Re: cpufreq: intel_pstate: map utilization into the pstate range

[Rafael J. Wysocki]

On Thu, Dec 30, 2021 at 7:21 PM Julia Lawall <julia.lawall@xxxxxxxx> wrote:
>
>
>
> On Thu, 30 Dec 2021, Rafael J. Wysocki wrote:
>
> > On Thu, Dec 30, 2021 at 6:54 PM Julia Lawall <julia.lawall@xxxxxxxx> 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.
>
> There's also no difference between 10 and 20.  This seems normal, because
> the same cores are either fully used or fully idle in both cases.  The
> idle ones are almost always in C6.

The turbostat output sent by you previously shows that the CPUs doing
the work are only about 15-or-less percent busy, though, and you get
quite a bit of C-state residency on them.  I'm assuming that this is
for 1 running thread.

Can you please run the 32 spinning threads workload (ie. on one
package) and with P-state locked to 10 and then to 20 under turbostat
and send me the turbostat output for both runs?