OS Lecture#4 - Process Scheduling

8/7/2019 OS Lecture#4 - Process Scheduling

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Operating SystemsProf. Navneet Goyal Department of Computer Science & Information SystemsBITS, Pilani


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Topics for TodayProcess SchedulingCriteria & Objectives

Types of SchedulingLong termMedium termShort term


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CPU SchedulerSelects from among the processes in memory that are ready to execute, and allocates theCPU to one of themCPU scheduling decisions may take placewhen a process:1. Switches from running to waiting state2. Switches from running to ready state3. Switches from waiting to ready

4. TerminatesScheduling under 1 and 4 is non-preemptiveA ll other scheduling is preemptive


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P reemptive & Non-preemptiveScheduling

PreemptiveCurrently running process may beinterrupted and moved to the ready stateby the OSWindows 95 introduced preemptiveschedulingUsed in all subsequent versions of windowsMac OS X uses it


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D ispatcherD ispatcher module gives control of theCPU to the process selected by the short-term scheduler; this involves:

switching contextswitching to user modejumping to the proper location in the userprogram to restart that program (IP)

D ispatch latency time it takes for thedispatcher to stop one process and startanother running


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Criteria & ObjectivesCPU utilization keep the CPU as busy as

possible (40 lightly loaded, 90 heavily loaded)Throughput # of processes that completetheir execution per time unitTurnaround time amount of time to executea particular process (total time spent on thesystem)Waiting time amount of time a process hasbeen waiting in the ready queueResponse time amount of time it takes fromwhen a request was submitted until the firstresponse is produced, not output (for time-sharing environment)


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OptimizationC

riteriaMax CPU utilizationMax throughputMin turnaround timeMin waiting timeMin response time


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Types of SchedulingLong-term scheduling

the decision to add to pool of processesto be executed

Mid-term schedulingthe decision to add to the number of processes that are partially or fully inmemory

Short-term schedulingdecision as to which available processwill be executed

I/O schedulingdecision as to which processs pendingrequest shall be handled by an availableI/O device


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Long- Term SchedulingD etermines which programs are admitted

to the system for processingControls the degree of multiprogrammingMore processes, smaller percentage of time each process is executed


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Medium- Term SchedulingPart of the swapping function

Based on the need to manage the degreeof multiprogramming


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Short- Term SchedulingK nown as the dispatcher

Executes most frequently Invoked when an event occursClock interruptsI/O interrupts

Operating system calls


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P rocess Scheduling Example


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F irst- Come- F irst-Served ( F C F S)0 5 10 15 20

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Each process joins the Ready queueWhen the current process ceases toexecute, the oldest process in the Ready queue is selected


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F irst- C ome- F irst-Served ( F C F S)Short jobs suffer penalty

Favors CPU-bound processes over I/Obound processesI/O processes have to wait until CPU-bound process completes

May result in inefficient use of both theprocessor & I/O devicesReasons?


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F irst- C ome- F irst-Served ( F C F S)Not an attractive alternative for single

processor systems. Must be combinedwith priority scheme


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Round RobinReduces the penalty that short hobssuffer with FCFS

Uses clock based preemptionA ready job is selected from the queue ona FCFS basisA lso called as Time Slicing


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Round- Robin (q=1)0 5 10 15 20

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Round- Robin (q=4)D O IT YOURSELF!!


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Round- RobinD rawback: relative treatment of CPU-bound

& I/O bound processesReason?Virtual Round Robin (VRR) avoids thisunfairness

SO what is VRR?


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Virtual Round- RobinWhen a running process is timed out, it joinsthe ready queue (FCFS queue)When a process is blocked, it joins an I/OqueueThis is normal!New queue: A uxiliary FCFS queueProcesses are moved to this queue after beingreleased from an I/O block When dispatching decision is made, processesin the auxiliary queue are given preferenceover those in main ready queueFigure


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Virtual Round- RobinProcess selected from the auxiliary queueruns for q-q1 timeq1 is the time it ran the last time whenselected from the ready queueWHY q q1??


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Shortest P rocess Next (S P N or SJ F )0 5 10 15 20

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Non-preemptive policy Process with shortest expectedprocessing time is selected nextShort process jumps ahead of longerprocesses


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Shortest P rocess NextPredictability of longer processes is

reducedIf estimated time for process not correct,the operating system may abort itPossibility of starvation for longer

processesReduces bias in favor of longer processes,it is still not desirable because of lack of preemptions


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Shortest Remaining T ime

Preemptive version of shortest processnext policy Must estimate processing time

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Analysis of Scheduling AlgorithmsPredictability of longer processes isreducedIf estimated time for process not correct,the operating system may abort itPossibility of starvation for longer

processes


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Normalized Turnaround T imeTA T ( T r ) does not tell us anything aboutthe breakup between the service time ( T

s)

and the waiting timeNormalized T A T (NT A T) = T r / T sIndicates the relative delay experienced

by a processMinimum possible value for NT A T isIncreasing value corresponds todecreasing level of service

1.0


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Normalized Turnaround T imeTA T = 1+1 = 2

NTA

T = 2/1 = 2TA T = 1+9 = 10NT A T = 10/9 = 1.1 (good service)By looking at T A T, we get no idea about

the quality of service!


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Some F inal RemarksFCFS and RR (including VRR) do notrequire the information about service

time!!SJF & SRT both need to know or at leastestimate the processing time of eachprocess

Exponential SmoothingFormula and discussion


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H ighest Response Ratio Next ( HRR N)NT A T is a better metric than T A TFor each individual process we would like toreduce this ratioMinimize its average value for all processesConsider

R = (w+s)/s, whereR is response ratio

w = time spent waiting for processors = expected service timeMin (R) = 1.0 (when w=0)


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H ighest Response Ratio Next ( HRR N)When current process completes or isblocked, choose the ready process with max.value of R HRRN is attractive as it incorporates age of aprocess (in terms of w)Shorter jobs are favored (small s)A ging without service (increase in w) will

increase R Like SJF & SRT, HRRN also needs toestimate expected service timeD O the running example with HRRN


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HRR N Example0 5 10 15 20

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F eedback SchedulingIf service time is not known, we can notuse any of the following :

SJF or SPNSRTHRRN

Focus on time spent on processorA nother way of penalizing that have beenrunning longer and giving preference toshorter jobs


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F eedback SchedulingQ uantum based preemptionD

ynamic priority mechanismWith each preemption, a process isdemoted to the next lower-priority queueA short process will complete quickly

without slipping very far down thehierarchy of ready queuesA longer process will gradually drift down


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F eedback SchedulingNewer shorter processes are favored overolder, longer processesIn each ready queue, except the lowestpriority queue, FCFS is usedLowest priority queue is treated in RR

fashionScheme is called as multi-level feedback scheme


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F eedback Scheduling: Example q=10 5 10 15 20

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Feedback Scheduling: Example q=2

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C omparisonTA T

NTA

T


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F air Share SchedulingA ll scheduling algorithms discussed so fartreat collection of ready processes as ahomogeneous collectionGenerally a user application or job consistsof many processesThis structure is not recognized by traditional schedulers

User is per se not interested in how anindividual process performsInterested in how set of processes whichconstitute a single application, performs


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F air Share SchedulingIntroduces two levels of abstraction viz.,users & groupsUsers of the same department - one groupA ttempt to give each group/user similarserviceIf large no. of users from one dept. log onto the system, we make sure that theresponse times of members of only thatdept. suffers rather than that of otherdepts.


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F air Share SchedulingFSS is all about making schedulingdecisions based on process sets ratherthan on basis of individual processesEach group/user is assigned a share of theprocessorConsiders the execution history of a relatedgroup of processes, along with theindividual execution history of eachprocess in making scheduling decisionsScheduling is based on priority


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F air Share SchedulingEach group is allocated a weight W k 0


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Revising P riorities


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P roblem4 processes3 groups

Processor is interrupted 60 times per secondD uring each interrupt the processor usagefield of the currently running process isincremented as is the corresponding groupprocessor usage fieldOnce per second the priorities are recalculatedusing FSS algorithm draw the Gantt chart forthe first 5 seconds


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Problem

Group name Group 1 Group2 Group3Process Id P1 P2 P3 P4

Groupweightage

0.4 0.3 0.3

Process basepriority

60 80 50 50


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Coming U p Process Coordination

SynchronizationD

eadlocks

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OS Lecture#4 - Process Scheduling