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5.4.3 Round Robin Scheduling

• A preemptive scheduling designed for Time Sharing Systems

• The Ready Queue is treated as a circular queue

• A small execution time interval is defined as the Time Quantum, or time slice

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Interrupts in Round Robin

• When the executing interval of a process reaches the time quantum, the timer will cause the OS to interrupt the process

• The OS carries out a context switch to the next selected process from the ready queue.

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Length of the Time Quantum

• Time quantum that is too short will generate many context switching and results in lower CPU efficiency.

• Time quantum that is too long may result in poor performance time.

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RR Scheduling (cont’d)Example: Five processes arrive at time 0, in the order: P1, P2, P3, P4, P5. Their CPU burst time are shown in the following table. Using RR with Quantum =40 msec, find the average turnaround time and average waiting time

Process CPU burst (Unit: ms)

P1 135

P2 102

P3 56

P4 148

P5 125

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First round (First turn): 0 to 200 ms Execution time of each Process Remaining time

P1 40 (run from 0 to 40) 135 – 40 = 95

P2 40 (run from 40 to 80) 102 – 40 = 62

P3 40 (run from 80 to 120) 56 – 40 = 16

P4 40 (run from 120 to 160) 148 – 40 = 108

P5 40 (run from 160 to 200) 125 – 40 = 85

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Second round (Second turn): 200 to 376 ms

Execution time of each Process Remaining time

P1 40 (run from 200 to 240) 95 – 40 = 55

P2 40 (run from 240 to 280) 62 – 40 = 22

P3 16 (run from 280 to 296) P3 runs to completionTa(P3) = 296

P4 40 (run from 296 to 336) 108 – 40 = 68

P5 40 (run from 236 to 376) 85 – 40 = 45

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Third round (third turn): 376 to 518 ms

Execution time of each Process Remaining time

P1 40 (run from 376 to 416) 55 – 40 = 15

P2 22 (run from 416 to 438) P2 runs to completionTa(P2) = 438

P4 40 (run from 438 to 478) 68 – 40 = 28

P5 40 (run from 478 to 518) 45 – 40 = 5

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Fourth round (fourth turn): 518 to 566 ms

Execution time of each Process Remaining time

P1 15 (run from 518 to 533) P1 runs to completionTa(P1) = 533

P4 28 (run from 533 to 561) P4 runs to completionTa(P4) = 561

P5 5 (run from 561 to 566) P5 runs to completionTa(P5) = 566

Average Turnaround Time = (Ta(P3)+Ta(P2) +Ta(P1)+Ta(P4)+Ta(P5)) / 5 =(296+438+533+561+566) / 5 = 478.8 ms

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Example with Round Robin

The chart for Round-Robin, with Quantum =40 msec., is:

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Wait Time:Wait time

P1 533 – 135 = 398P2 438 – 102 = 336P3 296 – 56 = 240P4 561 – 148 = 413P5 566 – 125 = 441

Average Wait Time = (Tw(P1)+Tw(P2) +Tw(P3)+Tw(P4)+Tw(P5)) / 5 =(398+336+240+413+441) / 5 = 365.6 ms

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5.4.4 Shortest Remaining Time First

• Shortest remaining time (SRTF) is a preemptive version of SPN scheduling.

• With this scheduling policy, a new process that arrives will cause the scheduler to interrupt the currently executing process if the CPU burst of the newly arrived process is less than the remaining service period of the process currently executing.

• There is then a context switch and the new process is started immediately.

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SRTF Scheduling

• When there are no arrivals, the scheduler selects from the ready queue the process with the shortest CPU service period (burst).

• As with SPN, this scheduling policy can be considered multi-class because the scheduler gives preference to the group of processes with the shortest remaining service time and processes with the shortest CPU burst.

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SRTF Example

Process Arrival time CPU burst P1 0 135 P2 0 102 P3 0 56 P4 0 148 P5 0 125 P6 200 65

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Gantt Chart for SRTF Example

Turnaround Time:Ta(p3)=56, Ta(p2)=56+102=158At time 200, P5 has executed during 42 msec and has remaining service time of 83 microseconds, which is greater than the CPU burst of P6 (i.e., 65 ms). Ta(p6) = 65, Ta(P5)= 56+102+42+65+83=348 Ta(P1) = 348+135 = 483Ta(P4) = 483 + 148 = 631

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Results of Example Using SRTProcessStart Completion Wait Turnaround Ntat P1 348 483 348 483 3.577 P2 56 158 56 158 1.549 P3 0 56 0 56 1.0 P4 483 631 483 631 4.263 P5 158 348 223 348 2.784 P6 200 265 0 65 1.0 The average wait period = (248+56+0+483+223+0)/6=185.0 microsec.The average turnaround time = (483+158+56+631+348+65) =290.16

microsec.

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5.4.6 Dynamic Priority Scheduling• Processes that request I/O service will typically start with CPU

service; for a short time; request another I/O operation; and release the CPU.

• If these processes are given higher priority, they can keep the I/O devices busy without using a significant amount of CPU time.

• This will tend to maximize I/O utilization while using a relatively small amount of CPU time.

• The remaining CPU capacity will be available for processes that are requesting CPU bursts.

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Dynamic Priority Scheduling (2)

• The CPU scheduler dynamically adjusts the priority of a process as the process executes.

• The typical approach is to adjust the priority based on the level of expectation that the process will carry out a system call (typically an I/O request).

• However, this requires the CPU scheduler to predict future process requests.

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5.4.7 Other Scheduling Policies

5.4.7.1 Longest Process Next Not attractive

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5.4.7.2 Real-Time Systems

• A system that maintains an on-going interaction with its environment

• One of the requirements of the system is its strict timing constraints

• The system depends on priorities and preemption

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Real-Time Scheduling Policies

• One of the goals of real-time scheduling is to guarantee fast response of the high-priority real-time processes.

• The second general goal is to guarantee the processes van be scheduled in some manner to meet their individual deadline.

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Real-time Processes

• The real-time processes, each with its own service demand, priority and deadline, compete for the CPU.

• Real-time processes must complete their service before their deadlines expire.

• The performance of the system is based on this guarantee.

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Real-time Processes (2)

• The real-time processes can be:– Periodic processes need to be executed every

specific interval (known as the period)– Sporadic processes can be started by external

random events.

• The operating system uses a real-time scheduling policy based on priorities and preemption.

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5.5 Multiple Processors

• For multiple-processor systems, there are several ways a system is configured.

• More advanced configurations and techniques for example, parallel computing, are outside the scope of this book.

•

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Single Queue-Multiple Processors

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Multiple Queues-Multiple Processors

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5.6 Summary

• FCFS• SJF• RR• SRT