| CPU Scheduling
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CPU Scheduling
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Objectives
• Describe various CPU-scheduling algorithms
• Describe various multi- processor CPU-scheduling
algorithms
• To discuss evaluation criteria for selecting a CPU-
scheduling algorithm for a particular system
CPU scheduling 2
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Basic Concepts
• Almost all computer resources are scheduled before use.
The CPU is one of the primary computer resources.
• CPU scheduling is the basis of multi-programmed operating
systems.
• What are multi-programmed operating systems?
• What are the benefits of multiprogramming?
CPU scheduling 3
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Basic Concepts
• In a single-processor system, only one process can run
at a time; any others must wait until the CPU is free
and can be rescheduled.
• The objective of multiprogramming is to have some
process running at all times, to maximize CPU
utilization. How is multiprogramming achieved?
CPU scheduling 4
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Cycle of CPU execution and I/O wait
• The success of CPU scheduling depends on an observed
property of processes:
» Process execution consists of a cycle of
CPU execution and I/O wait. Processes
alternate between these two states.
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Cycle of CPU execution and I/O wait
CPU scheduling 6
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Cycle of CPU execution and I/O wait
• Process execution begins with a CPU burst. That is
followed by an I/O burst, which is followed by another
CPU burst, then another I/O burst, and so on.
Eventually, the final CPU burst ends with a system
request to terminate execution.
CPU scheduling 7
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Short term Scheduler
• Selects from among the
processes in ready queue,
and allocates the CPU to one
of them
– Queue may be ordered in
various ways. What are
this ways? (think real life
queues)
CPU scheduling 8
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CPU scheduling
• 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
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Non-preemptive & preemptive
• CPU Scheduling algorithms can be categorized into two;
» Non-Preemptive
» Preemptive
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Non-preemptive & preemptive
• Under non preemptive scheduling, once the CPU has been
allocated to a process, the process keeps the CPU until it
releases the CPU either by terminating or by switching to
the waiting state……The process cannot be interrupted
• With Preemptive a process can be interrupted during its
course of execution
CPU scheduling 11
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Scheduling Algorithms
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CPU scheduling deals with the
problem of deciding which of
the processes in the ready
queue is to be allocated the
processor.
There are many different CPU
scheduling algorithms. These
are;
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First-Come, First-Served Scheduling
Shortest-Job-First Scheduling
Priority Scheduling
Round-Robin Scheduling
Multilevel Queue Scheduling
Multilevel Feedback-Queue Scheduling
CPU scheduling 14
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First-Come, First-Served (FCFS) Scheduling
• The simplest CPU-scheduling algorithm
• With this scheme, the process that requests the CPU
first is allocated the CPU first. The implementation of
the FCFS policy is easily managed with a FIFO queue.
• When a process enters the ready queue, its PCB is
linked onto the tail of the queue.
• When the CPU is free, it is allocated to the process at
the head of the queue. The running process is then
removed from the queue.
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First come First Served (FCFS)
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First-Come, First-Served (FCFS) Scheduling
Process Burst Time
P1 24
P2 3
P3 3
• Suppose that the processes arrive in the order: P1 , P2 , P3
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First-Come, First-Served (FCFS) Scheduling
The Gantt Chart for the schedule is:
• Waiting time for P1 = 0; P2 = 24; P3 = 27
• Average waiting time: (0 + 24 + 27)/3 = 17miliseconds
P1 P2 P3
24 27 30 0
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FCFS Scheduling (Cont.)
Suppose that the processes arrive in the order:
P2 , P3 , P1
Will there be a difference in the average waiting time?
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FCFS Scheduling (summary)
The FCFS scheduling algorithm is non-preemptive. What
does this mean?
• FCFS algorithm is troublesome for time-sharing systems
because Once the CPU has been allocated to a process, that
process keeps the CPU until it releases the CPU, either by
terminating or by requesting I/O.
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Class Exercise 1
Process l Time Burst Time
P1 0.0 6
P2 2.0 8
P3 4.0 7
P4 5.0 3
Determine the average waiting time if FCFS scheduling algorithm
was in use
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Shortest Job First (SJF)
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Shortest-Job-First (SJF) Scheduling
• This algorithm associates with each process the length of
the process's next CPU burst.
• When the CPU is available, it is assigned to the process
that has the smallest next CPU burst.
• What if two processes have the same CPU burst?
• If the next CPU bursts of two processes are the same,
FCFS scheduling is used to break the tie.
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Example of SJF
Process l Time Burst Time
P1 0.0 6
P2 2.0 8
P3 4.0 7
P4 5.0 3
• Average waiting time = (0 + 3 + 9 + 16) / 4 = 7 miliseconds
P4 P3 P1
3 16 0 9
P2
24
CPU scheduling 24
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Class Exercise 2
Process Burst Time
P1 24
P2 3
P3 3
Assume STF scheduling algorithm is used, what would the
order of the processes be?
Compute average waiting time if STF scheduling algorithm
was used
CPU scheduling 25
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Difficulties with SJF
• The real difficulty with the SJF algorithm is knowing the length
of the next CPU request.
• However For long-term (job) scheduling, we can use the length
of the process specified when the process is submited.
• SJF is much difficult to implement at the level of short-term
CPU scheduling. There is no way to know the exact length of
the next CPU burst.
• What are the possible solutions to address this issue?
CPU scheduling 26
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SJF cont…
• The SJF algorithm can be either preemptive or non-
preemptive.
• In Preemtive, The choice arises when a new process arrives at
the ready queue while a previous process is still executing.
The next CPU burst of the newly arrived process may be
shorter than what is left of the currently executing process
CPU scheduling 27
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SJF cont…
• A preemptive SJF algorithm will preempt the currently
executing process, whereas a non-preemptive SJF algorithm
will allow the currently running process to finish its CPU burst.
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SJF cont…
• Preemptive SJF scheduling is sometimes called shortest-
remaining-time-first scheduling.
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Shortest Remaining time First
(SRTF)
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Example of Shortest-remaining-time-first
ProcessAarrArrival TimeT Burst Time
P1 0 8
P2 1 4
P3 2 9
P4 3 5
Class Discussion Questions
• What will be the order of process execution?
• Compute the average waiting time
• Compute the average waiting time if the algorithm is non
preemptive
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Example of Shortest-remaining-time-first
ProcessAarri Arrival TimeTBurst Time
P1 0 8
P2 1 4
P3 2 9
P4 3 5
• Preemptive SJF Gantt Chart
• Average waiting time = 6.5 msec
P1 P1 P2
1 17 0 10
P3
26 5
P4
CPU scheduling 32
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Non-preemptive
ProcessAarri Arrival TimeTBurst Time
P1 0 8
P2 1 4
P3 2 9
P4 3 5
• Order of execution will be P1-P2-P4-P3
• P1 0
• P2 7
• P3 9
• P4 15
Average waiting time = 7.75 msec
CPU scheduling 33
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Class Exercise
ProcessAarri Arrival TimeTBurst Time
P1 0.0 7
P2 2.0 4
P3 4.0 1
P4 5.0 4
i. Draw a gantt chart to represent the following processes
ii. Compute the average waiting time of the following set of
processes using
» SJF (preemptive)-3m/s
» SJF (non-Preemptive)-4m/s
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Priority Scheduling
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Priority Scheduling
• A priority is associated with each process, and the CPU is
allocated to the process with the highest priority.
• Would we be right to say that the SJF algorithm is a special
case of the general priority scheduling algorithm?
• What would happen if two processes have the same priority?
• When two processes have the same priority, First come first
served algorithm is used to schedule
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Priority Scheduling
• Note that we discuss scheduling in terms of high priority and
low priority. Priorities are generally indicated by some fixed
range of numbers, such as 0 to 7 or 0 to 4,095. However, there
is no general agreement on whether 0 is the highest or lowest
priority.
• Some systems use low numbers to represent low priority;
others use low numbers for high priority. This difference can
lead to confusion. In our class, we assume that low numbers
represent high priority.
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Example of Priority Scheduling – assumption is that all processes are in the ready queue
ProcessAarriBurst TimeT Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2
Class discussion questions
Draw the Gantt chart for the processes above
What will the average waiting time be?
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Example of Priority Scheduling –Solution assumption is that all processes are in the ready queue
• Priority scheduling Gantt Chart
P2 P3 P5
1 18 0 16
P4
19 6
P1
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Priority Scheduling
• Priorities can be defined either internally or externally.
• Internally defined priorities use some measurable quantity or
quantities to compute the priority of a process. For example,
time limits, memory requirements, the number of open files,
and the ratio of average I/O burst to average CPU burst have
been used in computing priorities.
CPU scheduling 40
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Priority Scheduling
• External priorities are set by criteria outside the operating
system, such as the importance of the process. Examples?
• Priority scheduling can be either preemptive or non-
preemptive. Explain
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Problems with Priority Scheduling
• A major problem with priority scheduling algorithms is
indefinite blocking, or starvation.
• A process that is ready to run but waiting for the CPU can be
considered blocked.
• A priority scheduling algorithm can leave some low priority
processes waiting indefinitely. In a heavily loaded computer
system, a steady stream of higher-priority processes can
prevent a low-priority process from ever getting the CPU.
CPU scheduling 42
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Indefinite blocking solution
• A solution to the problem of indefinite blockage of low-priority
processes is aging.
• Aging is a technique of gradually increasing the priority of
processes that wait in the system for a long time.
• For example, if priorities range from 127 (low) to 0 (high), we
could increase the priority of a waiting process by 1 every 15
minutes.
CPU scheduling 43
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Class Exercise
Process Duration Priority Arrival
Time
P1 6 4 0.0
P2 8 1 2.0
P3 7 3 3.0
P4 3 2 5.0
• Draw a Gantt chart to
represent the following
processes
• Compute the average
waiting time of the
following set of processes
using preemptive priority
scheduling algorithm
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Round Robin (RR)
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Round Robin (RR)
• The round-robin (RR) scheduling algorithm is designed
especially for time sharing systems.
• It is similar to FCFS scheduling, but preemption is added to
switch between processes. A small unit of time, called a time
quantum or time slice, is defined.
• The CPU scheduler goes around the ready queue, allocating
the CPU to each process for a time interval of up to 1 time
quantum.
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Example of RR with Time Quantum = 4
Process Burst Time
P1 24
P2 3
P3 3
• The Gantt chart is:
• What will the average waiting time be?
• What will the average Turn-around time be?
P1 P2 P3 P1 P1 P1 P1 P1
0 4 7 10 14 18 22 26 30
CPU scheduling 47
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Class Exercise : Time Quantum = 4 Process Burst Time Arrival time P1 8 P2 4 P3 9 P4 5
• Draw the Gantt chart to represent the execution for a time quantum of 4
and 5
• Compute the average waiting time for a time quantum of 4 and 5
• Compute the average turn-around time for a time quantum of 4 and 5
(comment on the turn around time)
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Turnaround Time Vs Time Quantum
• Would we be right to conclude that turnaround time of a set of
processes improves as the time-quantum size increases?
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Turnaround Time Vs Time Quantum
• The average turnaround time of a set of processes does not
necessarily improve as the time-quantum size increases.
• In general, the average turnaround time can be improved if
most processes finish their next CPU burst in a single time
quantum.
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Multilevel Queue
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Multilevel Queue
• Multilevel queues are used for situations where processes
are easily classified into different groups.
• A common division is made between foreground
(interactive) processes and background processes. These
two types of processes have different response-time
requirements and so may have different scheduling needs
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Multilevel Queue
• A multilevel queue scheduling algorithm partitions the
ready queue into several separate queues.
• The processes are permanently assigned to one queue,
generally based on some property of the process, such as
memory size, process priority, or process type. Each queue
has its own scheduling algorithm.
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Multilevel Queue Scheduling
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Multilevel Feedback Queue
• In contrast to Multilevel queue, this algorithm allows a
process to move between queues. The idea is to
separate processes according to the characteristics of
their CPU bursts.
• If a process uses too much CPU time, it will be moved
to a lower-priority queue. This scheme leaves I/O-
bound and interactive processes in the higher-priority
queues.
CPU scheduling 55
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Multilevel Feedback Queues
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Scheduling Criteria
• Different CPU scheduling algorithms have different
properties, and the choice of a particular algorithm may
favor one class of processes over another.
• Many criteria have been suggested for comparing CPU
scheduling algorithms. Which characteristics are used for
comparison can make a substantial difference in which
algorithm is judged to be best. The criteria include the
following:
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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 – From the point of view of a particular
process 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 time-sharing environment)
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Scheduling Algorithm Optimization Criteria
• Max CPU utilization
• Max throughput
• Min turnaround time
• Min waiting time
• Min response time
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Multilevel Queue Scheduling-Example
• ML queue, 2 levels
– RR @ 10 units
– FCFS
RR gets priority over FCFS
• Process Arrival Burst Queue
P1 0 12 FCFS
P2 4 12 RR
P3 8 8 FCFS
P4 20 10 RR
• Non-preemptive and preemptive
• Draw a Gantt charts to represent this algorithm both for
preemptive and non-preemptive
• Compute average waiting and turn around time.
CPU scheduling 60
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