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

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Date:   Tue, 28 Dec 2021 18:40:18 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Julia Lawall <julia.lawall@...ia.fr>
Cc:     Francisco Jerez <currojerez@...eup.net>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        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 Tue, Dec 28, 2021 at 5:58 PM Julia Lawall <julia.lawall@...ia.fr> wrote:
>
> I looked a bit more into why pstate 20 is always using the least energy. I
> have just one thread spinning for 10 seconds, I use a fixed value for the
> pstate, and I measure the energy usage with turbostat.

How exactly do you fix the pstate?

> I tried this on a
> 2-socket Intel 6130 and a 4-socket Intel 6130.  The experiment runs 40
> times.
>
> There seem to be only two levels of CPU energy usage.  On the 2-socket
> machine the energy usage is around 600J up to pstate 20 and around 1000J
> after that.  On the 4-socket machine it is twice that.

These are the package power numbers from turbostat, aren't they?

> The change in RAM energy usage is similar, eg around 320J for the 2-socket
> machine up to pstate 20, and around 460J for higher pstates.
>
> On the 6130, pstate 21 is 2.1GHz, which is the nominal frequency of the
> machine.  So it seems that the most efficient thing is to be just below
> that.  The reduced execution time with pstate 20 as compared to pstate 10
> greatly outweighs any small increase in the energy usage due to changing
> the frequency.
>
> Perhaps there is something abnormal in how the machines are configured?