1. CPU Scheduling ( Basic Concepts)
Which process will get the CPU when it is available?
This is decided by the O.S. and is called CPU scheduling.
The part of the O.S. which performs scheduling is called Scheduler.
The scheduler selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them.
The objective is to maximize CPU utilization in multiprogramming.
CPU–I/O Burst Cycle
Process execution consists of a cycle of CPU execution and I/O wait.

2. Alternating Sequence of CPU And I/O Bursts
execute
CPU Burst
I/O Wait
I/O Burst
execute
CPU Burst
I/O Wait
I/O Burst

3. Important Definitions
Term
Description
Preemptive Scheduling
After allocation the CPU can be taken away from a process.
Non Preemptive Scheduling
After its allocation the CPU cannot be taken away from a process; process can voluntarily release the CPU.
CPU Bound Process
A process which heavily uses the CPU and performs a little I/O
I/O Bound Process
A process performs I/O very frequently
Throughput
No. of processes that complete their execution per time unit
Turnaround time
Amount of time to execute a particular process
Waiting time
Amount of time a process has been waiting in the ready queue
Response time
Time between submitting a request and getting the first response, not output (in a time-sharing environment)

4. Optimization Criteria
Max CPU utilization
Max throughput
Min turnaround time
Min waiting time
Min response time

5. Scheduling Algorithms
First-Come, First-Served (FCFS) Scheduling ( non preemptive)
Process
CPU Burst (B)
(msec)
Wait Time (W)
W/B P1 25 P2 40
0.625 P3 03 65
21.667 P4 12 68
5.667
This algorithm favors:
Longer jobs against the shorter jobs ( W/B is less for longer jobs)
CPU bound processes against I/O bound processes

6. Shortest Job first Scheduling
Now the jobs appear as:
Process
CPU Burst (B)
(msec)
Wait Time (W)
W/B P3 03 P4 12
0.25 P1 25 15
0.65 P2 40
1.00
Now:
Longer jobs are not favored against the shorter jobs ( W/B is less for longer jobs)
A longer job can face starvation
CPU bound processes are still favored against I/O bound processes

7. Round-Robin (RR) Scheduling (Preemptive)
A queue of ready processes is maintained
A time quantum q ( msec) is allocated to each process.
I f a process does not finish in the allocated time it goes back to the end of the queue (circular queue)
If there are n processes, each process gets turn at most after (n-1) q units.
Gannt Chart for q = 20 msec
CPU
CPU … P1 P4 P3 P2
Performance
If q is large, this scheme degenerates to FCFS
If q is small, more context switches (each process finishes in more turns)