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Silberschatz, Galvin and Gagne 2002 6.1 Operating System Concepts Schedulers Long-term scheduler (or job scheduler) – selects which processes should be.

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Presentation on theme: "Silberschatz, Galvin and Gagne 2002 6.1 Operating System Concepts Schedulers Long-term scheduler (or job scheduler) – selects which processes should be."— Presentation transcript:

1 Silberschatz, Galvin and Gagne Operating System Concepts Schedulers Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue. Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU.

2 Silberschatz, Galvin and Gagne Operating System Concepts Schedulers (Contd) Short-term scheduler is invoked very frequently (milliseconds) => must be fast Long-term scheduler is invoked very infrequently (seconds, minutes) => may be slow The long-term scheduler controls the degree of multiprogramming.

3 Silberschatz, Galvin and Gagne Operating System Concepts Basic Concepts CPU–I/O Burst Cycle – Process execution consists of a burst of CPU execution then some waiting time CPU-bound process – spends more time doing computations; few very long CPU bursts. I/O-bound process – spends more time doing I/O than computations; many short CPU bursts.

4 Silberschatz, Galvin and Gagne Operating System Concepts Histogram of CPU-burst Times CPU burst distribution

5 Silberschatz, Galvin and Gagne Operating System Concepts CPU Scheduler Selects from among the ready processes Highest priority process is selected Non-preemptive scheduling Process gives up the CPU voluntarily Waiting for IO activity Yields Terminates Can be implemented without a CPU clock Preemptive scheduling Interrupted process can be replaced Interrupts, including CPU clock Includes arrival of new processes

6 Silberschatz, Galvin and Gagne Operating System Concepts Scheduling Criteria CPU utilization – keep the CPU as busy as possible Throughput – # of processes that complete their execution per time unit Turnaround time – amount of time to execute a particular process Response time – amount of time it takes from when a request was submitted until the start of a response (not time for output) (for time-sharing environment) Waiting time – amount of time a process has been waiting in the ready queue. Quite representative.

7 Silberschatz, Galvin and Gagne Operating System Concepts First-Come, First-Served (FCFS) Scheduling Is non-preemptive ProcessBurst Time P 1 24 P 2 3 P 3 3 Suppose that the processes arrive in the order: P 1, P 2, P 3, all at the same time. The Gantt Chart for the schedule is: Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 Average waiting time: ( )/3 = 17 P1P1 P2P2 P3P

8 Silberschatz, Galvin and Gagne Operating System Concepts FCFS Scheduling (Cont.) Suppose that the processes arrive in the order P 2, P 3, P 1. The Gantt chart for the schedule is: Waiting time for P 1 = 6; P 2 = 0 ; P 3 = 3 Average waiting time: ( )/3 = 3 Much better than previous case. Convoy effect short process behind long process P1P1 P3P3 P2P

9 Silberschatz, Galvin and Gagne Operating System Concepts Shortest-Job-First (SJF) Scheduling Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time. Two schemes: Non-preemptive – once CPU given to the process it cannot be preempted until completes its CPU burst. Preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is know as the Shortest-Remaining-Time-First (SRTF).

10 Silberschatz, Galvin and Gagne Operating System Concepts ProcessArrival TimeBurst Time P P P P SJF (non-preemptive) Average waiting time = ( )/4 - 4 Example of Non-Preemptive SJF P1P1 P3P3 P2P P4P4 812

11 Silberschatz, Galvin and Gagne Operating System Concepts Example of Preemptive SJF ProcessArrival TimeBurst Time P P P P SJF (preemptive) Average waiting time = ( )/4 - 3 P1P1 P3P3 P2P P4P4 57 P2P2 P1P1 16

12 Silberschatz, Galvin and Gagne Operating System Concepts More SJF Examples SJF non-preemptiveProcArrivesBurst P 1 08 P 2 14 P 3 29 P 4 35 And then preemptive SJF non-preemptiveProcArrivesBurst P 1 12 P 2 07 P 3 27 P 4 53 P 5 61 And then preemptive

13 Silberschatz, Galvin and Gagne Operating System Concepts Determining Length of Next CPU Burst SJF is optimal – gives minimum average waiting time for a given set of processes. But we can only estimate the length of a CPU burst. Can be done by using the length of previous CPU bursts, using exponential averaging.

14 Silberschatz, Galvin and Gagne Operating System Concepts Prediction of the Length of the Next CPU Burst

15 Silberschatz, Galvin and Gagne Operating System Concepts Examples of Exponential Averaging =0 n+1 = n Recent history does not count. =1 n+1 = t n Only the actual last CPU burst counts. If we expand the formula, we get: n+1 = t n +(1 - ) t n -1 + … +(1 - ) j t n -1 + … +(1 - ) n=1 t n 0 Since both and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor.

16 Silberschatz, Galvin and Gagne Operating System Concepts Priority Scheduling A priority number (integer) is associated with each process, Internal, e.g., by resource needs External, e.g., by user priority The CPU is allocated to the process with the highest priority (smallest integer highest priority). Non-preemptive or Preemptive ExampleProcArrivalBurstPriority P P P P P SJF is a priority scheduling where priority is the predicted next CPU burst time. Problem Starvation – low priority processes may never execute. Solution Aging – as time progresses increase the priority of the process.

17 Silberschatz, Galvin and Gagne Operating System Concepts More Priority Examples ExampleProcArrivalBurstPriority P P P P P

18 Silberschatz, Galvin and Gagne Operating System Concepts Round Robin (RR) Each process gets a small unit of CPU time (time quantum), usually milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue. If there are n processes in the ready queue and the time quantum is 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.

19 Silberschatz, Galvin and Gagne Operating System Concepts Example of RR with Time Quantum = 20 ProcessBurst Time P 1 53 P 2 17 P 3 68 P 4 24 The Gantt chart is: P1P1 P2P2 P3P3 P4P4 P1P1 P3P3 P4P4 P1P1 P3P3 P3P

20 Silberschatz, Galvin and Gagne Operating System Concepts More RR Examples ProcArrivalBurst P P P P

21 Silberschatz, Galvin and Gagne Operating System Concepts Time Quantum and Context Switch Time Performance q large FIFO q small q must be large with respect to context switch, otherwise overhead is too high. Typically, higher average turnaround than SJF, but better response.

22 Silberschatz, Galvin and Gagne Operating System Concepts Multilevel Queue Ready queue is partitioned into separate queues: foreground (interactive) background (batch) Each queue has its own scheduling algorithm, e.g., foreground – RR background – FCFS Scheduling must be done between the queues. Fixed priority scheduling; (i.e., serve all from foreground then from background). Possibility of starvation. Time slice – each queue gets a certain amount of CPU time which it can schedule amongst its processes 80% to foreground in RR 20% to background in FCFS

23 Silberschatz, Galvin and Gagne Operating System Concepts Multilevel Queue Scheduling

24 Silberschatz, Galvin and Gagne Operating System Concepts Multilevel Queue Examples ML queue, 2 levels 10 units FCFS RR gets priority over FCFS ProcArrivalBurstQueue P 1 012FCFS P 2 412RR P 3 88FCFS P RR Non-preemptive and preemptive

25 Silberschatz, Galvin and Gagne Operating System Concepts Multilevel Feedback Queue A process can move between the various queues; aging can be implemented this way. Multilevel-feedback-queue scheduler defined by the following parameters: number of queues scheduling algorithms for each queue method used to determine which queue a process will enter when that process needs service method used to determine when to upgrade a process method used to determine when to demote a process

26 Silberschatz, Galvin and Gagne Operating System Concepts Example of Multilevel Feedback Queue Three queues: Q 0 – time quantum 8 milliseconds Q 1 – time quantum 16 milliseconds Q 2 – FCFS Scheduling A new job enters queue Q 0 which is served FCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q 1. At Q 1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q 2.

27 Silberschatz, Galvin and Gagne Operating System Concepts Multilevel Feedback Queues

28 Silberschatz, Galvin and Gagne Operating System Concepts Multilevel Feedback Queue Example Three levels RR at 8 units RR at 16 units FCFS ProcArrivalBurst P P P Non-preemptive and preemptive

29 Silberschatz, Galvin and Gagne Operating System Concepts Multiple-Processor Scheduling CPU scheduling more complex when multiple CPUs are available. Homogeneous processors within a multiprocessor. Load sharing Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing.

30 Silberschatz, Galvin and Gagne Operating System Concepts Real-Time Scheduling Hard real-time systems – required to complete a critical task within a guaranteed amount of time. Soft real-time computing – requires that critical processes receive priority over less fortunate ones.


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