Post on 06-Jan-2017
Baljit Singh Saini
Process Scheduling
Baljit Singh Saini
When CPU becomes idle which scheduler is invoked?
Short Term schedulerWhat it does?Selects one process from ready queue for
execution
Baljit Singh Saini
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.
Baljit Singh Saini
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
Baljit Singh Saini
IssuesFCFS – convoy effectPriority – Starvation
Solution - AgingRR – Time Quantum
Baljit Singh Saini
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.