SEMAPHORE

1
SEMAPHORE
By Wilson Lee
2

• Concurrency
• Task
• Synchronization
• Example of semaphore
• Language Support

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Concurrency

• First is instruction level
• executing two or more machine instructions
  simultaneously
• usually handled by optimizing compiler
• Second is statement level
• executing two or more statement instructions
  simultaneously
• Third is unit level
• executing two or more machine subprogram units
  simultaneously
• Fourth is programming level
• executing two or more machine program
  simultaneously
• usually handled by the operating system

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Concurrency Execution

• It can be physical
• on separate processor (single or multiple
  processors)
• It can be logical
• in some time-sliced method on a single processor
  computer system

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Task

• Task is a unit of a program that can be in
  concurrent execution with other units of the same
  program. Each task in a program can provide one
  thread of control.
• A task can communicate with other tasks through
  shared non-local variables, through message
  passing, or through parameters.
• Because tasks often work together to solve
  problems, they must use some form of
  communication to either synchronize their
  executions or shard data.

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Synchronization

• Two kind of synchronization
• cooperation synchronization
• when task A must wait for task B to complete some
  specific activity before task A can continue its
  execution.
• competition synchronization
• when two tasks require the use of some resource
  that cannot be simultaneously used

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Synchronization alternatives

• 1. Semaphores
• 2. Monitors
• 3. Message Passing

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Facts about semaphore

• It is a mechanism that can be used to provide
  synchronization of tasks.
• It is a low level synchronization mechanism.
• It was devised in 1965 by Edsger Dijkstra to
  provide competition synchronization and also
  cooperation synchronization.
• It is a data structure that contains an integer
  and a queue that store tasks descriptors.
• Semaphore has only two operations. They are
  pass/wait and release. Originally named P and V
  by Dijkstra, after two Dutch words passeren (to
  pass) and vrygeren (to release).

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Two semaphore operations

• wait (aSemaphore)
• if aSemaphores counter gt 0 then
• decrement aSemaphores counter
• else
• put the caller in aSemaphores queue
• attempt to transfer control to some ready task
• (if the task ready queue is empty, deadlock
  occurs)
• End
• release(aSemaphore)
• if aSemaphores queue is empty (no task is
  waiting) then
• increment aSemaphores counter
• else
• put the calling task in the task ready queue
• transfer control to a task from aSemaphores
  queue
• end

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Example of semaphore

• Producer and consumer problem
• The producer should stop producing when the
  warehouse is full. The consumer should stop
  consuming when the warehouse is empty
• Retrieving databases
• For example, we would initialize a semaphore to
  the number of database connections available. As
  each thread acquires the semaphore, the number of
  available connections is decremented by one. Upon
  consumption of the resource, the semaphore is
  released, incrementing the counter .

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Language Support

• PL/I and ALGOL 68 are the only languages that
  support semaphore
• ALGOL 68 has a data type name sema
• Java has no built in semaphore mechanism.
  Although, it is possible to construct one. (hand
  out)

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Famous quote from Per Brinch Hansen

• The semaphore is an elegant synchronization tool
  for an ideal programmer who never makes
  mistakes. Unfortunately, programmers of that
  kind are rare.1
• 1 Concepts of Programming Language, Robert W.
  Sebesta, Addison Wesley, 2002 pg. 528

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• http//www.informatik.uni-stuttgart.de/ipvr/bv/cpp
  vm/online-doc/node53.html
• http//www.javaworld.com/javaworld/javaqa/1999-11/
  02-qa-semaphore.html