This document discusses different CPU scheduling algorithms: First-Come, First-Served (FCFS) is the simplest non-preemptive algorithm that processes jobs in the order they arrive. Shortest Job First (SJF) selects the job with the shortest estimated run time, preemptively or not. Priority scheduling assigns priorities to processes and the highest priority job runs, allowing for preemption. Round-robin scheduling uses time quanta to give each process a fair share of CPU in a preemptive manner.

4. • In computer science, scheduling is the method
by which threads, processes or data flows are
given access to system resources
• Long-term scheduling
• Medium-term scheduling
• Short-term scheduling

5. • CPU utilization
• Throughput
• Turnaround time
• Waiting time
• Response time

6. • is the simplest scheduling algorithm, FIFO
simply queues processes in the order that
they arrive in the ready queue.
• The FCFS scheduling algorithm is non
preemptive. Once the CPU has been allocated
to a process, that process keeps the CPU until
it releases the CPU.

8. Process 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
P1 P2 P3

9. • Associate with each process the length of its next
CPU burst. When the CPU is available, it is assigned
to the process that has the smallest next CPU burst.

10. – 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 know
as the
Shortest-Remaining-Time-First (SRTF)

11. 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
P1 P3 P2 P4

12. 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
P1 P3P2 P4 P1P2

13. • A priority number (integer) is associated with each process
• The CPU is allocated to the process with the highest priority (smallest
integer  highest priority)
– Preemptive
– non preemptive
• SJF is a priority scheduling where priority is the predicted next CPU burst
time
• Problem  Starvation – low priority processes may never execute
• Solution  Aging – as time progresses increase the priority of the
process

14. Process Burst Time Arrival Time Priority
P1 10 0 3
P2 1 0 1
P3 2 0 4
P4 1 0 5
P5 5 0 2
P3P5P2
61 190 16 18
P3P5P2
61 190 16 18
P4P1P2 P5 P1 P3 P4

15. • Each process gets a small unit of CPU time (time quantum),
usually 10-100 milliseconds. After this time has elapsed, the
process is preempted and added to the end of the ready queue.
• If there are n processes in the ready queue and the time
quantum is q, then each process gets 1/n of the CPU time in
chunks of at most q time units at once. No process waits more
than (n-1)q time units.

16. Process Burst Time Arrival
P1 24 0
P2 3 0
P3 3 0
• The Gantt chart is:
• Average waiting time: (0+4+7+(10-4))/3 = 5.66
With FCFS: (0+24+27)/3 = 17
P1 P2 P3 P1 P1 P1 P1 P1
0 4 7 10 14 18 22 26 30
P2 P3 P1 P1P1 P1P1P1

17. • FCFS (First-come, First-Served)
– Non-preemptive
• SJF (Shortest Job First)
– Can be either
– Choice when a new (shorter) job arrives
– Can preempt current job or not
• Priority
– Can be either
– Choice when a processes priority changes or when a higher priority
process arrives