Chapter 5 Process Scheduling Operating System Concepts 9

CPU Scheduler n Short-term scheduler selects from among the processes in ready queue, and

Dispatcher n n Dispatcher module gives control of the CPU to the process selected

Scheduling Criteria n CPU utilization – keep the CPU as busy as possible n

Scheduling Algorithm Optimization Criteria n Max CPU utilization n Max throughput n Min turnaround

First-Come, First-Served (FCFS) Scheduling n Process Burst Time P 1 P 2 P 3

FCFS Scheduling (Cont. ) Suppose that the processes arrive in the order: P 2

Shortest-Job-First (SJF) Scheduling n Associate with each process the length of its next CPU

Example of SJF Process. Arriva n l Time Burst Time P 1 0. 0

Determining Length of Next CPU Burst n Can only estimate the length – should

Examples of Exponential Averaging n =0 l l n+1 = n Recent history does

Example of Shortest-remaining-time-first Now we add the concepts of varying arrival times and preemption

Priority Scheduling n A priority number (integer) is associated with each process n The

Example of Priority Scheduling Process. A n Priority P 1 10 3 P 2

Round Robin (RR) n Each process gets a small unit of CPU time (time

Example of RR with Time Quantum = 4 n 0 n n Burst Time

Time Quantum and Context Switch Time Operating System Concepts – 9 th Edition 6.

Multiple-Processor Scheduling n CPU scheduling more complex when multiple CPUs are available n Homogeneous

Multiple-Processor Scheduling – Load Balancing n If SMP, need to keep all CPUs loaded

Multicore Processors n Recent trend to place multiple processor cores on same physical chip

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CPU Scheduler n Short-term scheduler selects from among the processes in ready queue, and allocates the CPU to one of them l n Queue may be ordered in various ways CPU scheduling decisions may take place when a process: 1. Switches from running to waiting state 2. Switches from running to ready state 3. Switches from waiting to ready 4. Terminates n Scheduling under 1 and 4 is nonpreemptive n All other scheduling is preemptive l Consider access to shared data l Consider preemption while in kernel mode l Consider interrupts occurring during crucial OS activities Operating System Concepts – 9 th Edition 6. 2 Silberschatz, Galvin and Gagne © 2013

Dispatcher n n Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves: l switching context l switching to user mode l jumping to the proper location in the user program to restart that program Dispatch latency – time it takes for the dispatcher to stop one process and start another running Operating System Concepts – 9 th Edition 6. 3 Silberschatz, Galvin and Gagne © 2013

Scheduling Criteria n CPU utilization – keep the CPU as busy as possible n Throughput – # of processes that complete their execution per time unit n Turnaround time – amount of time to execute a particular process n Waiting time – amount of time a process has been waiting in the ready queue n 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) Operating System Concepts – 9 th Edition 6. 4 Silberschatz, Galvin and Gagne © 2013

Scheduling Algorithm Optimization Criteria n Max CPU utilization n Max throughput n Min turnaround time n Min waiting time n Min response time Operating System Concepts – 9 th Edition 6. 5 Silberschatz, Galvin and Gagne © 2013

First-Come, First-Served (FCFS) Scheduling n Process Burst Time P 1 P 2 P 3 24 3 3 Suppose that the processes arrive in the order: P 1 , P 2 , P 3 The Gantt Chart for the schedule is: P 1 0 n n P 2 24 P 3 27 30 Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17 Operating System Concepts – 9 th Edition 6. 6 Silberschatz, Galvin and Gagne © 2013

FCFS Scheduling (Cont. ) Suppose that the processes arrive in the order: P 2 , P 3 , P 1 n The Gantt chart for the schedule is: P 2 0 P 3 3 P 1 6 30 n Waiting time for P 1 = 6; P 2 = 0; P 3 = 3 n Average waiting time: (6 + 0 + 3)/3 = 3 n Much better than previous case n Convoy effect - short process behind long process l Consider one CPU-bound and many I/O-bound processes Operating System Concepts – 9 th Edition 6. 7 Silberschatz, Galvin and Gagne © 2013

Shortest-Job-First (SJF) Scheduling n Associate with each process the length of its next CPU burst l n Use these lengths to schedule the process with the shortest time SJF is optimal – gives minimum average waiting time for a given set of processes l The difficulty is knowing the length of the next CPU request l Could ask the user Operating System Concepts – 9 th Edition 6. 8 Silberschatz, Galvin and Gagne © 2013

Example of SJF Process. Arriva n l Time Burst Time P 1 0. 0 6 P 2 2. 0 8 P 3 4. 0 7 P 4 5. 0 3 SJF scheduling chart P 4 0 n P 3 P 1 3 9 P 2 16 24 Average waiting time = (3 + 16 + 9 + 0) / 4 = 7 Operating System Concepts – 9 th Edition 6. 9 Silberschatz, Galvin and Gagne © 2013

Determining Length of Next CPU Burst n Can only estimate the length – should be similar to the previous one l Then pick process with shortest predicted next CPU burst n Can be done by using the length of previous CPU bursts, using exponential averaging n Commonly, α set to ½ n Preemptive version called shortest-remaining-time-first Operating System Concepts – 9 th Edition 6. 10 Silberschatz, Galvin and Gagne © 2013

Examples of Exponential Averaging n =0 l l n+1 = n Recent history does not count n =1 n n+1 = tn l Only the actual last CPU burst counts If we expand the formula, we get: n+1 = tn+(1 - ) tn -1 + … +(1 - )j tn -j + … l +(1 - )n +1 0 n Since both and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor Operating System Concepts – 9 th Edition 6. 11 Silberschatz, Galvin and Gagne © 2013

Example of Shortest-remaining-time-first Now we add the concepts of varying arrival times and preemption to the analysis n Process. A Burst Time P 1 0 8 P 2 1 4 P 3 2 9 P 4 3 5 Preemptive SJF Gantt Chart n 0 1 P 4 P 2 P 1 n arri Arrival Time. T 5 10 P 3 17 26 Average waiting time = [(10 -1)+(17 -2)+5 -3)]/4 = 26/4 = 6. 5 msec Operating System Concepts – 9 th Edition 6. 12 Silberschatz, Galvin and Gagne © 2013

Priority Scheduling n A priority number (integer) is associated with each process n The CPU is allocated to the process with the highest priority (smallest integer highest priority) l Preemptive l Nonpreemptive n SJF is priority scheduling where priority is the inverse of predicted next CPU burst time n Problem Starvation – low priority processes may never execute n Solution Aging – as time progresses increase the priority of the process Operating System Concepts – 9 th Edition 6. 13 Silberschatz, Galvin and Gagne © 2013

Example of Priority Scheduling Process. A n Priority P 1 10 3 P 2 1 1 P 3 2 4 P 4 1 5 P 5 5 2 Priority scheduling Gantt Chart 0 P 1 P 5 P 2 n arri Burst Time. T 1 P 3 6 16 P 4 18 19 Average waiting time = 8. 2 msec Operating System Concepts – 9 th Edition 6. 14 Silberschatz, Galvin and Gagne © 2013

Round Robin (RR) n Each process gets a small unit of CPU time (time quantum q), usually 10 -100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue. n 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. n Timer interrupts every quantum to schedule next process n Performance l q large FIFO l q small q must be large with respect to context switch, otherwise overhead is too high Operating System Concepts – 9 th Edition 6. 15 Silberschatz, Galvin and Gagne © 2013

Example of RR with Time Quantum = 4 n 0 n n Burst Time P 1 P 2 P 3 24 3 3 The Gantt chart is: P 1 n Process P 2 4 P 3 7 P 1 10 P 1 14 P 1 18 P 1 22 P 1 26 30 Typically, higher average turnaround than SJF, but better response q should be large compared to context switch time q usually 10 ms to 100 ms, context switch < 10 usec Operating System Concepts – 9 th Edition 6. 16 Silberschatz, Galvin and Gagne © 2013

Multiple-Processor Scheduling n CPU scheduling more complex when multiple CPUs are available n Homogeneous processors within a multiprocessor n Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing n Symmetric multiprocessing (SMP) – each processor is self-scheduling, all processes in common ready queue, or each has its own private queue of ready processes l n Currently, most common Processor affinity – process has affinity for processor on which it is currently running l soft affinity l hard affinity l Variations including processor sets Operating System Concepts – 9 th Edition 6. 18 Silberschatz, Galvin and Gagne © 2013

Multiple-Processor Scheduling – Load Balancing n If SMP, need to keep all CPUs loaded for efficiency n Load balancing attempts to keep workload evenly distributed n Push migration – periodic task checks load on each processor, and if found pushes task from overloaded CPU to other CPUs n Pull migration – idle processors pulls waiting task from busy processor Operating System Concepts – 9 th Edition 6. 19 Silberschatz, Galvin and Gagne © 2013

Multicore Processors n Recent trend to place multiple processor cores on same physical chip n Faster and consumes less power n Multiple threads per core also growing l Takes advantage of memory stall to make progress on another thread while memory retrieve happens Operating System Concepts – 9 th Edition 6. 20 Silberschatz, Galvin and Gagne © 2013