Cooperation Process, Interprocess Communication - Operating Systems | CMSC 412, Study notes of Operating Systems

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Announcements

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Program #2– Info on the Web

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Reading– Chapter 7

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Cooperating Processes

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Often need to share information between processes– information: a shared file– computational speedup:

• break the problem into several tasks that can be run on different processors - requires several processors to actually get speedup - modularity: separate processes for different functions - compiler driver, compiler, assembler, linker - convenience: - editing, printing, and compiling all at once

Producer-consumer pair

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producer creates data and sends it to the consumer

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consumer read the data and uses it

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examples: compiler and assembler can be used as aproducer consumer pair

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Buffering– processes may not produce and consume items one by one– need a place to store produced items for the consumer

• called a buffer
  • could be fixed size (bounded buffer) or unlimited (un-

bounded buffer)

Message Passing

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What happens when a message is sent?– sender blocks waiting for receiver to receive– sender blocks until the message is on the wire– sender blocks until the OS has a copy of the message– sender blocks until the receiver responds to the message

• sort of like a procedure call• could be expanded into a remote procedure call (RPC) system

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Error cases– a process terminates:

• receiver could wait forever• sender could wait or continue (depending on semantics)
  • a message is lost in transit
    • who detects this? could be OS or the applications

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Special case: if 2 messages are buffered, drop the older one– useful for real-time info systems

Signals

Signal Handler

Table

SigIOHandler{ } SigAlarmHandler{ }

SetSigAction(sig, handler)

Shared Memory

Process 1

Process 2 Shared Region

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Like Threads, but only part of memory shared

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Allows communication without needing kernel action– Kernel calls setup shared region

Problems with the Producer-Consumer

Shared Memory Solution

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Consider the three address code for the counterCounter Increment

Counter Decrement

reg

= counter 1

reg

= counter 2

reg

= reg 1

reg

= reg 2

counter = reg

1

counter = reg

2

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Now consider an ordering of these instructionsT

0

producer

reg

= counter 1

{ reg

T^1

producer

reg

= reg 1

{ reg

T^2

consumer

reg

= counter 2

{ reg

T^3

consumer

reg

= reg 2

{ reg

T^4

producer

counter = reg

1

{ counter = 6 }

T^5

consumer

counter = reg

2

{ counter = 4 }

Thisshouldbe 5!