicro Workshop on Chandras, R. G Distributed message passing operating systems. SIGOPS Oper. Syst. Rev. 24, 1 (Jan. 1990), DOI=