Computer Systems and Systems Software Lecture 7

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Computer Systems and Systems Software Lecture 7

• We will cover in this lecture
• semaphores as a solution to the critical section
  problem
• semaphores as inter process synchronisation and
  co-ordination primitives
• Introduction to the concept of deadlock
• the use of resource allocation graphs

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• Reading associated with lecture
• A. Silbershatz, P. B. Galvin and G. Gagne,
  Operating Systems Concepts, 6th edition, chapter
  7 p.201-209 chapter 8 p. 243-248

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Semaphore

• More general solution to critical section
  problems
• Semaphore S integer variable that can only be
  accessed via two indivisible (atomic) operations
• wait (S) while S? 0 do no-op
  S S 1
• signal (S) S S 1
• This original semaphore definition requires busy
  waiting - wasting CPU cycles that could be used
  productively by other processes - often called a
  spinlock (process spins while waiting for lock)

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ExampleCritical Section of n Processes

• Shared variables
• var mutex semaphore
• initially mutex 1
• Process Pi
  
  repeat
• wait(mutex)
• critical section
• signal(mutex)
• remainder section
• until false

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Semaphore Implementation

• Rather than busy waiting we can define a
  semaphore to block a process that cannot proceed
  placing it on a queue of processes associated
  with the semaphore. signal(S) then removes a
  process from waiting queue
• Define a semaphore as a record
  type semaphore record
  value integer
  L list of
  process end
• Assume two simple operations
• block suspends the process that invokes it.
• wakeup(P) resumes the execution of a blocked
  process P.

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• Semaphore operations now defined as
• wait(S) S.value S.value 1
  if S.value lt 0
  
  then begin
  add this process to S.L block
  
  end
  signal(S) S.value S.value 1
  if S.value ? 0
  then begin
  remove
  a process P from S.L wakeup(P)
  end

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Semaphore for general synchronization

• Execute B in process Pj only after A executed in
  process Pi
• Use semaphore flag initialized to 0
• Code
• Pi Pj
• ? ?
• A wait(flag)
• signal(flag) B

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Deadlock and Starvation

• Deadlock two or more processes are waiting
  indefinitely for an event that can only be caused
  by one of the waiting processes - this could be
  a signal operation. e.g. let S and Q be two
  semaphores initialized to 1
• P0 P1
• wait(S) wait(Q)
• wait(Q) wait(S)
• ? ?
• signal(S) signal(Q)
• signal(Q) signal(S)
• Starvation indefinite blocking of a process on
  a queue

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Two Types of Semaphores

• Counting semaphore integer value can range over
  an unrestricted domain. Useful for controlling
  access to a type of resource with a some given
  number of instances of that resource e.g. with n
  resources of a given type, with semaphore value
  set to n, process will wait when n instances of
  resource are already allocated.
• Binary semaphore integer value can range only
  between 0 and 1. Useful for implementing mutual
  exclusion.

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Classical Problems of Synchronization - Bounded
Buffer Problem

• Shared data - as before
• buffer for shared data - an array organised as a
  circular buffer
• n is the size of the buffer
• BUT IN ADDITION now have semaphores
• full, empty, mutex initialised as below
• full 0 // meaning number of elements in
  buffer
• empty n // meaning no. of empty spaces in
  buf
• mutex 1

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Bounded-Buffer Problem (Cont.)

• Producer process
• repeat
• produce an item
• wait(empty)
• wait(mutex)
• put item in buffer
• signal(mutex)
• signal(full)
• until false

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Bounded-Buffer Problem (Cont.)

• Consumer process
• repeat
• wait(full)
• wait(mutex)
• remove an item from buffer
• signal(mutex)
• signal(empty)
• consume the item
• until false

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The Deadlock Problem

• A set of blocked processes each holding a
  resource and waiting to acquire a resource held
  by another process in the set.
• Example
• System has 2 tape drives
• P1 and P2 each hold one tape drive and each needs
  another one.
• Example - semaphores A and B, initialized to 1
• P0 P1
• wait (A) wait(B)
• wait (B) wait(A)

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Bridge Crossing Example

• Traffic only in one direction.
• Each section of a bridge can be viewed as a
  resource.
• If a deadlock occurs, it can be resolved if one
  car backs up (preempt resources and rollback).
• Several cars may have to be backed up if a
  deadlock occurs.
• Starvation is possible.

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Deadlock Characterization

• Deadlock can arise if four conditions hold
  simultaneously
• Mutual exclusion only one process at a time can
  use a resource.
• Hold and wait a process holding at least one
  resource is waiting to acquire additional
  resources held by other processes.
• No preemption a resource can be released only
  voluntarily by the process holding it, after that
  process has completed its task.
• Circular wait there exists a set P0, P1, ,
  Pn of waiting processes such that P0 is waiting
  for a resource that is held by P1, , Pn1 is
  waiting for a resource that is held by Pn, and Pn
  is waiting for a resource that is held by P0.

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Resource-Allocation Graph

• A set of vertices V and a set of edges E.
• V is partitioned into two types
• P P1, P2, , Pn, the set consisting of all
  the processes in the system.
• R R1, R2, , Rm, the set consisting of all
  resource types in the system.
• request edge directed edge Pi ? Rj
• assignment edge directed edge Rj ? Pi

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Resource-Allocation Graph (Cont.)

• Process
• Resource Type with 4 instances
• Pi requests instance of Rj
• Pi is holding an instance of Rj

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Example of a Resource Allocation Graph
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Resource Allocation Graph With A Deadlock
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Resource Allocation Graph With A Cycle But No
Deadlock
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Basic Facts

• If graph contains no cycles ? no deadlock.
• If graph contains a cycle ?
• if only one instance per resource type, then
  deadlock.
• if several instances per resource type,
  possibility of deadlock.