Process Scheduling in Windows XP, Linux, and UNIX

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Process Scheduling in Windows XP, Linux, and UNIX

• By Sarah G. Levinson
• CSC 4320

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Common Measures of Operating System Performance

• CPU Utilization
• Throughput
• Turnaround Time
• Response Time
• Wait Time

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Other Important Factors

• User-Perceived Performance This measure is
  concerned with how efficient the system seems to
  a human user. This deals less with overall
  efficiency and more with the time it takes for
  the user to see observable results of a response.
• Fairness Processes that are similar are given
  similar access to system resources. The property
  of fairness is exemplified by the Round Robin
  scheduling algorithm.

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The process scheduling algorithms used by an
operating system reflect its primary performance
objectives.

• Traditional UNIX Overall Efficiency, Fairness
  and User-Perceived Performance
• Multi-level feedback queue implementation is
  used, so processes with higher priority have
  first access to CPU time.
• Priority of a process is based on the type of
  process, where the swapper has highest priority
  and user processes have lowest priority.

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The process scheduling algorithms used by an
operating system reflect its primary performance
objectives.

• Traditional UNIX Overall Efficiency, Fairness
  and User-Perceived Performance
• Round Robin algorithm is implemented within
  individual priority queues, and a processs
  priority is decreased as it uses more CPU time.
• User programs that are processor-bound are
  penalized more than user programs that are
  I/O-bound.

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The process scheduling algorithms used by an
operating system reflect its primary performance
objectives.

• Modern UNIX Real-Time Processing and Overall
  Efficiency
• Processes can be assigned one of three priority
  levels Real Time, Kernel, or Time Shared.
• Real Time processes have the highest priority and
  may pre-empt processes at any other level,
  including kernel processes.

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The process scheduling algorithms used by an
operating system reflect its primary performance
objectives.

• Modern UNIX Real-Time Processing and Overall
  Efficiency
• If multiple real-time processes are waiting to
  run, the highest-priority process is selected to
  run until it has exhausted its time quantum.
• If a higher-level process has not completed but
  is not currently in a running status, a process
  is selected from the highest-priority ready queue
  that is not empty.

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The process scheduling algorithms used by an
operating system reflect its primary performance
objectives.

• Modern UNIX Real-Time Processing and Overall
  Efficiency
• The priorities of time shared processes are
  dynamically set by the OS.
• A processs priority is lowered when it uses up a
  time quantum and raised it if blocks on an event
  or resource.

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The process scheduling algorithms used by an
operating system reflect its primary performance
objectives.

• LinuxReal-Time Processing and Fairness
• Linux uses two process scheduling algorithms a
  real-time algorithm for processes that must be
  allowed to execute in a timely fashion, and a
  timesharing algorithm for all other processes.
• The Round Robin and FIFO scheduling algorithms
  are used to switch between real-time processes.

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The process scheduling algorithms used by an
operating system reflect its primary performance
objectives.

• LinuxReal-Time Processing and Fairness
• Timesharing algorithm is prioritized and
  credit-based.
• Credit-Based When a new process must be
  selected, the process with the most credits is
  selected. A process that is running loses a
  credit every time the timer interrupt occurs.
  When a process has no more credits, it may not
  run.

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The process scheduling algorithms used by an
operating system reflect its primary performance
objectives.

• LinuxReal-Time Processing and Fairness
• Timesharing algorithm is prioritized and
  credit-based.
• Prioritized When no processes have any
  remaining credits, the system re-assigns credits.
  Higher priority processes receive more credits.

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The process scheduling algorithms used by an
operating system reflect its primary performance
objectives.

• LinuxReal-Time Processing and Fairness
• Timesharing algorithm is prioritized and
  credit-based.
• This crediting system automatically prioritizes
  interactive and I/O-bound processes, which
  increases fairness and user-perceived
  performance.

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The process scheduling algorithms used by an
operating system reflect its primary performance
objectives.

• Windows XP Real Time Processing and
  User-Perceived Performance
• Real time processes have the highest priority and
  can pre-empt any process with lower priority.
• System processes may be starved by real time
  processes because a processs priority is never
  decreased.

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The process scheduling algorithms used by an
operating system reflect its primary performance
objectives.

• Windows XP Real Time Processing and
  User-Perceived Performance
• All processes receive a priority boost after a
  wait event, but processes that have experienced a
  keyboard I/O wait get a larger boost than those
  that have experienced a disk I/O wait.
• Foreground processes given higher priority.

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Questions?