The Wizard of OS: a Heartbeat for Legacy Multimedia Applications

RETIS Lab
Real-Time Systems Laboratory

7th IEEE Workshop on
Embedded Systems for Real-Time Multimedia
Grenoble, France
October, 15th – 16th, 2009

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

Tommaso Cucinotta, Fabio Checconi Luca Abeni, Luigi Palopoli
RealTime Systems Laboratory University of Trento
Scuola Superiore Sant'Anna
© 2008 Scuola Superiore Sant’Anna

Motivations

GeneralPurpose 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

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

Motivations
Overcoming 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

Motivations
Recent 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 objectives

This 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 approach

Proposed 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

Proposed approach

Comprehensive view
 Application tracing

 Period estimation (events analysis)

 Online WCET estimation

 Automagic provisioning of
scheduling guarantees


Realtime theory
Reservationbased scheduling: (Q, P)
Known results from the realtime theory
 Not all (Q, P) with
a given Q/P are
equal

 Highlighted by the
supplybound
function shown


Realtime theory

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

The minimum reservation
utilization is achieved

 when the reservation period

equals the task period

• or any integer sub
multiple

Core problem

How to detect the application period ?
 Of a legacy realtime application (no sourcecode
availability, no modifications)
Proposed approach
 Tracing the application behaviour at runtime

 For the purpose of identifying “periodicity patterns”


Legacy application tracing

Tracing 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 systemcall tracing


Legacy application tracing

Available 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 qostrace
 A lowoverhead systemcall tracer based on ptrace()

Period detection

The tracer produces a sequence of timestamps

Timestamps used to compute a Fouriertransform

A heuristic catches the first harmonic

Heuristic

Identification of local peaks

Selection of candidate peaks
(sensibly higher than average)

Perform a weighted linear regression
among the candidate peaks

The peak closest to the regression
line inclination coefficient wins


Experimental results
Setup
 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 metrics
 InterFrame time

 Allocated bandwidth


InterFrame times Cumulative Distribution Function


The bill

What it costs (tracing overhead)
 When using strace: +4,8%

 When using qostrace: +2,75%

42% overhead reduction with the custom tracer


Future work

Reducing further the tracing overhead
 Kernellevel tracer
Period 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 attention

Questions ?


The Wizard of OS: a Heartbeat for Legacy Multimedia Applications

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