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CPU Scheduling

CPU Scheduling. Basic Concepts Scheduling Criteria Scheduling Algorithms Scheduling in BSD UNIX. Scheduling Criteria. CPU utilization – Percent time CPU busy Throughput – # of processes that complete their execution per time unit

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CPU Scheduling

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  1. CPU Scheduling • Basic Concepts • Scheduling Criteria • Scheduling Algorithms • Scheduling in BSD UNIX

  2. Scheduling Criteria • CPU utilization – Percent time CPU busy • Throughput – # of processes that complete their execution per time unit • Turnaround time–time to execute a process: from start to completion. • Waiting time –time in Ready queue • Response time – amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment)

  3. Optimization Criteria • Max CPU utilization • Max throughput • Min turnaround time • Min waiting time • Min response time

  4. P1 P2 P3 0 24 27 30 First-Come, First-Served (FCFS) • Example: ProcessBurst Time P1 24 P2 3 P33 • Assume processes arrive as: P1 , P2 , P3 The Gantt Chart for the schedule is: • Waiting time forP1 = 0; P2= 24; P3= 27 • Average waiting time: (0 + 24 + 27)/3 = 17

  5. P2 P3 P1 0 3 6 30 FCFS Scheduling (Cont.) Suppose processes arrive as: P2 , P3 , P1 . • The Gantt chart for the schedule is: • Waiting time for P1 = 6;P2 = 0; P3 = 3 • Average waiting time: (6 + 0 + 3)/3 = 3 • Much better than previous case. • Convoy effector head-of-line blocking • short process behind long process

  6. Shortest-Job-First (SJR) Scheduling • Process declares its CPU burst length • Two schemes: • non-preemptive – once CPU assigned, process not preempted until its CPU burst completes. • Preemptive – if a new process with CPU burst less than remaining time of current, preempt. Shortest-Remaining-Time-First (SRTF). • SJF is optimal – gives minimum average waiting time for a given set of processes.

  7. P1 P3 P2 P4 0 3 7 8 12 16 Example of Non-Preemptive SJF Process Arrival Time Burst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 • SJF (non-preemptive) • Average waiting time • = (0 + 6 + 3 + 7)/4 = 4

  8. P1 P2 P3 P2 P4 P1 11 16 0 2 4 5 7 Example of Preemptive SJF ProcessArrival TimeBurst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 • SJF (preemptive) • Average waiting time • = (9 + 1 + 0 +2)/4 = 3

  9. Priority Scheduling • Priority associated with each process • CPU allocated to process with highest priority • Preemptive or non-preemptive • Example - SJF: priority scheduling where priority is predicted next CPU burst time. • Problem: Starvation • low priority processes may never execute. • Solution: Aging • as time progresses increase the priority of the process.

  10. Round Robin (RR) • Each process assigned a time quantum, usually 10-100 milliseconds. After this process moved to end of the Ready Q • n processes in ready queue, time quantum = q, then each process gets 1/n of the CPU time in chunks of at most q time units at once. No process waits more than (n-1)q time units. • Performance • q large  FIFO • q small  q must be large with respect to context switch, otherwise overhead too high.

  11. P1 P2 P3 P4 P1 P3 P4 P1 P3 P3 0 20 37 57 77 97 117 121 134 154 162 Example: RR, Quantum = 20 ProcessBurst Time P1 53 P2 17 P3 68 P4 24 • The Gantt chart is: • Typically, higher average turnaround than SJF, but better response.

  12. Small Quantum Increased Context Switches

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