linux-kernel - Re: [RFC v4 0/8] TurboSched: A scheduler for sustaining Turbo Frequencies for longer durations

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Message-ID: <20190731173225.GB24222@amd>
Date:   Wed, 31 Jul 2019 19:32:26 +0200
From:   Pavel Machek <pavel@....cz>
To:     Parth Shah <parth@...ux.ibm.com>
Cc:     peterz@...radead.org, mingo@...hat.com,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        patrick.bellasi@....com, dietmar.eggemann@....com,
        daniel.lezcano@...aro.org, subhra.mazumdar@...cle.com
Subject: Re: [RFC v4 0/8] TurboSched: A scheduler for sustaining Turbo
 Frequencies for longer durations

Hi!

> >> Abstract
> >> ========
> >>
> >> The modern servers allows multiple cores to run at range of frequencies
> >> higher than rated range of frequencies. But the power budget of the system
> >> inhibits sustaining these higher frequencies for longer durations.
> > 
> > Thermal budget?
> 
> Right, it is a good point, and there can be possibility of Thermal throttling
> which is not covered here.
> But the thermal throttling is less often seen in the servers than the throttling
> due to the Power budget constraints. Also one can change the power cap which leads
> to increase in the throttling and task packing can handle in such
> cases.

Ok. I thought you are doing this due to thermals. If I understand
things correctly, you can go over thermal limits for a few seconds
before the silicon heats up. What is the timescale for power budget?

> BTW, Task packing allows few more cores to remain idle for longer time, so
> shouldn't this decrease thermal throttles upto certain extent?

I guess so, yes.

> > >> These numbers are w.r.t. `turbo_bench.c` multi-threaded test benchmark
> >> which can create two kinds of tasks: CPU bound (High Utilization) and
> >> Jitters (Low Utilization). N in X-axis represents N-CPU bound and N-Jitter
> >> tasks spawned.
> > 
> > Ok, so you have description how it causes 13% improvements. Do you also have metrics how
> > it harms performance.. how much delay is added to unimportant tasks etc...?
> > 
> 
> Yes, if we try to pack the tasks despite of no frequency throttling, we see a regression
> around 5%. For instance, in the synthetic benchmark I used to show performance benefit,
> for lower count of CPU intensive threads (N=2) there is -5% performance drop.
> 
> Talking about the delay added to an unimportant tasks, the result can be lower throughput
> or higher latency for such tasks.

Thanks. I believe it would be good to mention disadvantages in the
documentation, too.

Best regards,
							Pavel
							
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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