the wizard of os: a heartbeat for legacy multimedia applications

© 2008 Scuola Superiore Sant’Anna

“The Wizard of OS: a Heartbeat for Legacy Multimedia Applications”

Real-Time Systems Laboratory

RETIS Lab

Tommaso Cucinotta, Fabio ChecconiRealTime Systems Laboratory

Scuola Superiore Sant'Anna

7th IEEE Workshop onEmbedded Systems for Real-Time Multimedia

Grenoble, FranceOctober, 15th – 16th, 2009

Luca Abeni, Luigi Palopoli University of Trento

© 2007 Scuola Superiore Sant’Anna Tommaso Cucinotta – Real Time Systems Laboratory (ReTiS) – Scuola Superiore Sant'AnnaTommaso Cucinotta – Real Time Systems Laboratory (ReTiS) – Scuola Superiore Sant'Anna

MotivationsMotivations

GeneralPurpose Operating SystemsGeneralPurpose Operating Systems

Very effective for storing & managing multimedia contents

Designed for

• averagecase performance

• serving applications on a besteffort basis

They are not the best candidate for serving realtime

applications with tight timing constraints

• nor for realtime multimedia



Overcoming limitations of a GPOS for multimediaOvercoming limitations of a GPOS for multimedia

Large buffers used to compensate unpredictability

• ==> no realtime interaction nor lowlatency multimedia

Oneapplication onesystem paradigm

• For example, for lowlatency realtime audio processing (jack),

gaming, CD/DVD burning, etc...

POSIX realtime extensions

• Prioritybased, no temporal isolation

• Not appropriate for deploying the multitude of (soft) realtime

applications populating the systems of tomorrow



Recent developments in GPOS schedulingRecent developments in GPOS scheduling

EDFbased realtime scheduling with temporal isolation

• Better utilization of resources

• Independent applications are isolated from the temporal perspective

Various APIs for accessing the enhanced functionality

• For example, the FRSH API from

• For example, the API from

They require modifications of the applications

• at the sourcecode level

What about legacy multimedia applications ?


Research objectivesResearch objectives

This research aims toThis research aims to

Allow legacy realtime applications to benefit of realtime

scheduling facilities available on a GPOS

Without any change in the application sourcecode


Proposed approachProposed approach


An appropriate tracing mechanism observes the

application, inferring main parameters affecting a (periodic)

multimedia application temporal behaviour:

• job execution time

• period

Scheduling guarantees are automagically provisioned by

the OS, according to proper scheduling parameters

• Based on sound arguments from realtime theory



Comprehensive viewComprehensive view

Application tracing

Period estimation (events analysis)

Online WCET estimation

Automagic provisioning of

scheduling guarantees


Realtime theoryRealtime theory

Reservationbased scheduling: (Q, P)Reservationbased scheduling: (Q, P)Known results from the realtime theoryKnown results from the realtime theory

Not all (Q, P) with

a given Q/P are

equal

Highlighted by the

supplybound

function shown


Realtime theoryRealtime theory

What are the reservation What are the reservation parameters needed for parameters needed for correctly scheduling a realcorrectly scheduling a realtime periodic task with time periodic task with assigned WCET and period ?assigned WCET and period ?

The The minimumminimum reservation reservation utilization is achievedutilization is achieved

when the reservation period

equals the task period

• or any integer sub

multiple


Core problemCore problem

How to detect the application period ?How to detect the application period ?

Of a legacy realtime application (no sourcecode

availability, no modifications)


Tracing the application behaviour at runtime

For the purpose of identifying “periodicity patterns”


Legacy application tracingLegacy application tracing

Tracing legacy multimedia applicationTracing legacy multimedia application

Tracing what the application does

• What system calls it periodically calls

– Waiting, reading time information, posting timers, disk

operations, network operations, etc...

Tracing how the application behaves temporally

• When it suspends and resumes

Results obtained with Results obtained with systemcall tracingsystemcall tracing


Legacy application tracingLegacy application tracing

Available mechanismsAvailable mechanisms

strace Linux utility

• Designed as debugging helper, too much overhead due to the

generation of unneeded data

ptrace() system call

• Allows an external process to trace the execution of a target

process, forcing it to stop at each entry and exit of a system call

We developed We developed qostraceqostrace

A lowoverhead systemcall tracer based on ptrace()


Period detectionPeriod detection

The tracer produces a sequence of The tracer produces a sequence of timestampstimestamps

Timestamps used to compute a Timestamps used to compute a FouriertransformFouriertransform

A A heuristicheuristic catches the catches the first harmonicfirst harmonic


HeuristicHeuristic

Identification of local peaksIdentification of local peaks

Selection of candidate peaksSelection of candidate peaks(sensibly higher than average)(sensibly higher than average)

Perform a weighted linear regression Perform a weighted linear regression among the candidate peaksamong the candidate peaks

The peak closest to the regression The peak closest to the regression line inclination coefficient winsline inclination coefficient wins


Experimental resultsExperimental results

SetupSetup

Linux OS, with an implementation of the CBS scheduler

Feedbackscheduling by means of LFS (prior paper)

Custom video player, using libavformat/libavcodec

• modified to monitor the InterFrame time

Application tracing by using qostrace

Validation metricsValidation metrics

InterFrame time

Allocated bandwidth



InterFrame times Cumulative Distribution FunctionInterFrame times Cumulative Distribution Function


The billThe bill

What it costs (tracing overhead)What it costs (tracing overhead)

When using strace: +4,8%

When using qostrace: +2,75%

42% overhead reduction with the custom tracer


Future workFuture work

Reducing further the tracing overheadReducing further the tracing overhead

Kernellevel tracer

Period detection heuristicPeriod detection heuristic

More comprehensive evaluation

Precision with respect to available parameters

Improvements for enhanced precision and reduced

overhead

Facing with multithread applications


Thanks for your attentionThanks for your attention

Questions ?Questions ?

the wizard of os: a heartbeat for legacy multimedia applications

Documents

embedded systems

retis lab

thefrshapifrom forexample

multimedia grenoble

disk operations