Transcript Module 6: CPU Scheduling

Chapter 5: CPU Scheduling
Scheduler
 What is the job of a scheduler
 Terms

CPU burst

IO burst (???)
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Scheduling
 Which processes get control of the CPU and for
how long
 One of the most important jobs for the OS
 Can have a huge impact on system
performance
 But why?
 To understand let’s examine the behavior of a
typical program…

Programs typically operate in bursts

Some bursts involve CPU intensive
operations

Some involve I/O operations
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What criteria?
 What makes a good scheduling algorithm?

Average wait time?

Processor utilization?
 What’s more important: user happiness or
processor utilization?
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Some 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: from time of job submission to job completion
 Waiting time – amount of time a process has been waiting
in the ready queue
 Response time – similar to waiting time, but instead of
measuring the total amount of time spent in the ready
queue, only count the time from a request for data (like a
mouse click) to the time when “begin” to respond.
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What control does the OS have?
 Determines who is next in line to get access to the CPU
 Determines how long they get the CPU
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Preemptive or Non-preemptive
 If allow a process to run until it is done or it performs I/O

Non-preemptive or cooperative
 If put a limit on the amount of time that a process can stay on
the CPU

Preemptive
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Dispatcher
 Conceptually broken up into two separate
tasks

Scheduling: determine which process
is next

Dispatch: actual process of context
switch

Save state

Move process to ready Q

Bring in next process (set PC)

Re-establish state
 Dispatch latency – time it takes for the
dispatcher to stop one process and start
another running
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Scheduling Algorithms
 There are a number of common scheduling algorithms

First come, first served

Shortest job first

Priority scheduling

Round robin
 We will examine strengths and weaknesses of each
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First-Come, First-Served (FCFS) Scheduling
 Note: no preemption
Process
P1
P2
P3
Burst Time
24
3
3
 Suppose that the processes arrive in the order: P1 , P2 , P3
The Gantt Chart for the schedule is:
P1
P2
0
24
P3
27
30
 Waiting time for P1 = 0; P2 = 24; P3 = 27
 Average waiting time: (0 + 24 + 27)/3 = 17
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FCFS Scheduling (Cont.)
Suppose that the processes arrive in the order
P2 , P3 , P1
 The Gantt chart for the schedule is:
P2
0
P3
3
P1
6
30
 Waiting time for P1 = 6; P2 = 0; P3 = 3
 Average waiting time: (6 + 0 + 3)/3 = 3
 Much better than previous case
 Convoy effect short process behind long process
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Round Robin (RR)
 Similar to FCFS but with
preemption

After quantum (10 to 100 ms)
go to back of line

Quantum must be large with
respect to context switch,
otherwise overhead is too
high
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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
 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
SJF Optimal for
Wait Time
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Example of Non-Preemptive SJF
Process
Arrival Time
Burst Time
P1
0.0
7
P2
2.0
4
P3
4.0
1
P4
5.0
4
 SJF (non-preemptive), P1 goes first because no other process is in
the Q when it arrives
P1
0
3
P3
7
P2
8
P4
12
16
 Average waiting time = (0 + 6 + 3 + 7)/4 = 4
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Example of Preemptive SJF
Process
Arrival Time
Burst Time
P1
0.0
7
P2
2.0
4
P3
4.0
1
P4
5.0
4
 SJF (preemptive)
P1
0
P2
2
P3
4
P2
5
P4
P1
11
7
16
 Average waiting time = (9 + 1 + 0 +2)/4 = 3
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Priority Scheduling
 A priority number (integer) is associated with each process
 The CPU is allocated to the process with the highest priority
(smallest integer  highest priority)

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
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Example of RR with Time Quantum = 20
Process
P1
P2
P3
P4
 The Gantt chart is:
P1
0
P2
20
P3
37
Burst Time
53
17
68
24
P4
57
P1
77
P3
97 117
P4
P1
P3
P3
121 134 154 162
 Typically, higher average turnaround (start to finish) than SJF, but
better response (first access to CPU)
 Why can’t we say this definitively? Why typically?
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Multilevel Queue
 Ready queue is partitioned into
separate queues:
foreground (interactive)
background (batch)
 Each queue has its own
scheduling algorithm

For example

foreground – RR

background – FCFS
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Multilevel Queue
 Scheduling must be done between the queues: which Q does the
OS pull-from

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; e.g.

80% to foreground in RR

20% to background in FCFS
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Multilevel Feedback Queue
 If allow process behavior to dictate Q…
 Each Q might have different:

Scheduling algorithms

Priority

Quantum
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Multiple-Processor Scheduling
 Can have Q’s for each processor or one Q to service all

Where does scheduler run?
 Must design for

Symmetric multiprocessing

Asymmetric multiprocessing


only one processor accesses the system data
structures, alleviating the need for data sharing
Load sharing
 If each proc has own cache…

Best if a process stays
with a given processor
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Operating System Examples

Solaris scheduling
 6 classes of apps each with different
scheduling algorithms and priorities
Good response time for interactive
processes
 Good throughput for CPU bound
 Windows XP scheduling
 Variable priority

All three support
•Priority scheduling
•Preemption
•Server threads
Window with focus gets high pri
 Give high-I/O procs higher pri’s
 CPU bound lower
 I/O devices stay busy
 CPU bound use CPU cycles in background
 Linux scheduling
 Two priority ranges: real-time and other
 Higher priority tasks get longer quanta
(unlike Solaris and Windows)

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Algorithm Evaluation
 How select a CPU-scheduling
algorithm?
 Best approach is to generate a “typical”
workload and run on simulator with
different schedulers

deterministic modeling

How describe

List of jobs including
–
start-time
–
list of times and durations
»
CPU and I/O bursts
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End of Chapter 5