Transcript CPU Scheduling

Chapter 5: CPU Scheduling
Operating System Concepts with Java – 8th Edition
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Chapter 5: CPU Scheduling
 Basic Concepts
 Scheduling Criteria
 Scheduling Algorithms
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Objectives
 To introduce CPU scheduling, which is the basis for
multiprogrammed operating systems
 To describe various CPU-scheduling algorithms
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Basic Concepts
 Maximum CPU utilization obtained with
multiprogramming
 CPU scheduling is central to operating system
design.
 CPU–I/O Burst Cycle – Process execution consists
of a cycle of CPU execution and I/O wait
 CPU burst distribution

I/O bound program has many short CPU bursts

CPU bound has few log CPU bursts.
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Alternating Sequence of CPU And I/O Bursts
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CPU Scheduler
 Selects from among the processes in memory that are ready to execute,
and allocates the CPU to one of them

Short term scheduler
 CPU scheduling decisions may 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
 Scheduling under 1 and 4 is nonpreemptive

Once the CPU has been allocated to a process, the process keeps
the CPU until it released the CPU either by terminating or waits
 All other scheduling is preemptive

Affect cost at sharing data and design of OS kernel
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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
(submit – complete)
 Waiting time – amount of time a process has been waiting in the ready
queue (not in CPU or I/O)
 Response time – amount of time it takes from when a request was
submitted until the first response is produced, not output (for timesharing environment)
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Scheduling Algorithm Optimization Criteria
 Max CPU utilization
 Max throughput
 Min turnaround time
 Min waiting time
 Min response time
In most cases, it optimize the average measure but some times need
to optimize the minimumor maximum values rather than average.
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Scheduling algorithms
CPU scheduling deals with the problem of deciding which of the
processes in the ready queue is to be allocated the CPU.
First-come, first served scheduling
Shortest-job first scheduling
Priority scheduling
Round-robin scheduling
Multilevel queue scheduling
Multilevel feedback queue scheduling
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1. First-Come, First-Served (FCFS) Scheduling
• Simple - process that requests the CPU first is allocated the CPU first
• Implementation : FIFO queue
Process
P1
P2
P3
Burst Time
24
3
3
 Suppose that the processes arrive in the order: P1 , P2 , P3
The Gantt Chart (a bar chart that illustrates a particular scheduler including
start and finish times of each processes) 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
 Note : the average waiting time under FCFS policy is often quite long
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1. FCFS Scheduling (Cont.)
Suppose that the processes arrive in the order:
P2 , P3 , P1
 The Gantt chart for the schedule is:
P2
P3
P1
0
3
6
 Waiting time for P1 = 6; P2 = 0; P3 = 3
30
 Average waiting time: (6 + 0 + 3)/3 = 3
 Much better than previous case
 Convoy effect short process behind long process

All the processes waits for the one big process to get off the CPU.
→ lower CPU device utilization
Note: FCFS is nonpreemptive
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2. Shortest-Job-First (SJF) Scheduling
In this algorithm :
Associate with each process the length of its next CPU burst. Use these lengths
to schedule the process with the shortest time – when CPU is available, it is
assigned the process that has the smallest next CPU burst.
Two schemes ( the choice arises when a new process arrives at the ready queue
while a previous process is still executing) :

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 know
as the shortest-Remaining-Time-First(SRTF)

It may called (Shortest Next CPU Burst) algorithm because it depends
on next CPU burst not total length
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
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Example of SJF
Process
Arrival Time
Burst Time
P1
0.0
6
P2
2.0
8
P3
4.0
7
P4
5.0
3
 SJF scheduling chart
P1
P4
0
3
P3
9
P2
16
24
 Average waiting time = (3 + 16 + 9 + 0) / 4 = 7
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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
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
P2
P3
P2
P4
11
2
4
5
7
 Average waiting time = (9 + 1 + 0 +2)/4 = 3
0
P1
16
 Refer to the book for more examples page 192
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3. Priority Scheduling
 SJF is a special case of general Priority scheduling algorithm.
 A priority number (integer) is associated with each process
 The CPU is allocated to the process with the highest priority
(smallest integer  highest priority → not a standard)

Preemptive

Nonpreemptive
 What about equal priority processes?

FCFS
 SJF is a priority scheduling where priority is the predicted next
CPU burst time
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3. Priority Scheduling(cont’)
 Problem

Starvation (indefinite blocking) – low priority processes may
never execute – leave it waiting

Note : a process is ready to run but waiting for the CPU can be
considered blocked.
 Solution

Aging
 Increase
the priority of processes that wait in the system for
a long time.
 As
time progresses increase the priority of the process
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Example of Priority scheduling
Process
Burst Time
Priority
P1
10
3
P2
1
1
P3
2
4
P4
1
5
P5
5
2
 priority scheduling chart
 Average waiting time = (6 + 0 + 16 +18 + 1)/5 = 8.2
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4. Round Robin (RR)
 Designed especially for tie- sharing systems .
 Similar to FCFS but preemption is added to enable switching.
 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 → treated as circular 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.

Ex: with 5 processes and time quantum of 20 ms → each process will
get up to 20 ms every 100 ms.
 Performance(depends on the size of the time quantum)

q large  FIFO

q small  q must be large with respect to context switch, otherwise
overhead is too high
 It is preemptive and waiting time is long often
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Example of RR with Time Quantum = 4
Process
P1
Burst Time
24
P2
P3
3
3
 The Gantt chart is:
P1
0
P2
4
P3
7
P1
10
P1
14
P1
18 22
P1
26
P1
30
 Typically, higher average turnaround than SJF, but better response
 Turnaround time depends on the size of the quantum
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Time Quantum and Context Switch Time
We need to consider the effect of context switching on performance of RR
Scheduling
How a smaller time quantum increase context switches?
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4. Multilevel Queue
 Classify processes in to different groups.
 Ready queue is partitioned into separate queues:

foreground (interactive)

background (batch)
 Each queue has its own scheduling algorithm. Example:

foreground – RR

background – FCFS
 Scheduling must be done between the queues. Possibilities:

Fixed priority scheduling(preemptive); (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; i.e., 80% to foreground in RR

20% to background in FCFS
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Multilevel Queue Scheduling
Five queues in order of priority using the first possibility
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Multilevel Feedback Queue
 In multilevel queues, processes do not move from one queue to another →
low scheduling overhead but inflexible.
 A process can move between the various queues; aging can be implemented
this way

Idea: separate processes according to the characteristic of this CPU
bursts

If process uses too much CPU time → lower priority queue if it waits too
long, may be moved to higher priority queue.
 Multilevel-feedback-queue scheduler defined by the following parameters:

number of queues

scheduling algorithms for each queue

method used to determine when to upgrade a process

method used to determine when to demote a process

method used to determine which queue a process will enter when that
process needs service
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Example of Multilevel Feedback Queue
 Three queues:

Q0 – RR with time quantum 8 milliseconds


Q1 – RR time quantum 16 milliseconds


High priority
If not completed → preempt and go to Q2
Q2 – FCFS

Run only if queue 0 & 1 are empty
 Scheduling

A new job enters queue Q0 which is served FCFS. When it gains CPU,
job receives 8 milliseconds. 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. If it still does not complete, it is preempted and moved to
queue Q2.
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Multilevel Feedback Queues
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End of Chapter 5
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