linux-kernel - Re: 5.2-rc2: low framerate in flightgear, cpu not running at full speed, thermal related?

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Date:   Thu, 13 Jun 2019 10:11:58 +0200
From:   Pavel Machek <pavel@....cz>
To:     Doug Smythies <dsmythies@...us.net>
Cc:     "'Rafael J. Wysocki'" <rafael@...nel.org>,
        'kernel list' <linux-kernel@...r.kernel.org>,
        'ACPI Devel Maling List' <linux-acpi@...r.kernel.org>,
        "'Zhang, Rui'" <rui.zhang@...el.com>,
        "'Rafael J. Wysocki'" <rjw@...ysocki.net>,
        'Viresh Kumar' <viresh.kumar@...aro.org>,
        'Linux PM' <linux-pm@...r.kernel.org>,
        'Thomas Gleixner' <tglx@...utronix.de>,
        'Ingo Molnar' <mingo@...hat.com>,
        'Borislav Petkov' <bp@...en8.de>,
        "'H. Peter Anvin'" <hpa@...or.com>,
        'the arch/x86 maintainers' <x86@...nel.org>
Subject: Re: 5.2-rc2: low framerate in flightgear, cpu not running at full
 speed, thermal related?

Hi!

> On 2019.06.12 14:25 Rafael J. Wysocki wrote:
> > On Wed, Jun 12, 2019 at 4:45 AM Doug Smythies <dsmythies@...us.net> wrote:
> >>
> >> So, currently there seems to be 3 issues in this thread
> >> (and I am guessing a little, without definitive data):
> >>
> >> 1.) On your system Kernel 5.4-rc2 (or 4) defaults to the intel_pstate CPU frequency
> >> scaling driver and the powersave governor, but kernel 4.6 defaults to the
> >> acpi-cpufreq CPU frequency scaling driver and the ondemand governor.
> >
> > Which means that intel_pstate works in the active mode by default and
> > so it uses its internal governor.
> 
> Note sure what you mean by "internal governor"?
> If you meant HWP (Hardware P-state), Pavel's processor doesn't have it.
> If you meant the active powersave governor code within the driver, then agreed.
> 
> > That governor is more performance-oriented than ondemand and it very
> > well may cause more power to be allocated for the processor - at the
> > expense of the GPU.
> 
> O.K. I mainly use servers and so have no experience with possible GPU
> verses CPU tradeoffs.
> 
> However, I did re-do my tests measuring energy instead of CPU frequency
> and found very little difference between the acpi-cpufreq/ondemand verses
> intel_pstate/powersave as a function of single threaded load. Actually,
> I did the test twice, one at 20 hertz work/sleep frequency and also
> at 67 hertz work/sleep frequency. (Of course, Pavel's processor might
> well have a different curve, but it is a similar vintage to mine
> i5-2520M verses i7-2600K.) The worst difference was approximately
> 1.1 extra processor package watts (an extra 5.5%) in the 80% to 85%
> single threaded load range at 67 hertz work/sleep frequency for
> the intel-pstate/powersave driver/governor. 
> 
> What am I saying? For a fixed amount of work to do per work/sleep cycle
> (i.e. maybe per video frame related type work) while the CPU frequency Verses load
> curves might differ, the resulting processor energy curve differs much less.
> (i.e. the extra power for higher CPU frequency is for less time because it gets
> the job done faster.) So, myself, I don't yet understand why only the one method
> would have hit thermal throttling, but not the other (if indeed it
> doesn't).

It seems there are serious differences in reporting :-(. How do I
determine which frequency CPU really runs at, in 4.6 kernel?

But it seems that your assumptions are incorrect for my workload.

flightgear is single-threaded, and in my configuration saturates the
CPU, because it would like to achieve higher framerate than my system
is capable of.

> Just for information: CPU frequency verses single threaded load curves
> for the conservative governor is quite different between the two drivers.
> (tests done in February, perhaps I should re-do and also look at energy
> at the same time, or instead of CPU frequency.)

So this might be my problem?

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

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