2. OUTLINES
1. SCHEDULING
2. NON PRE EMPTIVE SCHEDULING
3. ADVANTAGES & DISADVANTAGES
4. FIRST COME FIRST SERVE
5. SHORTEST JOB FIRST
6. NON PRE EMPTIVE PRIORITY
7. PREEMPTIVE VS NON PREEMPTIVE
8. CONCLUSION
3. 3
SCHEDULING
Scheduling is the process of selecting a process from a ready queue. And
allotting cpu to this process for execution.
Scheduler is a part of operating system that allots computer resources to
the processes.
Scheduler uses scheduling algorithms to decide which processes it must
allot to the cpu.
6. 6
NON PRE-EMPTIVE SCHEDULING
In non pre-emptive scheduling, once a process is selected, it runs to
completion until is blocked for an i/o devices or other events.
This scheduling does not interrupt a process running CPU in the middle
of the execution.
It wait till the process completes its CPU burst time, and it can allocate
the CPU to another process.
7. 7
ADVANTAGES OF NON PRE-EMPTIVE SCHEDULING
It has a minimal scheduling burden.
It is a very easy procedure.
Less computational resources are used.
It has a high throughput rate.
It is a very simple method.
8. 8
DISADVANTAGES OF NON PRE-EMPTIVE SCHEDULING
It has poor response time for process.
A machine can freeze up due to bugs.
Waiting time is high.
10. 10
FIRST COME FIRST SERVE
1) selection area
As the name suggests, the FCFS assign the CPU to the process that
arrive first in the ready queue.
This means that the process that requests the CPU for its execution first
will get the CPU allocated first.
2) decision mode
Once a process is selected, it run until either it is blocked for an io or
some event or it is terminated
11. 11
CONT...
3) implementation
This method can be easily implemented by fifo.
When CPU becomes free, a process from the first position in a queue is
selected to run.
13. 13
CONT..
Advantages of FCFS
1) it is a simplest form of scheduling algorithm.
2) its implementations is easy
Disadvantages of FCFS
1) not efficient because average waiting time is high.
14. 14
SHORTEST JOB FIRST
1) selection area
• The process, that requires shortest time to complete execution, is
served first.
2) decision mode
• Once a process is selected, it run until either it is blocked for io
some other event or it is terminated.
15. 15
3) implementation
This method can be easily implemented by FIFO.
All processes in queue are sorted in ascending order based on their required
CPU bursts.
When CPU becomes free, a process from the first position in a queue is
selected to run.
CONT...
17. 17
CONT..
Advantages
1) less waiting time.
2) good response for short process.
Disadvantages
1) it is difficult to estimate time required to complete execution.
2) starvation is possible for long process. Long process may wait forever.
18. 18
NON-PRE-EMPTIVE PRIORITY
1) selection area
The process, that has highest priority is served first.
2) decision mode
Once a process is selected, it runs until it blocks for an io or some event or it
terminates.
19. 19
CONT...
3) implementation
This method can be implemented by using sorted fifo queue.
All process in queue an sorted based on their priority with highest priority
process at front end.
When cpu becomes free, a process from the first position in a queue is
selected to run.
21. 21
CONT...
Advantages
1) priority is considered so critical process can get even better response time.
Disadvantages
1) starvation is possible for low priority process. It can be overcome by using
technique called “aging”.
Aging : gradually increases the priority of process that wait in
the system for a long time.
22. 22
DIFFERENCE
PRE-EMPTIVE
In this resources are allocated to a
process for a limited time.
Process can be interrupted in
between.
NON-PRE-EMPTIVE
Once resources are allocated to a
process the process holds it till it
completes its burst time or switches
to waiting state.
Process can not be interrupted until
it terminates itself or its time is up.
23. 23
CONT...
It has overheads of scheduling the
process.
Flexible
Cost associated
Cpu utilization is high.
Waiting time is less.
Response time is less.
It does not have overheads.
Rigid
No cost associated
Cpu utilization is low.
Waiting time is high.
Response time is high.
24. 24
CONCLUSION
Non-pre emptive scheduling algorithms offer efficiency, predictability,
fairness, lower overhead, and simplicity, making them suitable for various
computing environments, especially those with stringent timing
requirements or resource limitations.