# Average waiting time = [(10-1)+(1-1)+(17-2)+(5-3)]/4 = 26/4 = 6.5 msec

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1 Example of Shortest-remaining-time-first i.e. Preemptive SJF Now we add the concepts of varying arrival times and preemption to the analysis ProcessAarri Arrival TimeT Burst Time P P P P Preemptive SJF Gantt Chart P 1 P 2 P 4 P 1 P Average waiting time = [(10-1)+(1-1)+(17-2)+(5-3)]/4 = 26/4 = 6.5 msec 6.1 Silberschatz, Galvin and Gagne 2013

2 Priority Scheduling A priority number (integer) is associated with each process The CPU is allocated to the process with the highest priority (smallest integer highest priority) Can be preemptive Can be nonpreemptive SJF is priority scheduling where priority is the inverse of predicted next CPU burst time Problem Starvation low priority processes may never execute Solution Aging as time progresses increase the priority of the process There is a rumor that when the IBM 7094 at MIT was shut down in 1973 that a low-priority process from 1967 had not yet been run. 6.2 Silberschatz, Galvin and Gagne 2013

3 Example of Priority Scheduling ProcessAarri Burst Time(ms)T Priority P P P P P Priority scheduling Gantt Chart assuming all arrive at time 0 Average waiting time = ( )/5 = 8.2 msec 6.3 Silberschatz, Galvin and Gagne 2013

4 Round Robin (RR) Each process gets a small unit of CPU time (time quantum q), 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. Timer interrupts every quantum to schedule next process Performance q large FIFO q small q must be large with respect to context switch, otherwise overhead is too high 6.4 Silberschatz, Galvin and Gagne 2013

5 Example of RR with Time Quantum = 4 Process Burst Time P 1 24 P 2 3 P 3 3 The Gantt chart assuming all processes arrive at time 0 is: P P 2 P 3 P 1 P 1 P 1 P P Typically, higher average turnaround than SJF, but better response q should be large compared to context switch time q usually 10ms to 100ms, context switch < 10 µsec 6.5 Silberschatz, Galvin and Gagne 2013

6 Time Quantum and Context Switch Time 6.6 Silberschatz, Galvin and Gagne 2013

7 Turnaround Time Varies With The Time Quantum 80% of CPU bursts should be shorter than q Given a time quantun of 1, where does 11.0 come from? 6.7 Silberschatz, Galvin and Gagne 2013

8 Multilevel Queue Ready queue is partitioned into separate queues, eg: foreground (interactive) background (batch) Process permanently in a given queue Each queue has its own scheduling algorithm: 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; i.e., 80% to foreground in RR 20% to background in FCFS 6.8 Silberschatz, Galvin and Gagne 2013

9 Multilevel Queue Scheduling 6.9 Silberschatz, Galvin and Gagne 2013

10 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 when to upgrade a process method used to determine when to demote a process method used to determine which queue a process will enter when that process needs service 6.10 Silberschatz, Galvin and Gagne 2013

11 Example of Multilevel Feedback Queue Three queues: Q 0 RR with time quantum 8 milliseconds Q 1 RR 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 Silberschatz, Galvin and Gagne 2013

12 Thread Scheduling Distinction between user-level and kernel-level threads When threads are supported, threads are scheduled, not processes Many-to-one and many-to-many models, thread library schedules user-level threads to run on LWP Known as process-contention scope (PCS) since scheduling competition is within the process Typically done via priority set by programmer Kernel thread scheduled onto available CPU is system-contention scope (SCS) competition among all threads in system 6.12 Silberschatz, Galvin and Gagne 2013

13 Pthread Scheduling API allows specifying either PCS or SCS during thread creation PTHREAD_SCOPE_PROCESS schedules threads using PCS scheduling PTHREAD_SCOPE_SYSTEM schedules threads using SCS scheduling Can be limited by OS Linux and Mac OS X only allow PTHREAD_SCOPE_SYSTEM 6.13 Silberschatz, Galvin and Gagne 2013

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