Operating Systems

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OperatingSystems
CIS 432April 3, 2008

• David Goldschmidt, Ph.D.
• Computer Science
• The College of Saint Rose

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Semaphores

• A semaphore is a synchronization mechanism
  provided by many operating systems
• Semaphore S is a special integer variable
• Two atomic operations on S
• wait(S) or P(S)
• signal(S) or V(S)

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Semaphores

• The wait(S) operation decrementssemaphore S only
  if possible
• wait(S) while ( S lt 0 ) / no-op
  / S--

P1
wait(S)
S 1
wait(S)
wait(S)
P2
P3
Drawback here is busy waiting,which wastes CPU
cycles
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Semaphores

• The signal(S) operationincrements semaphore S
• signal(S) S

P1
wait(S)
S 1
wait(S)
wait(S) // CRITICAL // SECTION signal(S)
P2
P3
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Binary Semaphores

• A binary semaphore providesmutual exclusive
  accessto a shared resource
• Initialize semaphore S to 1
• Use wait(S) and signal(S)

P1
wait(S)
S 1
wait(S)
wait(S) // CRITICAL // SECTION signal(S)
P2
P3
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Counting Semaphores

• A counting semaphore controls access to afinite
  number of instances of a given resource
• e.g. access to open files, network
  connections, shared buffers, etc.
• Use wait(S) and signal(S) operations aswith
  binary semaphores

// n instances of finite resource semaphore S n
Write pseudocode for theproducer-consumer
problem usingsemaphores to synchronize accessto
the shared buffer of size N
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Deadlock

• A deadlock occurs when every processis waiting
  on another process
• Deadlock might occur when using multiple
  semaphores

semaphore S, Q
// P0 ... wait(S) wait(Q) ... signal(S) signal(Q)
...
// P1 ... wait(Q) wait(S) ... signal(Q) signal(S)
...
Deadlock!
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Starvation

• Starvation occurs when a processwaits
  indefinitely as other processesuse shared
  resources
• Also known as indefinite blocking

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Dining Philosophers Problem

• Five philosophers sit at a table
• Each philosophereither thinks or eats
• To eat, a philosopher must pickup the closest
  two chopsticks
• But a philosopher may onlypick up one chopstick
  at a time
• Represents the problem of allocating several
  resourcesto several processes without deadlock
  or starvation

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Dining Philosophers Problem

• Strategy

N 5 philosopher( int i ) while (true)
think() pickUp(i) pickUp((i1)N)
eat() drop(i) drop((i1)N)
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Dining Philosophers Problem
semaphore chopstick 1, 1, 1, 1, 1

N 5 philosopher( int i ) while (true)
think() wait(chopsticki)
wait(chopstick(i1)N) eat() // CRITICAL
SECTION signal(chopsticki)
signal(chopstick(i1)N)
Is deadlock possible?
Write pseudocode to solvethe dining
philosophersproblem using semaphores
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Reading Assignments

• Read for next week
• Chapters 7 and 8 of the Dinosaur Book