CPU Scheduling

Chapter 5

CPU Scheduling

• Basic Concepts

• Scheduling Criteria

• Scheduling Algorithms

• Multiple-Processor Scheduling

• Thread Scheduling

• UNIX example

Basic Concepts

• Maximum CPU utilization obtained with multiprogramming

• CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait

• CPU times are generally much shorter than I/O times.

CPU-I/O Burst Cycle

Process AProcess B

Histogram of CPU-burst Times

Schedulers

• Process migrates among several queues– Device queue, job queue, ready queue

• Scheduler selects a process to run from these queues• Long-term scheduler:

– load a job in memory– Runs infrequently

• Short-term scheduler:– Select ready process to run on CPU– Should be fast

• Middle-term scheduler– Reduce multiprogramming or memory consumption

CPU Scheduler

• CPU scheduling decisions may take place when a process:1. Switches from running to waiting state (by sleep).2. Switches from running to ready state (by yield).3. Switches from waiting to ready (by an interrupt).4. Terminates (by exit).

• Scheduling under 1 and 4 is nonpreemptive.• All other scheduling is preemptive.

Dispatcher

• Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves:– switching context– switching to user mode– 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

Scheduling Criteria

• CPU utilization – keep the CPU as busy as possible• Throughput – # of processes that complete their execution

per time unit• Turnaround time (TAT) – amount of time to execute a

particular process• 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)

“The perfect CPU scheduler”• Minimize latency: response or job completion time • Maximize throughput: Maximize jobs / time. • Maximize utilization: keep I/O devices busy.

– Recurring theme with OS scheduling

• Fairness: everyone makes progress, no one starves

Scheduling Algorithms FCFS

• First-come First-served (FCFS) (FIFO)– Jobs are scheduled in order of arrival– Non-preemptive

• Problem:– Average waiting time depends on arrival order– Troublesome for time-sharing systems

• Convoy effect short process behind long process

• Advantage: really simple!

First Come First Served Scheduling

• Example: Process Burst TimeP1 24

P2 3 P3 3

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

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

P1P3P2

63 300

P1 P2 P3

24 27 300

Waiting time for P1 = 0; P2 = 24; P3 = 27

Average waiting time: (0 + 24 + 27)/3 = 17

Waiting time for P1 = 6; P2 = 0; P3 = 3

Average waiting time: (6 + 0 + 3)/3 = 3

Shortest-Job-First (SJR) 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: – nonpreemptive – 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)

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

Shortest Job First Scheduling

• Example: Process Arrival Time Burst Time

P1 0 7

P2 2 4

P3 4 1

P4 5 4

• Non preemptive SJF

P1 P3 P2

7

P1(7)

160

P4

8 12

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

2 4 5

P2(4)

P3(1)

P4(4)

P1‘s wating time = 0

P2‘s wating time = 6

P3‘s wating time = 3

P4‘s wating time = 7

Shortest Job First SchedulingCont’d

• Example: Process Arrival Time Burst Time

P1 0 7

P2 2 4

P3 4 1

P4 5 4

• Preemptive SJFP1 P3P2

42 110

P4

5 7

P2 P1

16

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

P1(7)

P2(4)

P3(1)

P4(4)

P1‘s wating time = 9

P2‘s wating time = 1

P3‘s wating time = 0

P4‘s wating time = 2

P1(5)

P2(2)

Shortest Job First SchedulingCont’d

• Optimal scheduling

• However, there are no accurate estimations to know the length of the next CPU burst

• Optimal for minimizing queueing time, but impossible to implement. Tries to predict the process to schedule based on previous history.

• Predicting the time the process will use on its next schedule:

t( n+1 ) = w * t( n ) + ( 1 - w ) * T( n )

Here: t(n+1) is time of next burst.

t(n) is time of current burst.

T(n) is average of all previous bursts .

W is a weighting factor emphasizing current or

previous bursts.

Shortest Job First SchedulingCont’d

• A priority number (integer) is associated with each process• The CPU is allocated to the process with the highest priority

(smallest integer highest priority in Unix but lowest in Java).– Preemptive– Non-preemptive

• 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.

Priority Scheduling

Round Robin (RR)

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

• time quantum = 20Process Burst Time Wait TimeP1 53 57 +24 = 81 P2 17 20 P3 68 37 + 40 + 17= 94 P4 24 57 + 40 = 97

Round Robin Scheduling

P1 P2 P3 P4 P1 P3 P4 P1 P3 P3

0 20 37 57 77 97 117 121 134 154 162

Average wait time = (81+20+94+97)/4 = 73

57

20

37

57

24

40

40

17

P1(53)

P2(17)

P3(68)

P4(24)

P1(33) P1(13)

P3(48) P3(28) P3(8)

P4(4)

• Typically, higher average turnaround than SJF, but better response.

• Performance– q large FCFS– q small q must be large with respect to context switch,

otherwise overhead is too high.

Round Robin Scheduling

Turnaround Time Varies With The Time Quantum

TAT can be improved if most processfinish their next CPU burst in a singletime quantum.

Multilevel Queue• Ready queue is partitioned into separate queues:

EX:• foreground (interactive)

background (batch)

• Each queue has its own scheduling algorithmEX– 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;EX80% to foreground in RR20% to background in FCFS

Multilevel Queue Scheduling

Multi-level Feedback QueuesImplement multiple ready queues

– Different queues may be scheduled using different algorithms– Just like multilevel queue scheduling, but assignments are not static

• Jobs move from queue to queue based on feedback– Feedback = The behavior of the job,

• EX does it require the full quantum for computation, or • does it perform frequent I/O ?

• Need to select parameters for:– Number of queues– Scheduling algorithm within each queue– When to upgrade and downgrade a job

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 FCFS.

When it gains CPU, job receives 8 milliseconds (RR). If it does not finish in 8 milliseconds, job is moved to queue Q1.

– At Q1 job is again served FCFS and receives 16 additional milliseconds (RR). If it still does not complete, it is preempted and moved to queue Q2.

– AT Q2 job is served FCFS

Multilevel Feedback Queues

Multiple-Processor Scheduling• CPU scheduling more complex when multiple

CPUs are available Different rules for homogeneous or heterogeneous

processors.• Load sharing in the distribution of work, such that

all processors have an equal amount to do.• Asymmetric multiprocessing – only one processor

accesses the system data structures, alleviating the need for data sharing

• Symmetric multiprocessing (SMP) – each processor is self-scheduling Each processor can schedule from a common ready

queue OR each one can use a separate ready queue.

Thread Scheduling

• On operating system that support threads the kernel-threads (not processes) that are being scheduled by the operating system.

• Local Scheduling (process-contention-scope PCS )– How the threads library decides which thread to put onto an available LWP

• PTHREAD_SCOPE_PROCESS

• Global Scheduling (system-contention-scope SCS )– How the kernel decides which kernel thread to run next

• PTHREAD_SCOPE_PROCESS

Linux Scheduling• Two algorithms: time-sharing and real-time• Time-sharing

– Prioritized credit-based – process with most credits is scheduled next

– Credit subtracted when timer interrupt occurs– When credit = 0, another process chosen– When all processes have credit = 0, recrediting occurs

• Based on factors including priority and history

• Real-time– Defined by Posix.1b – Real time Tasks assigned static priorities. All other tasks

have dynamic (changeable) priorities.

The Relationship Between Priorities and Time-slice length

List of Tasks Indexed According to Prorities

Conclusion

• We’ve looked at a number of different scheduling algorithms.

• Which one works the best is application dependent.

– General purpose OS will use priority based, round robin, preemptive

– Real Time OS will use priority, no preemption.

References

• Some slides from– Text book slides– Kelvin Sung - University of Washington,

Bothell– Jerry Breecher - WPI– Einar Vollset - Cornell University