Operating SystemsCMPSC 473CPU Scheduling

September 07, 2010 - Lecture 5

Instructor: Bhuvan Urgaonkar

From last class

• Context switch: why store a subset of registers on process stack rather than on its PCB?– Memory reserved for kernel more precious than that for processes

Choosing the Right Scheduling

Algorithm/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)

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

• Turnaround time – amount of time to execute a particular process

• Fairness

Shortest-Job-First (SJF) Scheduling

• Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time

• SJF is optimal for avg. waiting time – gives minimum average waiting time for a given set of processes– In class: Compute average waiting time for the previous example with SJF

– Exercise: Prove the optimality claimed above

Why Pre-emption is Necessary

• To improve CPU utilization– Most processes are not ready at all times during their

lifetimes– E.g., think of a text editor waiting for input from the

keyboard– Also improves I/O utilization

• To improve responsiveness– Many processes would prefer “slow but steady progress”

over “long wait followed by fast process”

• Most modern CPU schedulers are pre-emptive

SJF: Variations on the theme

• Non-preemptive: once CPU given to the process it cannot be

preempted until completes its CPU burst - the SJF we already saw

• Preemptive: if a new process arrives with CPU length less

than remaining time of current executing process, preempt. This scheme is know as Shortest-Remaining-Time-First (SRTF) Also called Shortest Remaining Processing Time (SRPT)

• Why SJF/SRTF may not be practical CPU requirement of a process rarely known in advance

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.

• Performance– q large => FCFS– q small => q must be large with respect to context switch, otherwise overhead is too high

Example of RR with Time Quantum = 20

Process CPU Time

P1 53

P2 17

P3 68

P4 24

• The Gantt chart is:

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

P1 P2 P3 P4 P1 P3 P4 P1 P3 P3

0 20 37 57 77 97 117 121 134 154 162

Time Quantum and Context Switch Time

Turnaround Time Varies With Time Quantum

Proportional-Share Schedulers

• A generalization of round robin• Process Pi given a CPU weight wi > 0

• The scheduler needs to ensure the following– forall i, j, |Ti(t1, t2)/Tj(t1,t2) - wi/wj| ≤ e

– Given Pi and Pj were backlogged during [t1,t2]

• Who chooses the weights and how?

Lottery Scheduling[Carl Waldspurger, MIT,

~1995]• Perhaps the simplest proportional-share scheduler

• Create lottery tickets equal to the sum of the weights of all processes– What if the weights are non-integral?

• Draw a lottery ticket and schedule the process that owns that ticket– What if the process is not ready?

•Draw tickets only for ready processes– Exercise: Calculate the time/space complexity of the operations Lottery scheduling will involve

Lottery Scheduling Example

1 4

2 5

3 6

7 10

8 11

9 12

13

14

15

9

P1=6 P2=9

Schedule P2

1 4

2 5

3 6

7 10

8 11

9 12

13

14

15

3

P1=6 P2=9

Schedule P1

Lottery Scheduling Example

1 4

2 5 11

6

7 10

8

3 9 12

13

14

15

11• As t ∞, processes will get their share (unless they were

blocked a lot)• Problem with Lottery scheduling: Only probabilistic guarantee• What does the scheduler have to do

– When a new process arrives?– When a process terminates?

P1=6 P2=9

Schedule P2

Lottery Scheduling Example