A presentation on implementing CPU Scheduling Algorithms in C++

1. CPU
Scheduling
Algorithms
Group No. 2 1

2. Overview
 Basic Concepts
 Scheduling Criteria
 Scheduling Algorithms
 Implementation in C++
 Demonstration
 Involvement of Operating System
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3. Basic Concepts
 Main objective of multiprogramming is to keep on
running processes all the time for maximum CPU
utilization.
 Scheduling is fundamental function of OS.
 The task of selecting the processes in memory that
are ready to execute, and allocating them to the
CPU is performed by the CPU Scheduler.
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4. CPU Scheduler
 CPU scheduling decisions may take place when a
process:
o 1. Switches from running to waiting state
o 2. Switches from running to ready state
o 3. Switches from waiting to ready
o 4. Terminates
 Scheduling under 1 and 4 is non preemptive.
 All other scheduling is preemptive.
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5. CONT…
CPU Scheduler
 Nonpreemptive
Once a process is allocated the CPU, it does not
leave unless:
o it has to wait, e.g., for I/O request
o it terminates
 Preemptive
o OS can force (preempt) a process from CPU at
anytime
o E.g., to allocate CPU to another higher-priority
process
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6. 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)
◦ Minimize
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7. CONT…
Scheduling Criteria
 Waiting time: amount of time a process has been
waiting in the ready queue (sum of time waiting in
ready queue)
o 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)
o Minimize
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8. Scheduling Algorithms
 First Come, First Served
 Shortest Job First
 Priority
 Round Robin
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9. Implementation in C++
 Class: cpuschedule
 Attributes:
o n – number of processes
o Bu[ ] – Array to store Burst Time
o A[ ] – Array to store Arrival Time
o Wt[ ] – Array to store Waiting Time
o Twt – Total Waiting Time
o Awt – Average Waiting Time
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10. CONT…
Implementation in C++
 Operations:
o Getdata() – To get number of processes and
Burst Times from the user
o Fcfs() – First Come, First Served Algorithm
o Sjf() – Shortest Job First (normal) Algorithm
o SjfP() – Shortest Job First (Preemption)
Algorithm
o SjfNp() – Shortest Job First (non
preemption) Algorithm
o Priority() – Priority Algorithm
o RoundRobin() – Round Robin Algorithm
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11. First Come, First Served
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:
P1 P2 P3
0 24 27 30
 Waiting time for P1 = 0; P2 = 24; P3 = 27
 Average waiting time: (0 + 24 + 27)/3 = 17
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12. First Come First CONT…
Served
 Suppose that the processes arrive in the order :
P2 , P3 , P1 (P1:24,P2:3,P3:3)
 The Gantt chart for the schedule is:
P2 P3 P1
0 3 6 30
 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
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13. Shortest Job First
Normal SJF
Process Burst Time
P1 7
P2 3
P3 4
 The Gantt Chart for SJF (Normal) is:
P2 P3 P1
0 3 7 14
 Average waiting time = (0 + 3 + 7)/3 = 3.33
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14. Shortest Job First
CONT…
Non-Preemptive SJF
Process Arrival Time Burst Time
P1 0.0 7
P2 2.0 4
P3 4.0 1
P4 5.0 4
 The Gantt Chart for SJF (non-preemptive) is:
P1 P3 P2 P4
0 2 4 5 7 8 12 16
 Average waiting time = (0 + 6 + 3 + 7)/4 = 4
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15. Shortest Job First
CONT…
Preemptive SJF
Process Arrival Time Burst Time
P1 0.0 7
P2 2.0 4
P3 4.0 1
P4 5.0 4
 The Gantt Chart for SJF (preemptive) is:
P1 P2 P3 P2 P4 P1
0 2 4 5 7 11 16
 Average waiting time = (9 + 1 + 0 +2)/4 = 3
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16. Shortest Job First
CONT…
 Associate with each process the length of its next CPU
burst.
 Use these lengths to schedule the process with the
shortest time.
 Two schemes:
o Non-Preemptive: once CPU given to the process it
cannot be preempted until completes its CPU burst.
o 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.
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17. Priority
Process Burst Time Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2
 Gantt Chart
P2 P5 P1 P3 P4
0 1 6 16 18 19
 Average waiting time = (6 + 0 + 16 + 18 + 1)/5 = 8.2
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18. Priority
CONT…
A priority number (integer) is associated with each
process.
 Lager the CPU burst lower the priority.
 The CPU is allocated to the process with the highest
priority (smallest integer ≡ highest priority)
 Starvation (Infinity blocking): low priority processes
may never execute.
 Aging: as time progresses increase the priority of the
process.
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19. Round Robin
Process Burst Time
P1 24
P2 3
P3 3
 Quantum time = 4 milliseconds
 The Gantt chart is:
P1 P2 P3 P1 P1 P1 P1 P1
0 4 7 10 14 18 22 26 30
 Average waiting time = {[0+(10-4)]+4+7}/3 = 5.6
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20. Round Robin
CONT…
 Typically, higher average turnaround than SJF, but
better response
 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.
 Performance
o q large ⇒ FCFS
o q small ⇒ q must be large with respect to
context switch, otherwise overhead is too high
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21. Involvement of OS
Source Code (.c)
Compiler Conversion (.i , .o)
Executable (.exe)
Microsoft (load executable
Micro-kernel
Windows directly to memory)
Memory
CPU
Scheduling
Algorithms
Execut
e
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22. CPU
Scheduling
Algorithms
Group No. 2
Group No. 2 22