Sporadic Server Scheduling in Linux Theory vs. Practice Mark Stanovich Theodore Baker Andy Wang

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Sporadic Server Scheduling in Linux Theory vs. Practice Mark Stanovich Theodore Baker Andy Wang

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Sporadic Server Scheduling in Linux Theory vs. Practice Mark Stanovich Theodore Baker Andy Wang. Real-Time Scheduling Theory. Analysis techniques to design a system to meet timing constraints Schedulability analysis Workload models Processor models Scheduling algorithms. - PowerPoint PPT Presentation

Transcript of Sporadic Server Scheduling in Linux Theory vs. Practice Mark Stanovich Theodore Baker Andy Wang

Page 1: Sporadic Server Scheduling in Linux Theory vs. Practice Mark Stanovich Theodore Baker Andy Wang

Sporadic Server Scheduling in LinuxTheory vs. Practice

Mark Stanovich

Theodore Baker

Andy Wang

Page 2: Sporadic Server Scheduling in Linux Theory vs. Practice Mark Stanovich Theodore Baker Andy Wang

Real-Time Scheduling Theory Analysis techniques to design a system to

meet timing constraints Schedulability analysis

– Workload models– Processor models– Scheduling algorithms

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Real-Time Scheduling Theory Analysis techniques to design a system to

meet timing constraints Schedulability analysis

– Workload models– Processor models– Scheduling algorithms

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timePeriod = T Computation time

WCET = C

Deadline = D

Task

jobs (j1, j2, j3, …)

Release time 4

Task = {T, C, D}

Periodic Task

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Periodic Task

sched_setscheduler(SCHED_FIFO)

clock_nanosleep()

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Periodic Task Assumptions

WCET is reliable Arrivals are periodic

Not realistic for most tasks

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Polling Server

time

time

Job arrivals

Initialbudget

Replenishment period

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Polling Server Type of aperiodic server CPU time no worse than an equivalent

periodic task Can be modeled as a periodic task

WCET = Initial Budget Period = Replenishment Period

Budget consumed as CPU time is used CPU time forfeited if not used

Replenish budget every period

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Polling Server Good

Bounds CPU time Analyzable workload Simplicity

Can be better Faster response time if budget is not forfeited

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Sporadic Server

time

time

Job arrivals

Initialbudget

Replenishment period

replenishments

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Sporadic Server Originally proposed by Sprunt et. al. Parameters

– Initial budget– Replenishment period

Bounds CPU interference for other tasks Fits into the periodic task workload model Better avg. response time than polling server

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Sporadic Server Scheduling algorithm for fixed-task-priority

systems Can be used in UNIX priority model

SCHED_SPORADIC is a version of SS defined in POSIX definition

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Implementation Linux 2.6.38

Softirq threading patch ported from earlier RT patch

Sporadic server implementation Uniprocessor

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Sporadic Server Performance Metrics

Interference for lower priority tasks Average response time

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An experiment

Sends UDP packet withcurrent timestamp

Receives UDP packets

Calculate response time based on arrival at UDP layer

Measure CPU time for 10 second burst

A B

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Measuring CPU Time Regher's “hourglass” technique Constantly read time stamp counter

– Detect preemptions by larger gaps– Sum execution chunks

Hourglass thread lower than SS thread– Measures interference from SS thread

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Measuring CPU Time Network receive thread

Sporadic and polling server Budget = 1 msec Period = 10 msec

SCHED_FIFO Hourglass thread

SCHED_FIFO Lower priority than network receive thread

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CPU Utilization

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Response Time

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Interference SS budget limited to CPU demand Additional overheads lower priority tasks

– Context switch time– Cache eviction and reloading

Not in theoretical workload model Guarantees of theory require interference to

be included in the analysis

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Polling Server

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time

= aperiodic job CPU time

= aperiodic job arrival

timebudget CS+SS 2

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Sporadic Server

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time

= aperiodic job CPU time

= aperiodic job arrival

= replenishment period

max_repl2 timebudget CS+SS

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Over Provisioning All context switch time may not be used

– e.g., one replenishment per period Account for CS time on-line

– Charge SS for each preemption

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CPU Utilization

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Response Time

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Response Time

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Analysis Light load

– Sporadic Server• Low response time

– Polling Server• High response time

Heavy load– Sporadic Server

• High response time• Dropped packets

– Polling Server• Low response time• No dropped packets

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Can we get the best of both?

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Sporadic ServerLight loads

Polling ServerHeavy loads

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Hybrid Server How to switch

– Ensure bounded interference– SS with 1 replenishment is same as polling

server– Coalesce replenishments

• Push replenishments further into the future Switching point

– Server has work but no budget

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Sporadic Server

time

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Sporadic Server

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time

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Response Time

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CPU Utilization

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Switching Immediate coalescing may be too extreme

CPU time could be used for better response time Gradual approach

Coalesce a few replenishments

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Sporadic Server

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time

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Sporadic Server

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time

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Sporadic Server

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time

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Response Time

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CPU Utilization

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Conclusion Theoretical analysis provides solid guarantees Implementation must match abstract models

Additional interference terms need to be considered

SS can fit into the theoretical analysis

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Deferrable Server

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Deferrable Server Bandwidth Preserving

Allow server to retain budget Periodically replenish budget WCET != Budget

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Response Time

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Replenishment Policy

replenishment periodreplenishment

initial budget

time

arrival time(work available for server)

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Bandwidth Preservation

replenishment

initial budget

time

arrival time(work available for server)

replenishment period

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Sporadic Server

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time