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From: "Rafael J. Wysocki" <rafael@kernel.org>
To: Doug Smythies <dsmythies@telus.net>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>,
	Pavel Machek <pavel@ucw.cz>,
	kernel list <linux-kernel@vger.kernel.org>,
	ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
	"Zhang, Rui" <rui.zhang@intel.com>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Linux PM <linux-pm@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"the arch/x86 maintainers" <x86@kernel.org>
Subject: Re: 5.2-rc2: low framerate in flightgear, cpu not running at full speed, thermal related?
Date: Thu, 13 Jun 2019 10:52:57 +0200
Message-ID: <CAJZ5v0gRaDe6Fajdx3wqaLz9LQr6Z6iD+2yrrZeXJV+3HiAOZg@mail.gmail.com> (raw)
In-Reply-To: <008f01d52178$07b3be70$171b3b50$@net>

On Thu, Jun 13, 2019 at 1:40 AM Doug Smythies <dsmythies@telus.net> wrote:
>
> On 2019.06.12 14:25 Rafael J. Wysocki wrote:
> > On Wed, Jun 12, 2019 at 4:45 AM Doug Smythies <dsmythies@telus.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's what I mean.

> > 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.

I see.  Then this shouldn't matter.

> 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).
> Other differences between kernel 4.6 and 5.2-rc? might explain it.

Right.

I personally doubt that any thermal throttling is involved here.

> I did all my tests on kernel 5.2-rc3, except that one example from kernel 4.4 on my
> earlier reply, so that were not other variables than CPU scaling driver and
> governor changes.
>
> > The lower-than-expected frame rate may result from that, in principle.
>
> > One way to mitigate that might be to use intel_pstate in the passive
> > mode (pass intel_pstate=passive to the kernel in the command line)
> > along with either ondemand or schedutil as the governor.
>
> The CPU frequency verses load curves for this those two governors are very similar
> for both the acpi_cpufreq and intel_cpufreq (which is the intel_pstate driver
> in passive mode) drivers.

That's what I would expect.

Cheers!

  parent reply index

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-06-09 11:17 Pavel Machek
2019-06-09 11:23 ` Pavel Machek
2019-06-09 12:12   ` Pavel Machek
2019-06-09 13:55     ` Daniel Lezcano
2019-06-09 14:13     ` Zhang Rui
2019-06-11 10:12       ` Pavel Machek
2019-06-11 10:15         ` Viresh Kumar
2019-06-12  2:37 ` Doug Smythies
2019-06-12 21:44   ` Rafael J. Wysocki
2019-06-12 23:39     ` Doug Smythies
2019-06-13  8:11       ` Pavel Machek
2019-06-13  9:00         ` Rafael J. Wysocki
2019-06-13  8:52       ` Rafael J. Wysocki [this message]
2019-06-18  8:20         ` Doug Smythies
2019-06-18  8:34           ` Pavel Machek

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