Messages in this thread | | | Date | Mon, 21 Nov 2016 13:53:08 +0000 | From | Juri Lelli <> | Subject | Re: [PATCH] cpufreq: schedutil: add up/down frequency transition rate limits |
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On 21/11/16 13:26, Peter Zijlstra wrote: > On Mon, Nov 21, 2016 at 12:14:32PM +0000, Juri Lelli wrote: > > On 21/11/16 11:19, Peter Zijlstra wrote: > > > > So no tunables and rate limits here at all please. > > > > > > During LPC we discussed the rampup and decay issues and decided that we > > > should very much first address them by playing with the PELT stuff. > > > Morton was going to play with capping the decay on the util signal. This > > > should greatly improve the ramp-up scenario and cure some other wobbles. > > > > > > The decay can be set by changing the over-all pelt decay, if so desired. > > > > > > > Do you mean we might want to change the decay (make it different from > > ramp-up) once for all, or maybe we make it tunable so that we can > > address different power/perf requirements? > > So the limited decay would be the dominant factor in ramp-up time, > leaving the regular PELT period the dominant factor for ramp-down. >
Hmmm, AFAIU the limited decay will help not forgetting completely the contribution of tasks that sleep for a long time, but it won't modify the actual ramp-up of the signal. So, for new tasks we will need to play with a sensible initial value (trading off perf and power as usual).
> (Note that the decay limit would only be applied on the per-task signal, > not the accumulated signal.) >
Right, and since schedutil consumes the latter, we could still suffer from too frequent frequency switch events I guess (this is where the down threshold thing came as a quick and dirty fix). Maybe we can think of some smoothing applied to the accumulated signal, or make it decay slower (don't really know what this means in practice, though :) ?
> It could be an option, for some, to build the kernel with a PELT window > of 16ms or so (half its current size), this of course means regenerating > all the constants etc.. And this very much is a compile time thing. >
Right. I seem to remember that helped a bit for mobile type of workloads. But never did a thorough evaluation.
> We could fairly easy; if this is so desired; make the PELT window size a > CONFIG option (hidden by default). > > But like everything; patches should come with numbers justifying them > etc.. >
Sure. :)
> > > Also, there was the idea of; once the above ideas have all been > > > explored; tying the freq ram rate to the power curve. > > > > > > > Yep. That's an interesting one to look at, but it might require some > > time. > > Sure, just saying that we should resist knobs until all other avenues > have been explored. Never start with a knob.
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