CPU Scheduling
Tanenbaum Ch 2.4
Silberchatz and Galvin Ch 5
Basic Concepts
• CPU - I/O Burst Cycle
• CPU Burst Distribution
160
140
120
100
80
60
40
20
0
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8
16
24
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2
CPU Scheduling
• Decision points:
–
–
–
–
Process switch from running to waiting state
Process switch from running to ready state
Process switch from waiting to ready state
Process terminates
• Scheduling types
– non-preemptive
– preemptive
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Task Dispatcher
• Functions:
– Switching context
– Switching to user mode
– Jumping to PC location
• The time needed to stop one process and start
up another is known as dispatch latency.
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Categories of Scheduling Algorithms
Batch
Interactive
Real time
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
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Scheduling Algorithm Goals
Figure 2-39. Some goals of the scheduling algorithm under
different circumstances.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
Scheduling in Batch Systems
First-come first-served
Shortest job first
Shortest remaining Time next
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
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Scheduling Algorithms
(First-come, First-served)
• Determine avg waiting time if:
– (P1, P2, P3) or (P3, P2, P1)
Process
Burst Time
P1
P2
P3
24
3
3
0
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10
20
30
8
Scheduling Algorithms
(First-come, First-served)
• Determine avg waiting time if:
– (P1, P2, P3)
Process
Burst Time
P1
P2
P3
24
3
3
0
10
Waiting time:
P1: 0
P2: 24
P3: 27
avg: 17
20
30
P1 ……………………………………………..P2…..P3…….
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Scheduling Algorithms
(First-come, First-served)
• Determine avg waiting time if:
–
(P3, P2, P1)
Process
Burst Time
P1
P2
P3
24
3
3
0
10
Waiting time:
P1: 6
P2: 3
P3: 0
avg: 3
20
30
P3….P2….P1………………………………………………...
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Scheduling Algorithms
(Shortest Job First)
• Schedule jobs based on their length.
• May be preemptive
– If a job is queued that is shorter than the remaining
time on the current job, there is a switch.
• May be non-preemptive.
– Once a job has started, it works to its normal switch.
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Scheduling Algorithms
(Shortest Job First)
• Non-Preemptive / preemptive SJF
Process
P1
P2
P3
P4
0
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Arr. Time Burst Time
0.0
7
2.0
4
4.0
1
5.0
4
10
20
30
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Scheduling Algorithms
(Shortest Job First)
• Preemptive SJF
Process
P1
P2
P3
P4
Arr. Time Burst Time
0.0
7
2.0
4
4.0
1
5.0
4
P1
|
0
10
20
30
P1..
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Scheduling Algorithms
(Shortest Job First)
• Preemptive SJF
Process
P1
P2
P3
P4
P1
|
Arr. Time Burst Time
0.0
7
2.0
4
4.0
1
5.0
4
P2
|
0
10
20
30
P1..P2..
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Scheduling Algorithms
(Shortest Job First)
• Preemptive SJF
Process
P1
P2
P3
P4
P1
|
P2
|
Arr. Time Burst Time
0.0
7
2.0
4
4.0
1
5.0
4
P3
|
0
10
20
30
P1..P2..P3
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Scheduling Algorithms
(Shortest Job First)
• Preemptive SJF
Process
P1
P2
P3
P4
P1
|
P2
|
Arr. Time Burst Time
0.0
7
2.0
4
4.0
1
5.0
4
P3P4
| |
0
10
20
30
P1..P2..P3P2..
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Scheduling Algorithms
(Shortest Job First)
• Preemptive SJF
Process
P1
P2
P3
P4
P1
|
P2
|
0
Arr. Time Burst Time
0.0
7
2.0
4
4.0
1
5.0
4
P3P4
| |
10
20
30
P1..P2..P3P2..P4…….
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Scheduling Algorithms
(Shortest Job First)
• Preemptive SJF
Process
P1
P2
P3
P4
P1
|
P2
|
0
Arr. Time Burst Time
0.0
7
2.0
4
4.0
1
5.0
4
P3P4
| |
10
20
30
P1..P2..P3P2..P4…….P1……….
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Scheduling Algorithms
(Shortest Job First)
•
Non-preemptive SJF
Process
P1
P2
P3
P4
P1
|
P2
|
0
Arr. Time Burst Time
0.0
7
2.0
4
4.0
1
5.0
4
P3P4
| |
10
20
30
P1…………..P3P2…….P4…….
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Scheduling Algorithms
(Shortest Job First)
• Determining the length of the jobs.
• Use weighted average burst lengths
n 1 * tn (1 ) * n
0 1
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Scheduling Algorithms
(Shortest Job First)
1
2
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Scheduling in Interactive Systems
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Round-robin scheduling
Priority scheduling
Multiple queues
Shortest process next
Guaranteed scheduling
Lottery scheduling
Fair-share scheduling
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
22
Round Robin Scheduling
• Developed in response to time-sharing
systems.
• Each job is given control of the CPU for a
short period - a time quantum. (In the range
of 10-100 milliseconds.)
• Control then passes to the next job (FIFO?)
• Average waiting time dependent on job size,
quantum size.
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Round-Robin Scheduling
Figure 2-41. Round-robin scheduling.
(a) The list of runnable processes. (b) The list of runnable
processes after B uses up its quantum.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
Round Robin Scheduling
Process
Burst Time
P1
P2
P3
6
3
1
P4
7
0
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All processes arrive at T=0
Vary Quantum time from
1 to 7. Calculate average
turnaround time (to job
completion).
20
30
25
Round Robin Scheduling
Process
Burst Time
P1
P2
P3
6
3
1
P4
7
0
All processes arrive at T=0
Quantum Average
Size
Turnaround
1
11
10
20
30
P1P2P3P4P1P2P4P1P2P4P1P4P1P4P1P4P4
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Round Robin Scheduling
Process
Burst Time
P1
P2
P3
6
3
1
P4
7
0
All processes arrive at T=0
Quantum Average
Size
Turnaround
1
11
2
11.5
10
20
30
P1..P2..P3P4..P1..P2P4..P1..P4..P4
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Round Robin Scheduling
Process
Burst Time
P1
P2
P3
6
3
1
P4
7
0
All processes arrive at T=0
Quantum
Average
Size
Turnaround
1
11
2
11.5
3
10.75
10
20
30
P1. . P2. . P3P4. . P1. . P4. . .P4
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Round Robin Scheduling
avg. turnaround
13
12
11
10
9
8
1
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2
3
4
5
6
7
Quantum
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Priority Scheduling
• Each process (or class of processes) is
given a priority.
• Jobs are executed in priority order
• Issue: If there are sufficient high priority
jobs, lower priority jobs may never get
scheduled.
• One solution: Aging of processes. After a
job has been in queue for a given period of
time, raise its priority.
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Priority Scheduling
Figure 2-42. A scheduling algorithm with four priority classes.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
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Multi-level queue Scheduling
• Partition the job space into distinct classes or
queues
– foreground / background
– system / interactive / editing / batch / student
– etc.
• Independently assign queueing service
disciplines
• Assign queue priorities
– Highest to lowest (empty one queue before starting next)
– Divide time between queues
• 80 / 20
• 30 / 20 / 20 / 20 / 10
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Multilevel Feedback Queue
Scheduling
• Similar to Multilevel queue scheduling, but
jobs are allowed to move between queues.
• Avoids process starvation by allowing
neglected jobs to move up to a higher
queue.
• Differentiate queues by (for example)
quantum size
– queue 1 = 8 ms
– queue 2 = 16 ms
– queue 3 = FCFS
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Thread Scheduling (1)
Figure 2-43. (a) Possible scheduling of user-level threads with a
50-msec process quantum and threads that run 5 msec per
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CPU burst.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
Thread Scheduling (2)
Figure 2-43. (b) Possible scheduling of kernel-level threads with
the same characteristics as (a).
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
Multiple Processor Scheduling
• CPU scheduling more complex when
multiple CPUs are available
• Homogeneous processors within a
multiprocessor are usual (heterogeneous
processors found in distributed systems).
• Load sharing used with SMP.
– Single queue, multiple servers.
• Asymmetric Multiprocessing (AMP)
– Assign roles (system control, application processing)
– Avoids system data sharing
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Benefits of Multiprocessors
Process Scheduling
– Process inter-arrival time variation can be characterized
by coefficient of variation = s / s
• standard deviation of interarrival time / mean service time
• ratio of 1 = exponential distribution. (your mileage may vary…)
– Differences between scheduling algorithms become
much less important in multiprocessor systems
RR to FCFS
thruput ratio
Single processor
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Dual processor
Coefficient of Variation
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Benefits of Multiprocessors
Thread Scheduling
• Allows true parallel processing within an
application
• However, if there is significant interaction
among threads, small differences in thread
management & scheduling can have big
results.
• General Approaches used
– Load Sharing - (not necessarily load balancing)
– Gang Scheduling - Schedule related threads together
– Dynamic Scheduling - allow # of threads to vary
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Real-Time Scheduling
• System Response Classifications
– Hard real-time System - requires response
guarantees
– Soft real-time System – response not guaranteed
• Dispatch Latency
Interrupt
processing
Response interval
Dispatch latency
conflicts
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Real-time
process
execution
dispatch
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Summary
• Primary job of an OS is to schedule
processes
– Batch
– Interactive
– Real Time
• Selection of scheduling algorithm
depends on optimization criteria
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Questions
•
•
•
•
•
On a system with multilevel queue scheduling, would you expect to
see the same scheduling algorithms used on all levels, or would you
expect them to be different? Justify your answer.
Consider the following set of processes, with the length of the CPU
burst time given in milliseconds. The processes are assumed to have
arrived in the order P1, P2, P3, P4.
Process
Job Size
P1
10
P2
1
P3
2
P4
5
Of the scheduling algorithms FCFS, SJF, and RR(qt=2) which
algorithm offers the best waiting time?
In many multiprocessing systems, although there is a fixed limit to the
amount of time that a job can keep control of the CPU, in practice, the
jobs are released earlier. Why?
The book talked about the multiple queueing system used by CTSS. If
a process needed 30 quanta to complete, how many times would it be
swapped in to complete?
How does Lottery Scheduling work? What is its principle advantage
over, say, Guaranteed Scheduling?
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