The document discusses different CPU scheduling algorithms: 1. First Come First Served scheduling allocates CPU to the longest waiting process first, which can result in longer processes waiting behind shorter ones (convoy effect). 2. Shortest Job First scheduling allocates CPU to the process with the shortest estimated run time, minimizing average wait time. Preemptive SJF allows interrupting the current process if a shorter one arrives. 3. Priority scheduling assigns priority levels and allocates CPU to the highest priority ready process. Preemption and aging policies address starvation of lower priority processes. 4. Round Robin scheduling allocates a time quantum (e.g. 10-100ms) to each ready process

1. ِ‫ن‬ ٰ‫م‬ْ‫ح‬َّ‫الر‬ ِ‫هللا‬ ِ‫م‬ْ‫س‬ِ‫ب‬ِ‫ْم‬‫ي‬ ِ‫ح‬َّ‫الر‬

2. GROUP MEMBERS
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1. Muhammad Waqas
4. Waqar Iqbal
2. Sheroz Aftab
5. Shahzaib Ali
3. Hammad Ali
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3. Topi
c
CPU Scheduling
Basic Concepts
Maximum CPU utilization obtained with
multiprogramming
CPU–I/O Burst Cycle – Process execution
consists of a cycle of CPU execution and I/O
wait
CPU burst distribution

4. ALTERNATING SEQUENCE OF CPU AND I/O BURSTS
Load store
Add store
Read from file
Store increment
index write to
file
Load store
Add store
Read from file
Wait for I/O
Cpu burst
I/O burst
Cpu
burst
Wait for I/O I/O burst
Cpu
burst
Wait for I/O I/O burst
Histogram of CPU-burst Times

5. CPU SCHEDULER
 Selects from among the processes in ready queue, and allocates the CPU to
one of them
 Queue may be ordered in various ways
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
Scheduling under 1 and 4 is non-preemptive
All other scheduling is preemptive
 Consider access to shared data
Consider preemption while in kernel mode
Consider interrupts occurring during crucial OS activities

6. DISPATCHER
Scheduler: selects one process to run on CPU
Dispatcher: allocates CPU to the selected process, which involves:
1. Switching context
2. switching to user mode
3. jumping to the proper location (in the selected process) and restarting it
Dispatcher should be fast enough.
 Dispatch latency – time it takes for the dispatcher to stop one process and start
another running.
Types of Scheduler
1. Non-preemptive
2. Preemptive

7. Non-preemptive
• Once a process is allocated the CPU, it does not leave unless:
1. it has to wait, e.g., for I/O request or for a child to terminate
2. it terminates
Preemptive
OS can force (preempt) a process from CPU at anytime
Say, to allocate CPU to another higher-priority process

8. SCHEDULING CRITERIA
CPU utilization – keep the CPU as busy as possible
• Maximize
Throughput – No of processes that complete their execution per time unit
• Maximize
Turnaround time – amount of time to execute a particular process (time from submission to termination) i.e
(waiting in memory + waiting time in ready queue + executing time in CPU + doing IO)
• Minimize
Waiting time – amount of time a process has been waiting in the ready queue (sum of time waiting in ready
queue)
• Minimize
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)
• Minimize

9. TYPES OF SCHEDULING ALGORITHM
1. First Come, First Served (FCFS)
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
Process Burst
Time
P1 24
P2 3
P3 3
P1 P2 P3
0 24 27
30

10.  Suppose that the processes arrive in the order : P2 , P3 , P1
(P1:24,P2:3,P3:3)
The Gantt chart for the schedule is:
 Waiting time for P1 = 6; P2 = 0; P3 = 3
Average waiting time: (6 + 0 + 3)/3 = 3
Much better than previous case
Convoy effect short process behind long process
P2 P3 P1
0 3 6
30

11. 2. SHORTEST-JOB-FIRST SCHEDULING
Associate with each process the length of its next CP burst.
Use these lengths to schedule the process with the shortest time
Two schemes:
• 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)
SJF is optimal – gives minimum average waiting time for a given set of
processes

12. Example of Non-Preemptive SJF
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 P4
0 3 7 8 12 16

13. Example of Preemptive SJF
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 P2 P3 P2 P4 P1
0 2 4 5 7 11 16

14. 3. PRIORITY SCHEDULING
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 priority scheduling where priority is the inverse of predicted next
CPU burst time
Problem ≡ Starvation – low priority processes may never execute
Solution ≡ Aging – as time progresses increase the priority of the
process

15. EXAMPLE OF PRIORITY SCHEDULING
Example of Non-Preemptive SJF
Process Burst Time Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2
Priority scheduling Gantt Chart
Average waiting time = 8.2 M - sec
P1 P3 P2 P4
0 1 6 16 18 19

16. 4. ROUND-ROBIN SCHEDULING
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.
 Performance
• q large ⇒ FIFO
• q small ⇒ q must be large with respect to context switch, otherwise overhead
is too high

17. EXAMPLE OF ROUND-ROBIN SCHEDULING
Process Burst
Time
P1 24
P2 3
P3 3
 Quantum time 4millisec
The Gantt chart is:
P1 P2 P3 P1 P1 P1 P1 P1
0 4 7 10 14 18 22 26
30
 Typically, higher average turnaround than SJF, but better
response

18. Thank You
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