SchedulingDecide what processes should be executed by the processorLong-term schedulingDeals with creating a new processControls degree of multiprogrammingMay be FCFS or priority basedInteractive systems tend to accept users unless the system is swampedMedium-term schedulingDeals with swapping processes in/outShort-term schedulingWhat process should we run next?Invoked on:clock or I/O interruptsystem call, signal

Short-term CriteriaOrientation: User vs. SystemPerformance Related: measurable?Criteria (table 9.2, pg 400):Response Time Start to first outputImportant in interactive systemsTurnaround Time Start to finishDeadlinesPredictability Time doesnt depend on what else is going onThroughput Jobs finished/unit timeProcessor UtilizationFairness Processes treated equallyEnforcing PrioritiesBalancing Resources Try to keep all system resources busy

SchedulingPrioritiesAre some processes more important than others?Dont want to starve low-priority processesDecision ModeWill we suspend the currently active process if it can continue?No:NonpreemptiveYes:PreemptiveSome systems (Win 3.1, early Mac) used cooperative multitasking (processes voluntarily give up the CPU)Preemption incurs more O.S. overhead, but prevents monopolizing the processorAlso helps with infinite loops

AlgorithmsFirst Come First ServedProcesses queued in order of arrivalRuns until finished or blocks on I/OTends to penalize short processesHave to wait for earlier long processesFavors processor-bound processes I/O processes block quicklyRound RobinFCFS with preemptionSize of ticks affects performanceWant processes to block in one tickFavors processor-bound processesVirtual Round Robin (figure 9.7)Second queue for formerly blocked processes given priorityAt end of time slice, add to end of standard queue

Algorithms 2Shortest Process NextSelect process with shortest expected running time (nonpreemptive)Difficult to estimate required timeCan estimate from previous runsTends to be less predictableCan starve long processesShort processes may still wait if a long process has just startedShortest Remaining TimePreemptive version of Shortest Process NextMay switch processes when a new process arrivesStill may starve long processes

Algorithms 3Highest Response Ratio NextNonpreemptive, tries to get best average normalized turnaround timeDepends on Response RatioW = time spent waitingS = expected service timeRR = (W + S) / SSelect process with highest RRFeedbackStarts in high-priority queue, moves down in priority as it executesLower-priority queues often given longer time slicesCan starve long processesMove up processes if they wait too long

Starvation

Effect on Processes

Overhead

Response Time

Throughput

Preemptive

Selection Function

Characteristics of Various Scheduling Policies (Table 9.3, page 402)

No

Penalizes short and I/O processes

Minimum

May be high

Not Emphasized

No

Max time waiting

FCFS

No

Fair

Low

Good for short processes

Low if quantum too small

At time quantum

-

Round Robin

Possible

Penalizes Long processes

Can be High

Good for short processes

High

No

Shortest process

SPN

Possible

Penalizes long processes

Can be High

Good

High

At arrival

Min time left

SRT

No

Good balance

Can be High

Good

High

No

See Notes

HRRN

Possible

May favor I/O bound processes

Can be High

Not emphaisized

Not emphaisized

At time quantum

See Notes

Feedback

Comparison (fig 9.5)P1: arrives at time 0, requires 3 unitsP2: arrives at time 2, requires 6 unitsP3: arrives at time 4, requires 4 unitsP4: arrives at time 6, requires 5 unitsP5: arrives at time 8, requires 2 units

Comparison (Table 9.5)

Sheet1

P1P2P3P4P5Mean

Arrival02468

Serv Time36452

FCFS

Finish39131820

Turn.37912128.60

Norm T.1.001.172.252.406.002.56

RR (q=1)

Finish418172015

Turn.4161314710.80

Norm T.1.332.673.252.803.502.71

RR (q=4)

Finish317112019

Turn.3157141110.00

Norm T.1.002.501.752.805.502.71

SPN

Finish39152011

Turn.37111437.60

Norm T.1.001.172.752.801.501.84

SRT

Finish31582010

Turn.31341427.20

Norm T.1.002.171.002.801.001.59

HRRN

Finish39132015

Turn.3791478.00

Norm T.1.001.172.252.803.502.14

Feedback (q=1)

Finish420161911

Turn.4181213310.00

Norm T.1.333.003.002.601.502.29

Feedback ( q=2^(i-1) )

Finish417182014

Turn.4151414610.60

Norm T.1.332.503.502.803.002.63

Sheet2

Sheet3

Performance ComparisonBased on queuing theoryAssume Poisson arrivals and exponential service timesExample:Two priority levelsLow-priority processes five times longer on averageOverall Normalized Response Times (Figure 9.11, Page 416)

Performance ComparisonNormalized response times for shorter processes (Fig 9.12, p. 417)

Normalized response times for longer processes (Fig 9.13, p. 417)

Performance ComparisonSimulation modeling50,000 simulated processes, grouped by size, 80% CPU UtilizationNormalized turnaround time (Fig 9.14, p. 418)

Waiting time (Fig 9.15, p. 419)

Fair Share SchedulingThink of processes as part of a groupEach group has a specified share of the machine it is allowed to usePriority is based on the time this processes is active, and the time the processes in the group have been activeValuesPjPriority of process jUjjProcessor use by process jGUkProcessor use by group kCPUjExponentially weighted average for process jGCPUkExponentially weighted average for group kWkWeight assigned to group kCalculations (done each second):Pj := Basej + CPUj/2 + GCPUk/(4*Wk)CPUj := Uj-1/2 +CPUj-1/2GCPUk := GUk-1/2 + GCPUk-1/2

Fair Share ExampleFigure 9.16, page 421Three processes A, B, CB,C are in one group, A is by itselfBoth groups get 50% weighting

Sheet1

Process AProcess BProcess C

PriorityProcessGroupPriorityProcessGroupPriorityProcessGroup

t=0600060006000

A+60+600000

t=190303060006000

B00+60+600+60

t=274151590303075030

A+60+600000

t=396373774151567015

C000+60+60+60

t=478181881737933037

A+60+600000

t=598393970318761518

B00+60+600+60

t=678191994313982739

A+60+600000

t=798393976151970319

C000+60+60+60

t=878191982739943139

A+60+600000

t=998393970319761519

B00+60+600+60

t=1078191994313982739

A+60+600000

t=1198393976151970319

C000+60+60+60

t=1278191982739943139

Sheet2

Sheet3

Unix SchedulingSame for both SVR3 and 4.3 BSD (SVR4 is different)Similar to Fair-share schedulingValuesPjPriority of process jUjProcessor use by jCalculations (done each second):CPUj := Uj-1/2 +CPUj-1/2Pj := Basej + CPUj/2 + nicejRestrictions on CPU and Nice to prevent a process from getting too far away from its base priority bandSwapperBlock I/O Device controlFile ManipulationCharacter I/O Device controlUser Processes

Response TimeIf the system gets too slow to a user, they may slow down or abort the operationMust balance response time with the cost requiredFaster/more expensive hardware may be requiredPriorities that may penalize certain processesRanges (table 9.7, page 429)15 seconds or greaterRules out conversational interaction.Most users will not wait this longIf it cannot be avoided, allow the user to continue on to something elseForeground/background threads

Response Time (Tab 9.7)4 to 15 secondsGenerally user loses track of the current operation in short-term memoryOk after end of a major closure2 to 4 secondsInhibits user concentrationBothers a user who is focused on the jobOk after end of a minor closure1 to 2 secondsImportant when information has to be remembered over several responsesImportant limit for terminal activitiesLess than 1 secondFor keeping users attention for thought-intensive activitiesExample: graphicsLess than 1/10 secondResponse to key presses or mouse clicks

Response TimeUser productivity tends to improve with a more rapid response timeEspecially true for expert usersBecomes very noticeable as response time drops below 1 secondUser time think time, time user spends thinking about the responseSystem time Time system takes to generate its responseShort response times are importantUser response time tends to decrease as system response time decreasesWeb Pages Loading a page in 3 seconds or less increases the users attentionUser may abort after 10s or more

Response TimeResponse time results for High-Function Graphics (Figure 9.20, page 430)

Response time requirements (Figure 9.21, page 431)