Operating Systems COMP 4850CISG 5550

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Operating SystemsCOMP 4850/CISG 5550

• Interprocess Communication, Part II
• Dr. James Money

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Semaphores

• In 1965, Dijkstra introduced the idea of a
  semaphore for the producer-consumer problem.
• A semaphore is an integer variable to count the
  number of wakeups.
• The semaphore is 0 if there are no wakeups saved,
  or some positive value is there are some waiting.

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Semaphores

• There are two operations on the semaphore
• up
• down
• These generalize sleep and wakeup

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Semaphore - down

• The down function does the following
• If the value of the semaphore is greater than 0,
  it decrements the value
• If the value is 0, the process it put to sleep
  without completing down
• This is done as an atomic operation

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Semaphore up

• The up function does the following
• Increments the value of the semaphore
• If one of the processes running down are
  sleeping, it is chosen by the system to complete
  its down operation. In this case, the semaphore
  remains 0.

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Solving P-C Problem with Semaphores

• We can solve the producer-consumer problem with
  semaphores
• We need three semaphores
• full counting the number of slots full
• empty counting the slots that are empty
• mutex to ensure the producer and consumer do
  not access the buffer at the same time

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P-C Problem with Semaphores

• Initially
• full 0
• emptyN
• mutex 1
• Semaphores that are set to 1, and used by 2
  processes to ensure only one of them enter their
  critical region at a time are called binary
  semaphores.

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P-C Problem with Semaphores
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Mutexes

• When the counting ability of a semaphore is not
  needed, we use a binary version called a mutex.
• A mutex is a variable that can be in one of two
  states
• locked
• unlocked

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Mutexes

• There are two functions
• mutex_lock
• mutex_unlock

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mutex_lock

• If the mutex is unlocked, the mutex is locked and
  the program can enter its critical regions
• If the mutex is locked, the calling thread is
  blocked until mutex_unlock has been called by
  another thread
• If multiple threads are blocked mutex_lock, then
  one of them is chosen to be given the lock.

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mutex_unlock

• mutex_unlock setp the mutex to 0, and unblocks a
  process if needed

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Mutexes
Mutexes are simple enough to implement in user
space
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Message Passing

• When multiple machines are involved, none of the
  prior solutions work.
• This new IPC method is called message passing.
• There are two functions
• send(destination, message)
• receive(source,message)

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Message Passing

• Design Issues
• Messages can be lost there must be some
  acknowledgement of the received message
• What if a message is received twice? Use sequence
  numbers or message tags
• How to name processes and ensure authentication?

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P-C Problem with Message Passing
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Barriers

• Barriers are used when one process may not
  proceed until another process reaches a certain
  point
• When a process reaches a barrier call, it blocks
  until all the processes call barrier

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Barriers