Lecture 5, 6 and 7 cpu scheduling

Rushdi Shams, Dept of CSE, KUET 1
Operating Systems
CPU Scheduling
Version 1.0

Basic Concepts
The objective of multiprogramming is to run
processes all time, keeping the CPU busy.
Uniprocessing system is different. Only one process
can run at a time- means processes have to wait until
the CPU is free.

Basic Concepts
A process is executed until the completion of an IO
request
The CPU then sits idle and all this waiting time is
wasted
With multiprogramming, we keep several processes
in memory at one time.
When one process has to wait, the OS takes the CPU
away from that process and gives the CPU to another
process

Basic Concepts
Almost all computer resources are scheduled before
use. The CPU is one of the primary resources. Thus,
its scheduling is central to operating systems

CPU burst and IO burst

CPU Scheduler
 Selects from the processes in memory that are ready to execute,
and allocates the CPU to one of them
 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
 All other scheduling is preemptive

CPU Scheduler
In case of Non-preemptive scheduling, the process
does not release the CPU unless it is finished (or
terminated) or going to the waiting state.

Dispatcher
Dispatcher module gives control of the CPU to the
process selected by the short-term scheduler; this
involves:
switching context
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

Scheduling Criteria
CPU utilization
How much of the CPU is in operation, how much of it
is utilized to serve processes.
Measured in Percentage
Should be maximized

Scheduling Criteria
Throughput
Number of processes executed and served per unit
time.
Measured in process per unit time
Should be maximized

Scheduling Criteria
Turnaround time
Amount of time to execute a particular process
Interval between the submission of the process and
its completion
Should be minimized

Scheduling Criteria
Waiting time
amount of time a process has been waiting in the
ready queue
Should be minimized

Scheduling Criteria
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)
Should be minimized

First Come, First Served (FCFS)
The simplest CPU Scheduling algorithm
The process that requests CPU first is allocated the
CPU first
Easily managed with a FIFO queue

FCFS
Proces Burst Time
P1 24
P2 3
P3 3
 Suppose that the processes arrive in the order: P1 , P2 , P3
The Gantt Chart for the schedule is:
 Waiting time for P1 = 0; P2 = 24; P3= 27
 Average waiting time: (0 + 24 + 27)/3 = 17
P1 P2 P3
24 27 300

FCFS
Proces Burst Time
P1 24
P2 3
P3 3
Suppose that the processes arrive in the order
P2 , P3 , P1
 The Gantt chart for the schedule is:
 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
P1P3P2
63 300

Shortest Job First (SJF)
 Associate with each process the length of its next CPU burst.
Use these lengths to schedule the process with the shortest
time
 Two schemes:
 non-preemptive – 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 also called Shortest-Remaining-Time-First (SRTF)
 SJF is optimal – gives minimum average waiting time for a given
set of processes

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)
 Average waiting time = (0 + 6 + 3 + 7)/4 = 4
P1 P3 P2
73 160
P4
8 12

SRTF
Process Arrival Time Burst Time
P1 0.0 7
P2 2.0 4
P3 4.0 1
P4 5.0 4
 SJF (preemptive)
• Average waiting time = (9 + 1 + 0 +2)/4 = 3
P1 P3P2
42 110
P4
5 7
P2 P1
16

Priority Scheduling
The SJF Algorithm is special case of Priority
Scheduling
A priority is associated with each process and the CPU
is given to the process with higher priority
If processes have equal priority, FCFS is followed
SJF is priority algorithm where
priority p = inverse of CPU burst
Priorities are generally some fixed range of numbers
such as 0 to 7 or 0 to 4,095

Priority Scheduling
However, there is no agreement on the highest
priority- if it would be 0 or 4,095
Priorities can be external or internal
Internal priorities are number of files required to
execute the process, its memory usage, resources
required, time to compute, etc.
External priorities are set by outside computer-
mainly by human based on the choice of process,
economical factors, etc.

Priority Scheduling
The Average Waiting Time is (6+0+16+18+1)/5=8.2 ms

Priority Scheduling: Starvation
 Higher priority schedules prohibits lower priority
schedules for getting the CPU
 There are two possibilities-
1. If no mechanism is applied, lower priority schedules
will get CPU on Friday morning (that’s the holiday)
2. If mechanism applied, lower priority schedules will
be swapped away by the CPU- those that did not get
a chance for a specific period
 IBM 7094 was shut down in 1973… during theIBM 7094 was shut down in 1973… during the
shut down, MIT found a lower prioritizedshut down, MIT found a lower prioritized
process still ignored that was submitted in 1967!process still ignored that was submitted in 1967!

Priority Scheduling: Aging
Technique to increase the priority of the processes
If priorities are ranged from 0 to 127 (127 is the highest
priority), then we can increase the priority of a process
for every 15 minutes.
The lowest prioritized process will now take no more
than 32 hours to get the CPU! 

Round-Robin (RR) Scheduling
Similar to FCFS but pre-emption is added to switch
between processes
A smaller unit of time called time quantum (or time
slice) is defined generally from 10 to 100 ms.
The ready queue is treated as circular queue
Scheduler goes around the ready queue, allots CPU to
each process for a time interval of up to 1 time
quantum

Round-Robin (RR) Scheduling
We need to treat the ready queue as FIFO
New processes are added at the end of the queue.
The CPU scheduler picks the first process from the
queue, sets a timer to interrupt the process after 1
time quantum and dispatches the process

RR Scheduling
2 things may happen-
The process may have a CPU burst less than time
quantum- the process will release the CPU
voluntarily.
CPU burst of the process is longer than time
quantum-the process will be put at the end of the
queue.

RR Scheduling
Time quantum is of 4 ms
Average waiting time: ((10-4)+4+7)/3 = 5.66 ms
Typically, higher average turnaround than SJF, but
better response

RR Scheduling
The performance heavily depends on the size of time
quantum
If the time quantum is very large, at one extreme, it
would be similar to FCFS
If the time quantum is very small, the RR approach is
called processor sharing- as it seems every process has
its own CPU
Smaller time quantum requires more context
switching.

Context switch with Quantum

Turnaround time with Quantum

80% of the CPU bursts
should be shorter than the
time quantum

Lecture 5, 6 and 7 cpu scheduling

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