Baljit Singh Saini

Process Scheduling

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When CPU becomes idle which scheduler is invoked?

Short Term schedulerWhat it does?Selects one process from ready queue for

execution

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CPU-scheduling decision circumstances1. Process switches from running state to waiting state.2. Process switches from running state to ready state.3. Process switches from waiting state to ready state.4. Process terminates.

1 and 4 are Non-Preemptive.2 and 3 are Preemptive.Non-Preemptive scheduling – once the CPU has been allocated to a process, the process keeps the CPU until it releases the CPU either by terminating or by switching to the waiting state.

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DispatcherGives control of the CPU to the process.It involves

Context switchingSwitching to user modeJumping to the proper location in the user

program to restart that process.

Dispatch Latency : time taken to stop one process and start another process.

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Scheduling CriteriaCPU utilization – high (40-90%)Throughput – number of process completed per unit timeTurnaround time – interval from time of submission to the

time of completion.Waiting time – time spent in the waiting in the ready

queue.Response time – time from the submission until the first

response is produced.

Maximize – CPU utilization and throughputMinimize – turnaround, waiting and response time

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Predicting Burst Time in SJFSJF is used for Long Term SchedulerCan not be used with STS because it is difficult to know

the length of next CPU burstPredict:

= length of the nth CPU burst = predicted value of next CPU burst0 <= <=1 contains most recent information contains the past history

𝜏 𝑛+1=𝛼𝑡𝑛+(1−𝛼)𝜏𝑛

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If =0, then recent history has no effectIf =1, then only the most recent CPU burst

mattersGenerally =1/2 is defined as constant or an overall system

average.

𝜏 𝑛+1=𝛼𝑡𝑛+(1−𝛼)𝜏𝑛

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IssuesFCFS – convoy effectPriority – Starvation

Solution - AgingRR – Time Quantum

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ReferencesSilberschatz, Abraham, et al. Operating

system concepts. Edition-8. Reading: Addison-Wesley.

Note : For more details on all the concepts mentioned here refer to the above listed book.