Concurrency

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Concurrency

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• Modern OS central themes

• Multiprogramming
• "the management of multiple processes within a
  single computer system"

• Distributed processing
• "the management of multiple processes executing
  on multiple, distributed computer systems"

Fundamental to both themes is concurrency
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• The concept of concurrency

• Processes are concurrent if they exist at the
  same time
• Processes that are either interleaved or
  overlapped in time to yield the appearance of
  simultaneous execution

P1
P2
P3
time
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• A simple example

• A program to provide character echo input
  character from keyboard and output character on
  display

repeat parbegin out in output
(out, display) input (in, keyboard) parend
This program will work or not depending on the
sequence the concurrent operations take place
(copy is wrong)
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• How to implement concurrency

• Programming language structures and mechanisms
• most programs can be decomposed in at least three
  processes input, process, output
• Operating systems structures and mechanisms
• process interaction
• mutual exclusion

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• Process Interaction

Degree of Awareness
Relationship
Influence one process has on another
Control problems
unaware
competition
results idependent timing may be affected
mutual exclusion deadlock (renewable) starvation
indirectly aware
cooperation- sharing
results may depend timing may be affected
mutual exclusion deadlock (renewable) starvation
coherence
directly aware
cooperation- communication
results may depend timing may be affected
deadlock (consumable) starvation
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• Competition for resources

• Mutual exclusion
• critical resources (nonsharable)
• a process manages each critical resource
• Deadlock
• a process is holding a critical resource (c1)
  and waiting for another critical resource (c2)
• another process is holding (c2) and waiting for
  (c1)
• Starvation
• one process is never granted access to a
  critical resource for other processes always get
  higher priority (and access) to the critical
  resource

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• Mutual exclusion requirements

• only one process at a time is allowed access to a
  critical resource
• a process that halts in a noncritical section
  should not affect others
• a process should not be allowed to deadlock or
  starve
• no delays should occur when a process requires a
  critical resource no others are using
• a process remains in its critical section for a
  finite time only

Technical Dekker and Peterson algorithms,
and Dijkstra semaphores
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• Monitors

• " a concurrency construct that contains both the
  data and procedures to perform allocation of a
  particular serially reusable shared resource or
  group of resources"
• only one process is allowed to enter the monitor
• if a process calls the monitor and a resource is
  already allocated the monitor calls wait (outside
  of monitor)
• when a process releases a resource the monitor
  calls signal (and grants the resource to a
  process waiting for it)
• it gives priority to waiting versus new

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• Deadlock

• "the permanent blocking of a set of processes
  that either compete for systems resources or
  communicate with each other"
• resources
• reusable (not depleted with use)
• consumable (produced and consumed)
• conditions for deadlock
• mutual exclusion
• hold-and-wait
• no preemption
• circular wait (deadly embrace)

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• Deadlock management

Principle
Resource allocation policy
schemas
advantages
disadvantages
Prevention
conservative
request all resources at once preemption reso
urce ordering
processes with single burst of activity resources
whose state can be saved and restored
easily problem solved by design
inefficient, delays process initiation preempts
more than necessary, needs to be
restarted preempts without need
Detection
very liberal
invoked periodically to test deadlock
does not delay process initiation
preemption losses
Avoidance
midway between conservative and liberal
search to find at least one safe path
no preemption needed
resources must be known, long blockings