[RFC] Scheduler recorder and playback

Frederic Weisbecker fweisbec at gmail.com
Thu Mar 8 15:45:21 UTC 2012

On Thu, Mar 08, 2012 at 03:20:53PM +0200, Pantelis Antoniou wrote:
> Hi there,
> There's considerable activity in the subject of the scheduler lately and how to 
> adapt it to the peculiarities of the new class of hardware coming out lately,
> like the big.LITTLE class of devices from a number of manufacturers.
> The platforms that Linux runs are very diverse, and run differing workloads.
> For example most consumer devices will very likely run something like Android, 
> with common use cases such as audio and/or video playback. Methods to achieve
> lower power consumption using a power aware scheduler are under investigation.
> Similarly for server applications, or VM hosting, the behavior of the scheduler
> shouldn't have adverse performance implications; any power saving on top of that
> would be a welcome improvement.
> The current issue is that scheduler development is not easily shared between
> developers. Each developer has their own 'itch', be it Android use cases, server
> workloads, VM, etc. The risk is high of optimizing for one's own use case and
> causing severe degradation on most other use cases.
> One way to fix this problem would be the development of a method with which one
> could perform a given use-case workload in a host, record the activity in a 
> interchangeable portable trace format file, and then play it back on another
> host via a playback application that will generate an approximately similar load
> which was observed during recording.
> The way that the two hosts respond under the same load generated by the playback
> application can be compared, so that the performance of the two scheduler implementations
> measured in various metrics (like performance, power consumption etc.) can be
> evaluated.
> The fidelity of the this approximation is of great importance but it is debatable
> if it is possible to have a fully identical load generated, since details of the hosts
> might differ in such a way that such a thing is impossible. 
> I believe that it should be possible at least to simulate a purely CPU load, and the
> blocking behavior of tasks, in such a way that it would result in scheduler decisions 
> that can be compared and shared among developers.  
> The recording part I believe can be handled by the kernel's tracing infrastructure,
> either by using the existing tracepoints, or need be adding more; possibly even
> creating a new tracer solely for this purpose.
> Since some applications can adapt their behavior according to insufficient system
> resources (think media players that can drop frames for example), I think it would
> be beneficial to record such events to the same trace file.
> The trace file should have a portable format so that it can be freely shared between
> developers. An ASCII format like we currently use should be OK, as long as it
> doesn't cause too much of an effect during execution of the recording.
> The playback application can be implemented via two ways. 
> One way, which is the LinSched way would be to have the full scheduler implementation
> compiled as part of said application, and use application specific methods to evaluate
> performance. While this will work, it won't allow comparison of the two hosts in a meaningful
> manner.
> For both scheduler and platform evaluation, the playback application will generate the load
> on the running host by simulating the source host's recorded work load session.
> That means emulating process activity like forks, thread spawning, blocking on resources
> etc. It is not clear to me yet if that's possible without using some kind of kernel
> level helper module, but not requiring such is desirable.
> Since one would have the full trace of scheduling activity: past, present and future; there would
> be the possibility of generating a perfect schedule (as defined by best performance, or best
> power consumption), and use it as a yardstick of evaluation against the actual scheduler.
> Comparing the results, you would get an estimate of the best case improvement that could be
> achieved if the ideal scheduler existed.
> I know this is a bit long, but I hope this can be a basis of thinking on how to go about
> developing this.
> Regards
> -- Pantelis


May be you could have a look at the perf sched tool.
It has a replay feature. I think it perform well basic replay but it can
certainly be enhanced.
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