Operating Systems

CPU Scheduling

Agenda for Today

What is Scheduler and its typesShort-term schedulerDispatcherReasons for invoking schedulerOptimization criteriaFCFS, SJF, SRTF, RR, Multi

level Queues With Examples

CPU Scheduling

Scheduling processes in the ready queue

Short-term schedulerDifferent types of schedulers

Life of a Process

Histogram of CPU-burst Times

CPU SchedulerShort-term schedulerSelects a process from among

the processes in the ready queue

Invokes the dispatcher to have the CPU allocated to the selected process

Dispatcher Dispatcher gives control of the CPU

to the process selected by the short-term scheduler; this involves:switching contextswitching to user mode jumping to the proper location in

the user program to start (or restart) it

Dispatcher

Dispatch latency – time it takes for the dispatcher to stop one process and start another running.

Typically, a few microseconds

CPU Scheduler CPU scheduling decisions may

take place when a process:1.Switches from running to

waiting state2.Switches from running to

ready state3.Switches from waiting to ready4.Terminates

CPU Scheduler

Scheduling under 1 and 4 is nonpreemptive.

All other scheduling is preemptive.

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

Scheduling Criteria

Waiting time – amount of time a process has been waiting in the 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)

Optimization Criteria

Maximize CPU utilizationMaximize throughputMinimize turnaround time Minimize waiting time Minimize response time

FCFS Scheduling The process that enters the ready

queue first is scheduled first, regardless of the size of its next CPU burst

Example: Process Burst Time P1 24 P2 3 P3 3

Suppose that processes arrive into the system in the order: P1, P2 , P3

FCFS Scheduling Processes are served in the order: P1,

P2, P3 The Gantt Chart for the schedule is:

Waiting times P1 = 0; P2 = 24; P3 = 27 Average waiting time: (0+24+27)/3 = 17

P1 P2 P3

24 27 300

Suppose that processes arrive in the order: 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 Convoy effect short process behind long

process

P1P3P2

63 300

FCFS Scheduling

Shortest-Job-First (SJF) Scheduling

Process with the shortest CPU burst is scheduled first.

Non-preemptive – once CPU given to a process it cannot be preempted until completes its CPU burst.

Shortest-Job-First (SJF) Scheduling

Preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt it—Shortest-Remaining-Time-First (SRTF).

SJF is optimal non-preemptive scheduling algorithm – gives minimum average waiting time for a given set of processes.

Non-Preemptive SJF Process Arrival Time Burst

Time

P1 0.0 7

P2 2.0 4

P3 4.0 1

P4 5.0 4

Gantt chart

Average waiting time = (0+6+3+7)/4 = 4

P1 P3 P2

7 160

P4

12

Preemptive SJF Process Arrival Time Burst Time

P1 0.0 7

P2 2.0 4

P3 4.0 1

P4 5.0 4

Gantt chart

Average waiting time = (9 + 1 + 0 +2)/4 = 3

P3P2

42 110

P4

5 7

P2 P1

16

P1

Priority SchedulingA priority number (integer) is

associated with each processThe CPU is allocated to the

process with the highest priority (smallest integer highest priority).PreemptiveNon-preemptive

Priority SchedulingSJF 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.

Round Robin (RR)Each process gets a small unit

of CPU time, called time slice or quantum, which is usually 10-100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue.

Round Robin (RR) If there are n processes in the

ready queue, the time quantum is q, and context switch time is tcs, then no process waits more than (n-1)(q+tcs) time units

Used in time-sharing systems where response time is an important performance criteria

Performance q large FCFS q small q must be large with

respect to context switch, otherwise overhead is too high.

Round Robin (RR)

Round Robin ExampleProcess Burst Time

P1 53 — 33 — 13P2 17

P3 68 — 48 — 28 — 8

P4 24 — 4 The Gantt chart with quantum 20 is:

P1 P2 P3 P4 P1 P3 P4 P1 P3 P3

0 20 37 57 77 97 117 121 134 154 162

Round Robin ExampleProcess Turnaround Time Waiting Time

P1 134 134 – 53 = 81P2 37 37 – 17 = 20P3 162 162 – 68 = 94P4 121 121 – 24 = 97

Average waiting time = 73 Average waiting time for SJF = 38 Typically, higher average turnaround

than SJF, but better response.

Quantum vs Context Switch

Process Time = 10 Quantum Context Switches

12 0

6 1

1 9

Multilevel QueuesReady queue is partitioned into

separate queues:- foreground (interactive)- background (batch)

Each queue has its own priority and scheduling algorithm: - foreground – RR- background – FCFS

Multilevel Queues Scheduling must be done across

queues.Fixed priority scheduling; i.e.,

serve all from foreground then from background.

Time slice – each queue gets a certain percentage of CPU time, e.g., 80% to foreground in RR and 20% to background in FCFS

Multilevel Queues

Multilevel Feedback Queues

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 queuesScheduling algorithms for each

queue

Multilevel Feedback Queues

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

Multilevel Feedback Queues