linux-kernel - Re: cpufreq: intel_pstate: map utilization into the pstate range

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Date:   Thu, 30 Dec 2021 18:58:41 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Julia Lawall <julia.lawall@...ia.fr>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Francisco Jerez <currojerez@...eup.net>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Len Brown <lenb@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: cpufreq: intel_pstate: map utilization into the pstate range

On Thu, Dec 30, 2021 at 6:54 PM Julia Lawall <julia.lawall@...ia.fr> wrote:
>
> > > The effect is the same.  But that approach is indeed simpler than patching
> > > the kernel.
> >
> > It is also applicable when intel_pstate runs in the active mode.
> >
> > As for the results that you have reported, it looks like the package
> > power on these systems is dominated by package voltage and going from
> > P-state 20 to P-state 21 causes that voltage to increase significantly
> > (the observed RAM energy usage pattern is consistent with that).  This
> > means that running at P-states above 20 is only really justified if
> > there is a strict performance requirement that can't be met otherwise.
> >
> > Can you please check what value is there in the base_frequency sysfs
> > attribute under cpuX/cpufreq/?
>
> 2100000, which should be pstate 21
>
> >
> > I'm guessing that the package voltage level for P-states 10 and 20 is
> > the same, so the power difference between them is not significant
> > relative to the difference between P-state 20 and 21 and if increasing
> > the P-state causes some extra idle time to appear in the workload
> > (even though there is not enough of it to prevent to overall
> > utilization from increasing), then the overall power draw when running
> > at P-state 10 may be greater that for P-state 20.
>
> My impression is that the package voltage level for P-states 10 to 20 is
> high enough that increasing the frequency has little impact.  But the code
> runs twice as fast, which reduces the execution time a lot, saving energy.
>
> My first experiment had only one running thread.  I also tried running 32
> spinning threads for 10 seconds, ie using up one package and leaving the
> other idle.  In this case, instead of staying around 600J for pstates
> 10-20, the pstate rises from 743 to 946.  But there is still a gap between
> 20 and 21, with 21 being 1392J.
>
> > You can check if there is any C-state residency difference between
> > these two cases by running the workload under turbostat in each of
> > them.
>
> The C1 and C6 cases (CPU%c1 and CPU%c6) are about the same between 20 and
> 21, whether with 1 thread or with 32 thread.

I meant to compare P-state 10 and P-state 20.

20 and 21 are really close as far as the performance is concerned, so
I wouldn't expect to see any significant C-state residency difference
between them.