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Chapter 6: CPU Scheduling
Operating System Concepts – 9th Edition
Modified by Dr. Neerja Mhaskar for CS 3SH3
Silberschatz, Galvin and Gagne ©2013
Basic Concepts
 OS schedules almost all resources available to the
system.
 CPU being the primary resource, scheduling is central
to OS design.
 Process alternate between the following two states in
a continuing cycle represented by a CPU-I/O Burst
Cycle.

CPU execution (CPU burst )

I/O wait (I/O burst)
Operating System Concepts – 9th Edition
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CPU Scheduler
 Short-term scheduler (aka CPU scheduler) selects from
among the processes in ready queue, and allocates the CPU to
one of them

Queue may be ordered in various ways

Nodes are PCBs (Process Control Blocks)
 CPU scheduling decisions take place when a process
1. Switches from running to waiting state
2. Switches from running to ready state
3. Switches from waiting to ready
4.
Terminates
Operating System Concepts – 9th Edition
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CPU Scheduler Cont.
 When scheduling takes place under conditions 1 and 4
the scheduling scheme is called nonpreemptive.

Process assigned to CPU, keeps the CPU till it
terminates or changes its state to waiting.
 When scheduling takes place under conditions 2 and 3
the scheduling scheme is called preemptive.

Processes can be removed from the CPU.
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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
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Scheduling 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

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 timesharing environment)

Scheduling Algorithm Optimization Criteria

Max CPU utilization

Max throughput

Min turnaround time

Min waiting time

Min response time
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First- Come, First-Served (FCFS) Scheduling
 Process requesting the CPU first is allocated the CPU first.

The implementation of the FCFS policy can be achieved with a
FIFO queue.
 FCFS scheduling algorithm is non preemptive.
 Disadvantage: Average CPU waiting time for a process to use CPU is
long.
 Example: Suppose processes P1, P2, P3 have CPU burst time
24,3,3 respectively.
 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
 The waiting time for P1 = 0; P2 = 24; P3 = 27
 Average waiting time: (0 + 24 + 27)/3 = 17
Operating System Concepts – 9th Edition
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Silberschatz, Galvin and Gagne ©2013
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

Results in lower CPU and device utilization
Operating System Concepts – 9th Edition
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Silberschatz, Galvin and Gagne ©2013
Shortest-Job-First (SJF) Scheduling
 Associated with each process is the length of its next CPU burst
 Use these lengths to schedule the process with the shortest time
 SJF is optimal – gives minimum average waiting time for a given
set of processes
 The difficulty is knowing the length of the next CPU request, and
therefore it is generally not implemented at the level of short-term
CPU scheduling.
 Can however estimate the length of the next CPU burst. It is
done by exponential averaging.
 SJF algorithm can either be preemptive or nonpreemptive
 Preemptive version of SJF is called shortest-remaining-time-
first
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Example of SJF
ProcessArrival Time
CPU Burst Time
P1
0.0
6
P2
2.0
8
P3
4.0
7
P4
5.0
3
 SJF scheduling chart
P4
0
P1
3
P3
9
P2
16
24
 Average waiting time = (3 + 9 + 16 + 0) / 4 = 7
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Example of Shortest-remaining-time-first
 Now we add the concepts of varying arrival times and preemption to
the analysis
ProcessAarri Arrival TimeT
Burst Time
P1
0
8
P2
1
4
P3
2
9
P4
3
5
 Preemptive SJF Gantt Chart
P1
0
P2
1
P4
5
P1
10
P3
17
26
 Average waiting time = [(10-1)+(1-1)+(17-2)+5-3)]/4 = 26/4 = 6.5
msec
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Priority Scheduling
 A priority number (integer) is associated with each process

Equal priority process scheduled in FCFS order.
 The CPU is allocated to the process with the highest priority
(smallest integer  highest priority)

Preemptive

Nonpreemptive
 SJF is priority scheduling where priority is the inverse of 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 Priority Scheduling
ProcessAarri Burst Time(ms)T
Priority
P1
10
3
P2
1
1
P3
2
4
P4
1
5
P5
5
2
 Priority scheduling Gantt Chart
P2
P5
P1
P3
P4
 Average waiting time = (0 + 1 + 6 + 16 + 18 )/5 = 8.2 ms (milliseconds)
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Round Robin (RR)
 Each process gets a small unit of CPU time (time quantum q),
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.
 Preemptive Scheduling
 Timer interrupts every quantum to schedule next process
 Performance

q large  FIFO

q small  q must be large with respect to context switch,
otherwise overhead is too high
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Example of RR with Time Quantum = 4
Process
P1
P2
P3
Burst Time
24
3
3
 The Gantt chart is:
P1
0
P2
4
P3
7
P1
10
P1
14
P1
18
P1
22
P1
26
30
 Typically, higher average turnaround than SJF, but better
response
 q should be large compared to context switch time
 q usually 10ms to 100ms, context switch < 10 usec
Operating System Concepts – 9th Edition
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Multilevel Queue and Multilevel Feedback Queue
 Multilevel Queue Scheduling: Ready queue is partitioned into
various separate queues. Process permanently in a given queue.

Each queue has its own scheduling algorithm

Scheduling must be done between the queues:


Fixed priority preemptive scheduling: Serve processes of
highest priority first. Possibility of starvation.

Time slice – each queue gets a certain amount of CPU time
which it can schedule amongst its processes.
Disadvantage: Since processes are permanently assigned to a
given queue, it is inflexible.
 In contrast, in Multilevel Feedback Queue Scheduling a process
can move between the various queues.

Process in low priority queue can be moved to high priority
queue, and aging can be implemented this way.
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Thread Scheduling
 When threads supported, threads scheduled, not
processes
 To run on CPU user level threads must be mapped
to an associated kernel level thread.
 POSIX Pthreads allows setting scheduling
parameters during thread creation.
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Multiple-Processor Scheduling
 CPU scheduling more complex when multiple CPUs are
available
 Asymmetric multiprocessing – only one processor
accesses the system data structures, alleviating the need
for data sharing
 Symmetric multiprocessing (SMP) – each processor is
self-scheduling, all processes in common ready queue, or
each has its own private queue of ready processes

Currently, most common
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Issues with SMP
 Issues with SMP:


Cache invalidated with process migration. High cost involved.

Process Affinity: Process has an affinity for the processor on which
it is currently running.

Scheduler schedules processes based on their affinity.
Load balancing attempts to keep workload evenly distributed

Push migration – periodic task checks load on each processor, and
if found pushes task from overloaded CPU to other CPUs

Pull migration – idle processors pulls waiting task from busy
processor
 Load balancing often counteracts the benefits of process affinity.
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Real-Time CPU Scheduling
 Real-time Operating Systems need to immediately respond to a
real-time process as soon as it requires CPU.

Soft real-time systems – no guarantee as to when critical
real-time process will be scheduled

Hard real-time systems – task must be serviced by its
deadline
 Therefore for real-time scheduling, scheduler must support
preemptive, priority-based scheduling algorithm.

Guarantees only soft real-time functionality.
 For hard real-time must also provide ability to meet deadlines
Operating System Concepts – 9th Edition
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Priority-based Scheduling
 Processes have new characteristics: periodic ones require CPU at
constant intervals

Has processing time t, deadline d, period p

0≤t≤d≤p

Rate of periodic task is 1/p
 Based on the above characteristics schedulers can assign priorities
according to processes deadline and rate requirements.
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Rate Monotonic Scheduling
 This algorithm schedules periodic tasks using static priority policy
with preemption.
 A priority is assigned based on the inverse of its period
 Shorter periods = higher priority and longer periods = lower priority
 Rate monotonic considered optimal, that is if a set of processes
cannot be scheduled by this algorithm it cannot be scheduled by
any other algorithm.
 For a process Pi if pi = period of Pi, ti= processing time of Pi, then
CPU utilization of Pi = ti/ pi
 CPU utilization bounded and not always possible to fully to
maximize CPU utilization.
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Rate – Monotonic Scheduling Example
Missed Deadlines with Rate Monotonic Scheduling
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Earliest Deadline First Scheduling (EDF)
 Priorities are assigned according to deadlines:
the earlier the deadline, the higher the priority;
the later the deadline, the lower the priority
 Theoretically optimal as theoretically CPU utilization is 100%, but
in practice it is impossible to achieved due to cost of context
switching and interrupt handling.
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Algorithm Evaluation (Optional)
 How to select CPU-scheduling algorithm for an OS?
 Determine criteria, then evaluate algorithms.
 Four evaluation methods are discussed:
 Deterministic modeling: Type of Analytic evaluation.

Given a predetermined load, computes performance of each
algorithm for it. For example, given the processes and their CPU
bursts, one can compute the average waiting times and evaluate
which algorithm gives least waiting times.

This type of modelling is simple and fast

Most cases, processes running on a system vary, hence this type
of modelling is not useful.
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Algorithm Evaluation Cont.
 Queuing Models: Describes the arrival of
processes, and CPU and I/O bursts probabilistically

Computer system described as network of
servers, each with queue of waiting processes

Knowing arrival rates and service rates

Computes utilization, average queue length,
average wait time, etc.
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Little’s Formula
 n = average queue length
 W = average waiting time in queue
 λ = average arrival rate into queue
 Little’s law – in steady state, processes leaving queue must equal
processes arriving, thus:
n=λxW

Valid for any scheduling algorithm and arrival distribution
 For example, if on average 7 processes arrive per second, and
normally 14 processes in queue, then average wait time per
process = 2 seconds
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Simulation and Implementation
 Simulations:

Provide more accurate evaluation of scheduling algorithm

Running Simulations involves programming a model of the
system and maintaining a clock.

Simulator modifies system state and logs the activities of the
devices, processes and scheduler.
 Implementation:

Most accurate way of evaluating a scheduling algorithm is to
implement it and put it in the Operating system and test it in
real time!
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End of Chapter 6
Operating System Concepts – 9th Edition
Silberschatz, Galvin and Gagne ©2013