CPU Scheduling: Dispatcher, CPU scheduling is a process which allows one process to use the CPU while the execution of another process is on hold(in waiting state) due to unavailability of any resource like I/O etc., thereby making full use of CPU. The aim of CPU scheduling is to make the system efficient, fast and fair.
The dispatcher is the module that gives control of the CPU to the process selected by the
short-term scheduler.
2. CONTENTS
Introduction
CPU Scheduling: Dispatcher
Types of CPU Scheduling
Non – Preemptive Scheduling
Preemptive Scheduling
CPU Scheduling: Scheduling Criteria
Scheduling Algorithms
First Come First Serve(FCFS) Scheduling
Shortest-Job-First(SJF) Scheduling
Shortest Remaining Time First (SRTF) Scheduling
Priority Non - Preemptive Scheduling
Priority Preemptive Scheduling
Round Robin(RR) Scheduling
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3. Introduction
CPU scheduling is a process which allows one process to use the CPU while the execution
of another process is on hold(in waiting state) due to unavailability of any resource like
I/O etc., thereby making full use of CPU. The aim of CPU scheduling is to make the
system efficient, fast and fair.
Whenever the CPU becomes idle, the operating system must select one of the processes in
the ready queue to be executed. The selection process is carried out by the short-term
scheduler (or CPU scheduler). The scheduler selects from among the processes in
memory that are ready to execute, and allocates the CPU to one of them.
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4. CPU Scheduling: Dispatcher
The dispatcher is the module that gives control of the CPU to the process selected by the
short-term scheduler. This function involves:
Switching context
Switching to user mode
Jumping to the proper location in the user program to restart that program from
where it left last time.
The dispatcher should be as fast as possible, given that it is invoked during every process
switch. The time taken by the dispatcher to stop one process and start another process is
known as the Dispatch Latency.
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5. Types of CPU Scheduling
CPU scheduling decisions may take place under the following four circumstances:
When a process switches from the running state to the waiting state (for I/O request or
invocation of wait for the termination of one of the child processes).
When a process switches from the running state to the ready state (for example, when
an interrupt occurs).
When a process switches from the waiting state to the ready state (for example,
completion of I/O).
When a process terminates.
In circumstances 1 and 4, there is no choice in terms of scheduling. A new process(if one
exists in the ready queue) must be selected for execution. There is a choice, however in
circumstances 2 and 3.
When Scheduling takes place only under circumstances 1 and 4, we say the scheduling
scheme is Non-preemptive; otherwise the scheduling scheme is Preemptive.
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6. Types of CPU Scheduling
Non-Preemptive Scheduling:
In this type of 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.
It is the only method that can be used on certain hardware platforms, because It does
not require the special hardware(for example: a timer) needed for preemptive
scheduling.
Preemptive Scheduling:
In this type of Scheduling, the tasks are usually assigned with priorities. At times it is
necessary to run a certain task that has a higher priority before another task although
it is running.
Therefore, the running task is interrupted for some time and resumed later when the
priority task has finished its execution.
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7. CPU Scheduling: Scheduling Criteria
There are many different criteria's to check when considering the "best" scheduling
algorithm, they are:
CPU Utilization: To make out the best use of CPU and not to waste any CPU cycle, CPU
would be working most of the time(Ideally 100% of the time). Considering a real
system, CPU usage should range from 40% (lightly loaded) to 90% (heavily loaded.)
Throughput: It is the total number of processes completed per unit time or rather say
total amount of work done in a unit of time. This may range from 10/second to
1/hour depending on the specific processes.
Turnaround Time: It is the amount of time taken to execute a particular process, i.e.
The interval from time of submission of the process to the time of completion of the
process(Wall clock time).
Waiting Time: It is the total time spent by the process in the ready state waiting to get
control on the CPU.
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8. CPU Scheduling: Scheduling Criteria
Load Average: It is the average number of processes residing in the ready queue
waiting for their turn to get into the CPU.
Response Time: Amount of time it takes from when a request was submitted until the
first response is produced. Remember, it is the time till the first response and not the
completion of process execution(final response).
In general CPU utilization and Throughput are maximized and other factors are
reduced for proper optimization.
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9. Scheduling Algorithms
To decide which process to execute first and which process to execute last to achieve
maximum CPU utilization, we have various scheduling algorithms as follows :
First Come First Serve(FCFS) Scheduling
Shortest Job First(SJF) Scheduling
Priority Scheduling
Round Robin(RR) Scheduling
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10. First Come First Serve(FCFS)
Scheduling
In the "First come first serve" scheduling algorithm, the process which arrives first, gets
executed first, or we can say that the process which requests the CPU first, gets the CPU
allocated first.
It is a Non-preemptive Scheduling algorithm.
Easy to understand and implement.
Its implementation is based on FIFO queue.
Poor in performance as average wait time is high.
This is used in Batch Systems.
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11. First Come First Serve(FCFS)
Scheduling
Example 1: Find out Average Turn – around time (T.A.T.) and Average Wait time (W.T.) for
the given jobs:
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Jobs Arrival Time (A.T.) Burst Time (B.T.)
J1 1 5
J2 0 3
J3 2 2
J4 3 4
J5 2 8
13. First Come First Serve(FCFS)
Scheduling
Example 2: Find out Average Turn – around time (T.A.T.) and Average Wait time (W.T.) for
the given process:
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Process Arrival Time (A.T.) Burst Time (B.T.)
P1 1 5
P2 0 6
P3 1 2
P4 0 4
Avg. T.A.T. = 11.5 ms
Avg. W.T. = 7.2 ms
14. Shortest Job First(SJF) Scheduling
Shortest Job First scheduling works on the process with the shortest burst time or duration
first.
This is the best approach to minimize waiting time.
Easy to implement in Batch systems where required CPU time is known in advance.
Impossible to implement in interactive systems where required CPU time is not known.
Processer should know in advance how much time process will take.
It is of two types:
Non Pre-emptive (SJF)
Pre-emptive (SRTF)
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15. Shortest Job First (SJF)
Non - Preemptive
In this type of scheduling CPU allocated to the process(min burst time) cannot be
preempted until completes its CPU burst.
Example 1: Find out Average Turn – around time (T.A.T.) and Average Wait time (W.T.) for
the given jobs:
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Jobs Arrival Time (A.T.) Burst Time (B.T.)
J1 0 4
J2 1 1
J3 2 2
J4 3 1
17. Shortest Job First (SJF)
Non - Preemptive
Example 2: Find out Average Turn – around time (T.A.T.) and Average Wait time (W.T.) for
the given process:
17
Jobs Arrival Time (A.T.) Burst Time (B.T.)
P1 3 5
P2 0 2
P3 4 2
P4 5 3
19. Shortest Job First (SJF)
Non - Preemptive
Example 3: Find out Average Turn – around time (T.A.T.) and Average Wait time (W.T.) for
the given process:
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Process Arrival Time (A.T.) Burst Time (B.T.)
P1 1.5 5
P2 0 1
P3 2 2
P4 3 4
Avg. T.A.T. = 4.875 ms
Avg. W.T. = 1.875 ms
20. Shortest Remaining Time First
(SRTF) Preemptive
In Preemptive Shortest Job First Scheduling, jobs are put into ready queue as they arrive,
but as a process with short burst time arrives, the existing process is preempted or removed
from execution, and the shorter job is executed first.
Example 1: Find out Average Turn – around time (T.A.T.) and Average Wait time (W.T.) for
the given jobs:
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Jobs Arrival Time (A.T.) Burst Time (B.T.)
J1 0 4
J2 1 1
J3 2 2
J4 3 1
22. Shortest Remaining Time First
(SRTF) Preemptive
Example 2: Find out Average Turn – around time (T.A.T.) and Average Wait time (W.T.) for
the given process:
22
Process Arrival Time (A.T.) Burst Time (B.T.)
P1 1.5 5
P2 0 1
P3 2 2
P4 3 4
24. Shortest Remaining Time First
(SRTF) Preemptive
Example 3: Find out Average Turn – around time (T.A.T.) and Average Wait time (W.T.) for
the given process:
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Jobs Arrival Time (A.T.) Burst Time (B.T.)
P1 3 5
P2 0 2
P3 4 2
P4 5 3
Avg. T.A.T. = 4.5 ms
Avg. W.T. = 1.5 ms
25. Priority Scheduling
Priority scheduling is one of the most common scheduling algorithms in batch systems.
Each process is assigned a priority. Process with highest priority is to be executed first and
so on.
Processes with same priority are executed on first come first serve basis.
Priority can be decided based on memory requirements, time requirements or any other
resource requirement.
It is of two types:
Non – Preemptive Scheduling
Preemptive Scheduling
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26. Non –Preemptive Priority
Scheduling
In case of non – preemptive priority scheduling algorithm if a new process arrives with a
higher priority than the current running process, the incoming process is put at the head
of the ready queue, which means after the execution of the current process it will be
processed.
Example 1: Find out Average Turn – around time (T.A.T.) and Average Wait time (W.T.) for
the given jobs:
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Jobs Arrival Time (A.T.) Burst Time (B.T.) Priority
J1 0 10 3
J2 4 5 0 (High)
J3 3 2 1
J4 5 16 2
J5 2 8 4 (Low)
28. Non –Preemptive Priority
Scheduling
Example 2: Find out Average Turn – around time (T.A.T.) and Average Wait time (W.T.) for
the given process:
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Jobs Arrival Time (A.T.) Burst Time (B.T.) Priority
P1 1 5 1 (High)
P2 0 6 2
P3 1 2 1
P4 0 4 3 (Low)
P5 2 3 2
30. Non –Preemptive Priority
Scheduling
Example 3: Find out Average Turn – around time (T.A.T.) and Average Wait time (W.T.) for
the given process:
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Jobs Arrival Time (A.T.) Burst Time (B.T.) Priority
P1 0 4 3
P2 2 3 1(Low)
P3 1 7 4 (High)
P4 3 15 2
Avg. T.A.T. = 16 ms
Avg. W.T. = 8.75 ms
31. Preemptive Priority Scheduling
If the new process arrived at the ready queue has a higher priority than the currently
running process, the CPU is preempted, which means the processing of the current process
is stopped and the incoming new process with higher priority gets the CPU for its
execution.
Example 1: Find out Average Turn – around time (T.A.T.) and Average Wait time (W.T.) for
the given jobs:
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Jobs Arrival Time (A.T.) Burst Time (B.T.) Priority
J1 0 10 3
J2 4 5 0 (High)
J3 3 2 1
J4 5 16 2
J5 2 8 4 (Low)
33. Preemptive Priority Scheduling
Example 2: Find out Average Turn – around time (T.A.T.) and Average Wait time (W.T.) for
the given process:
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Jobs Arrival Time (A.T.) Burst Time (B.T.) Priority
P1 1 5 1 (High)
P2 0 6 2
P3 1 2 1
P4 0 4 3 (Low)
P5 2 3 2
35. Preemptive Priority Scheduling
Example 3: Find out Average Turn – around time (T.A.T.) and Average Wait time (W.T.) for
the given process:
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Jobs Arrival Time (A.T.) Burst Time (B.T.) Priority
P1 0 4 3
P2 2 3 1(Low)
P3 1 7 4 (High)
P4 3 15 2
Avg. T.A.T. = 17 ms
Avg. W.T. = 9.75 ms
36. Round Robin Scheduling
Round Robin is the preemptive process scheduling algorithm.
Each process is provided a fix time to execute called quantum.
Once a process is executed for given time period. Process is preempted and other process
executes for given time period.
Context switching is used to save states of preempted processes.
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37. Round Robin Scheduling
Example 1: Find out Average Turn – around time (T.A.T.) and Average Wait time (W.T.) for
the given process with Time Slice (Quantum) = 3ms.
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Jobs Arrival Time (A.T.) Burst Time (B.T.)
P1 1 5
P2 0 7
P3 3 3
P4 2 10
39. Round Robin Scheduling
Example 2: Find out Average Turn – around time (T.A.T.) and Average Wait time (W.T.) for
the given process with Time Slice (Quantum) = 2ms.
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Jobs Arrival Time (A.T.) Burst Time (B.T.)
P1 1 5
P2 0 3
P3 2 2
P4 3 4
P5 13 2
41. Round Robin Scheduling
Example 2: Five jobs arrive at 0 arrival time the order is given below. Find out Average
Turn – around time (T.A.T.) and Average Wait time (W.T.) for the given process with
Time Slice (Quantum) = 10ms.
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Jobs Burst Time (B.T.)
J1 10
J2 29
J3 3
J4 7
J5 12
Avg. T.A.T. = 35.2 ms
Avg. W.T. = 23 ms