Chapter 5: Process Scheduling

Chapter 5: Process Scheduling Basic Concepts Scheduling Criteria Scheduling Algorithms (6)

Multiple-Processor Scheduling Thread Scheduling OS Examples Algorithm Evaluation

Basic Concepts In a multiprogramming system, multiple processes exist

concurrently in main memory. Each process alternates between using a processor and

waiting for some event to occur, such as the completion of an I/O operation.

The processor is kept busy by executing one process while the others wait.

The key to multiprogramming is scheduling.

Process Scheduling is the basis of multi-programmed operating system a fundamental function of operating-system.

Basic Concepts In a single-processor system

Only one process can run at a time Any others must wait until the CPU is free and can be rescheduled. When the running process goes to the waiting state,

the OS may select another process to assign CPU to improve CPU utilization.

Every time one process has to wait, another process can take over use of the CPU

Process scheduling is to select a process from the ready queue and assign the CPU

Diagram of Process State from ch.3

It is important to realize that only one process can be running on any processor at any instant.

Many processes may be ready and waiting states.

Process Scheduling from ch.3

• The process selection is carried out by the short-term scheduler (or CPU scheduler).

• The scheduler selects a process from the processes in memory that are ready to execute and allocates the CPU to that process.

CPU - I/O burst Cycle Process execution consists of

a cycle of CPU execution (CPU burst) and I/O wait (I/O burst)

Process alternate between these two states Process execution begins with a CPU burst, which is followed by

an I/O burst, and so on. Eventually, the final CPU burst ends with an system call to termi-

nate execution.

CPU burst distribution of a process varies greatly from process to process and from computer to com-

puter

Alternating Sequence of CPU & I/O Bursts

CPU burst time

I/O burst time

CPU burst time

CPU burst time

Histogram of CPU-burst Times

CPU burst distribution is generally characterized as exponential or hyper-exponential with large number of short CPU burst and small number of long CPU burst

I/O bound process has many short CPU bursts CPU bound process might have a few long CPU bursts.

Process Scheduler selects one of the processes in memory that are ready to

execute, and allocates the CPU to the selected process.

CPU scheduling decisions may take place when a process:1. switches from running to waiting state: I/O request, invocation of

wait() for the termination of other process2. switches from running to ready state: when interrupt occurs3. switches from waiting to ready: at completion of I/O4. terminates

Process Scheduler(cont.)

Scheduling under 1 and 4 is non-preemptive Scheduling under 2 and 3 is preemptive

Non-preemptive vs. Preemptive Non-preemptive scheduling

Once the CPU has been allocated to a process, the process keeps the CPU until it releases the CPU

either by terminating or by switching to the waiting state. used by Windows 3.x

Preemptive scheduling Current running process can be switched with another at any time

because interrupt can occur at any time Most of modern OS provides this scheme. (Windows XP, Max OS, UNIX)

Dispatcher Dispatcher module gives control of the CPU to the

process selected by the short-term scheduler; this in-volves: switching context switching to user mode jumping to the proper location in the user program to restart that pro-

gram

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

CPU-scheduling function

? ?

Context Switch from ch. 3

Scheduling Criteria Based on the scheduling criteria, the performance of various schedul-

ing algorithm could be evaluated. Different scheduling algorithms have different properties.

CPU utilization –keep the CPU as busy as possible. i.e., ratio (%) of the amount of time while the CPU was busy per time unit.

Throughput – # of processes that complete their execution per time unit.

Turnaround time – the interval from the time of submission of a process to the time of completion. Sum of the periods spent waiting to get into memory, waiting in the ready queue, executing on the CPU, and doing I/O

Waiting time – Amount of time a process has been waiting in the ready queue, which is affected by scheduling algorithm

Response time – In an interactive system, 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 It is desirable to maximize:

The CPU utilization The throughput

It is desirable to minimize: The turnaround time The waiting time The response time

However in some circumstances, it is desirable to opti-mize the minimum or maximum values rather than the average. Interactive systems, it is more important to minimize the variance

in the response time than minimize the average response time.

Process Scheduling Algorithms First-Come, First-Served Scheduling (FCFS) Shortest-Job-First Scheduling (SJF) Priority Scheduling Round-Robin Scheduling

Our measure of comparison is the average waiting time.

First-Come, First-Served (FCFS) Scheduling The process that request the CPU first is allocated the

CPU first.Process Burst Time(ms)

P1 24

P2 3

P3 3

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

The Gantt Chart for the schedule is:

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

P1 P2 P3

24 27 300

FCFS SchedulingSuppose that the 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

Much better than previous case

P1P3P2

63 300

FCFS Scheduling FCFS scheduling algorithm is non-preemptive

Once the CPU has been allocated to a process, that process keeps the CPU until it releases the CPU, either by terminating or by requesting I/O.

is particularly troublesome for time-sharing systems (response time ).

Convoy effect (short process behind long process)occurs: When one CPU-bound process with long CPU burst and many

I/O-bound process which short CPU burst. All I/O bound process waits for the CPU-bound process to get off

the CPU while I/O is idle All I/O- and CPU- bound processes executes I/O operation while

CPU is idle. results in low CPU and device utilization

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

CPU burst. use these lengths to schedule the process with the short-

est time

Two schemes: non-preemptive – once CPU given to the process it cannot be pre-

empted 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 known as the Shortest-Remaining-Time-First (SRTF)

SJF is optimal – gives minimum average waiting time for a given set of processes

Example of Non-Preemptive SJF

Process Burst Time

P1 6

P2 8

P3 7

P4 0 3

SJF scheduling chart (non-preemptive)

Average waiting time = (3 + 16 + 9 + 0) / 4 = 7

P4P3P1

3 160 9

P2

24

Example of Preemptive SJF (SRTF)Process Arrival Time Burst 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

P1 P3P2

42 110

P4

5 7

P2 P1

16

How do we know the length of the next CPU burst? by computing an approximation of the length of the next

CPU burst (estimate the length of the next CPU burst) can be done by using the length of previous CPU bursts,

using exponential averaging

The value of tn contains our most recent information. n+1 stores the past history The parameter controls the relative weight of recent and

past history in our prediction.

.1 : Define4.

10 number, reala is 3.

burst CPUnext for the valuepredicted 2.

burst CPU oflenght actual 1.

1

1

nnn

n

thn

t

nt

Prediction of the Length of the Next CPU Burst

In this example, 0 = 10, = ½ , t1=6 1 = x t0 + (1- ) x 0 = ½ x 6 + ½ x 10 = 8 2 = x t2 + (1- ) x 1 = ½ x 4 + ½ x 8 = 6

Examples of Exponential Averaging = 0

n+1 = n = n-1 = n-2 . … = 0

Recent history does not count = 1

n+1 = tn

Only the actual last CPU burst counts i.e., the most recent CPU burst

If we expand the formula, we get:n+1 = tn + (1 - ) tn - 1 + …

+ (1 - )j tn -j + …

+ (1 - )n +1 0

Since both and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor

Priority Scheduling A priority number (integer) is associated with each process The CPU is allocated to the process with the highest prior-

ity (smallest integer highest priority) Preemptive Non-preemptive

Process Burst Time Priority arrival time

P1 10 3 0

P2 1 1 0

P3 2 4 0

P4 1 5 0

P5 5 2 0

Priority Scheduling (non-preemptive)

Average waiting time = (0 + 1 + 6 + 16 + 18)/5 = 8.2

Example 1 of Non-Preemptive Priority

P2 P1 P3

1610

P4

18

P5

6 19

Example 2 of Non-Preemptive Priority Process Ready queue arrive time CPU burst time Priority

P1 0 3 ms 5

P2 1 7 ms 3

P3 4 2 ms 4

P4 2 3 ms 3

P5 6 4 ms 1

P1 P5P2 P3P4

0 3 171410 19

scheduling chart (non-preemptive)

Average waiting time= 31/5 = 6.2 ms

Total Waiting time = (0-0) + (3-1) +(17-4) + (14-2) + (10-6) = 0+2+13+12+4=31 ms

Total Turnaround time= (3-0) + (10-1) + (19-4) + (17-2) + (14- 6) = 50 ms

Average Turnaround time= 50/5 = 10ms

Example of Preemptive Priority Process Ready queue arrive time Cpu burst time Priority

P1 0 3 ms 5

P2 1 7 ms 3

P3 4 2 ms 4

P4 2 3 ms 3

P5 6 4 ms 1

P3P5P2P1 P4 P2 P1

0 6 131 1710 15 19

scheduling chart (preemptive)

Average waiting time= 42/5= 8.4 ms

Total Waiting time = (0-0+17-1) + (1-1+13-6) +(15-4) + (10-2) + (6-6) =42ms

Total Turnaround time= (19-0) + (15-1) + (17-4) + (13-2) + (10-6) = 61ms

Average Turnaround time= 61/5 = 12.2ms

Process P1 preempted by p2 because p2 has higher priority

Priority Scheduling 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

127

0

Round Robin (RR) Scheduling It is similar to FCFS scheduling, but preemption is

added to enable the system to switch between process.

Each process gets a small unit of CPU time (time quantum), usually 10-100 milliseconds.

After this time has elapsed, the process is pre-empted and added to the end of the ready queue.

If CPU cycle (CPU burst time ) > time quantum Job is preempted and put at the end of the READY Q

If CPU cycle < time quantum If job finished resources released

If interrupted by I/O request Info saved in PCB Linked at end of appropriate I/O queue When I/O complete, job returns to READY Q

Example of RR with Time Quantum = 4

Process Burst Time Arrival Time

P1 24 0

P2 3 0

P3 3 0

The Gantt chart is:

P1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30

Average waiting time= 17/3= 5.66 ms

Total Waiting time = (0-0+10-4) + (4-0) +(7-0) =17ms

Total Turnaround time= (30-0) + (7-0) + (10-0) = 47ms

Average Turnaround time= 47/5= 9.4 ms

Context switch ???

Example of RR with Time Quantum = 10

The Gantt chart is:

Process Ready queue arrive time CPU burst time

P1 0 7 ms

P2 1 13 ms

P3 5 25 ms

P4 6 22 ms

P5 8 40 ms

0 17 747 27 37 47 50 60 64

P2 P2P1 P3 P4 P5 P3 P4 P5

79

P3 P5

89

P5

99

Average waiting time= 180/5= 36 ms

Total Waiting time = (0-0) + (7-1 + 47-17 )+(17-5+ 50-27+ 74- 60)+(27-6+60-37)+ (37-8+64-47+79-64 ) = 180 ms

Total Turnaround time= (7-0) + (50-1) + (79-5)+(64-6)+(99-8) = 279ms

Average Turnaround time= 279/5= 55.8 ms

Round Robin (RR) Scheduling

Performance depends on the size of the time quantum. If the time quantum is extremely large, the RR policy is

the same as the same as the FCFS policy

If the time quantum is extremely small (say 1 millisec) , the RR approach is called processor sharing i.e., pro-vides high concurrency: each of n processes has its own processor running at 1/n the speed of the real pro-cessor

Time Quantum and Context Switch Time

In (a) the job finishes before the time quantum expires. In (b) and (c), the time quantum expires first, interrupting the job

The effect of context switching on the performance of RR scheduling, for example one process of 10 time quantum. quantum = 12 time units, finished in less than 1 time quantum quantum = 6 time units, requires 2 quanta, 1 context switch quantum = 1 time units, requires 10 quanta, 9 context switch

a

b

c

Round Robin (RR) Scheduling The time quantum q must be large with respect to context

switch, otherwise overhead is too high

If the context switching time is 10% of the time quantum, then about 10% of the CPU time will be spent in context switching

Most modern OS have time quanta ranging from 10 to 100 milliseconds,

The time required for a context switch is typically less than 10 microseconds; thus the context-switch time is a small fraction of the time quantum.

Turnaround Time varies with the Time Quantum The turnaround time depends on the size of

the time quantum The average turnaround

time of a set of processes

dose not necessarily improve

as the time quantum size

Increased.

The average turnaroundtime can be improvedif most processes finish their next CPU burst in a single time quantum.

Scheduling Algorithm with multi-Queues Multi-level Queue Scheduling Multi-level Feedback Queue Scheduling

Multilevel Queue Ready queue is partitioned into separate queues:

foreground (interactive)background (batch)

The processes are permanently assigned to one queue, generally based on some property, or process type.

Each queue has its own scheduling algorithm foreground – RR background – FCFS

Scheduling must be done between the queues Fixed priority scheduling - serve all from foreground then from

background, Possibility of starvation. Time slice scheduling – 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

Multilevel Queue Scheduling

No process in the batch queue could run unless the queues with high priority were all empty.

If an interactive editing process entered the ready queue while a batch process was running, the batch process would be preempted.

Multilevel Feedback Queue A process can move between the various queues; aging

can be implemented in this way

Multilevel-feedback-queue scheduler defined by the fol-lowing 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

Example of Multilevel Feedback Queue Three queues:

Q0 – RR with time quantum 8 milliseconds

Q1 – RR time quantum 16 milliseconds

Q2 – FCFS

Scheduling A new job enters queue Q0 which is served RR. When it gains

CPU, job receives 8 milliseconds. If it does not finish in 8 millisec-onds, job is moved to queue Q1.

At Q1 job is again served RR and receives 16 additional millisec-onds. If it still does not complete, it is preempted and moved to queue Q2.

The job is serverd based on FCFS in queue Q2

Multilevel Feedback Queues

Summary CPU scheduling is the task of selecting a waiting process from the

ready queue and allocating the CPU to it. The CPU is allocated to the selected process by the dispatcher. FCFS scheduling is simple, cause short processes to wait for long time SJF scheduling is provably optimal, providing the shortest averaging

waiting time. But predicting the length of the next CPU bursts is difficult. Priority scheduling allocates the CPU to the heights priority process. Both priority and SJF may suffer from starvation. Aging is a technique

to prevent starvation. RR scheduling is more appropriate for a time-shared system. Major problem of RR scheduling is the selection of the time quantum. FCFS is non-preemptive, RR is preemptive, SJF and Priority may be

preemptive and non-preemptive. Multilevel queue allows different scheduling algorithm for each queue. Multilevel feedback queue allow process to move from one queue to

another.

End of Chapter 5