Lecture 04 Process Interaction

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Lecture 04Process Interaction

• Competition among Processes for Resources
• Cooperation among Processes by Sharing
• Cooperation among Processes by Communication

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Operating System Concerns

• Keep track of active processes
• Allocate and deallocate resources
• processor time memory files I/O devices
• Protect data and resources
• Result of process must be independent of the
  speed of execution since other processes share
  the processor time

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Process Interaction

• Processes unaware of each other
• Competition (example, multiprogramming)
• Processes indirectly aware of each other
• Cooperation by sharing (example, processes
  sharing a common object such as an I/O buffer)
• Process directly aware of each other
• Cooperation by communication (example, processes
  that are able to communicate with each other)

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Competition Among Processes for Resources

• Execution of one process may affect the behavior
  of competing processes
• If two processes wish access to a single
  resource, one process will be allocated the
  resource and the other will have to wait
• Possible the blocked process will never get
  access to the resource and never terminate

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Control Problems I

• Mutual Exclusion
• critical sections
• only one program at a time is allowed in its
  critical section
• example only one process at a time is allowed to
  send command to the printer

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Control Problems IIDeadlock

• process P uses R1 uses R2
• process Q uses R2 uses R1
• parbegin PQ parend
• S.O. R1 -gt P
• S.O. R2 -gt Q
• P needs R2 but it does not release R1
• Q needs R1 but it does not release R2

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Control Problems IIIStarvation

• Processes P1, P2, P3 require periodically
  resource R
• R -gt P1
• P2 and P3 are delayed waiting for R
• R -gt P2
• (R -gt P1 R-gt P2)
• P3 is starving

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Cooperation Among Processes by Sharing

• Processes use and update shared data such as
  shared variables, files, and data bases
• Writing must be mutually exclusive
• Critical sections are used to provide data
  integrity

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RequirementData coherence

• a b
• P1 a a 1
• b b 1
• P2 b 2 b
• a 2 a

• a b
• P1 a a 1
• P2 b 2 b
• P1 b b 1
• P2 a 2 a
• a ! b

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Cooperation Among Processes by Communication

• Communication provides a way to synchronize, or
  coordinate, the various activities
• Possible to have deadlock
• each process waiting for a message from the other
  process
• Possible to have starvation
• two processes sending message to each other while
  another process waits for a message

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Requirements for Mutual Exclusion

• Only one process at a time is allowed in the
  critical section for a resource
• If a process halts in its critical section, it
  must not interfere with other processed
• A process requiring the critical section must not
  be delayed indefinitely no deadlock or starvation

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Requirements for Mutual Exclusion

• A process must not be delayed access to a
  critical section when there is no other process
  using it
• No assumptions are made about relative process
  speeds or number of processes
• A process remains inside its critical section for
  a finite time only