linux-kernel - RE: [RFC/RFT][PATCH v2] cpuidle: New timer events oriented governor for tickless systems

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Date:   Fri, 2 Nov 2018 08:39:42 -0700
From:   "Doug Smythies" <dsmythies@...us.net>
To:     "'Rafael J. Wysocki'" <rjw@...ysocki.net>,
        "'Giovanni Gherdovich'" <ggherdovich@...e.cz>
Cc:     "'Srinivas Pandruvada'" <srinivas.pandruvada@...ux.intel.com>,
        "'Peter Zijlstra'" <peterz@...radead.org>,
        "'LKML'" <linux-kernel@...r.kernel.org>,
        "'Frederic Weisbecker'" <frederic@...nel.org>,
        "'Mel Gorman'" <mgorman@...e.de>,
        "'Daniel Lezcano'" <daniel.lezcano@...aro.org>,
        "Doug Smythies" <dsmythies@...us.net>,
        "'Linux PM'" <linux-pm@...r.kernel.org>
Subject: RE: [RFC/RFT][PATCH v2] cpuidle: New timer events oriented governor for tickless systems

On 2018.10.26 02:12 Rafael J. Wysocki wrote:

...[snip]...

> The v2 is a re-write of major parts of the original patch.
>
> The approach the same in general, but the details have changed significantly
> with respect to the previous version.  In particular:
> * The decay of the idle state metrics is implemented differently.
> * There is a more "clever" pattern detection (sort of along the lines
>   of what the menu does, but simplified quite a bit and trying to avoid
>   including timer wakeups).
> * The "promotion" from the "polling" state is gone.
> * The "safety net" wakeups are treated as the CPU might have been idle
>   until the closest timer.

...[snip]...

I have been testing this V2 against a baseline that includes all
of the pending menu patches. My baseline kernel is somewhere
after 4.19, at 345671e.

A side note:
Recall that with the menu patch set tests, I found that the baseline
reference performance for the pipe test on one core had changed
significantly (worse - Kernel 4.19-rc1). Well, now it has changed
significantly again (better, and even significantly better than it
was for 4.18). 4.18 ~4.8 uSec/loop; 4.19 ~5.2 uSec/loop; 4.19+
(345671e) 4.2 uSec/loop.

This V2 is pretty good. All of the tests that I run gave similar
performance and power use between the baseline reference and V2.
I couldn't find any issues with the decay stuff, and I tried.
(sorry, I didn't do pretty graphs.)

After reading Giovanni's reply the other day, I tried the
Phoronix dbench test: 12 clients resulted in similar performance,
But TEOv2 used a little less processor package power; 256 clients
had about -7% performance using TEOv2, but (my numbers are not
exact) also used less processor package power.

On 2018.10.31 11:36 Giovanni Gherdovich wrote:

> Something I'd like to do now is verify that "teo"'s predictions
> are better than "menu"'s; I'll probably use systemtap to make
> some histograms of idle times versus what idle state was chosen
> -- that'd be enough to compare the two.

I don't know what a "systemtap" is, but I have (crude) tools to
post process trace data into histograms data. I did 5 minute
traces during the 12 client Phoronix dbench test and plotted
the results, [1]. Sometimes, to the right of the autoscaled
graph is another with fixed scaling. Better grouping of idle
durations with TEOv2 are clearly visible.

... Doug

[1] http://fast.smythies.com/linux-pm/k419p/histo_compare.htm