Re: [PATCH V2 0/3] Introduce Thermal Pressure

Ingo Molnar
2019-04-25T17:44:32Z

* Ingo Molnar <mingo@kernel.org> wrote:

>
> * Peter Zijlstra <peterz@infradead.org> wrote:
>
> > On Wed, Apr 17, 2019 at 08:29:32PM +0200, Ingo Molnar wrote:
> > > Assuming PeterZ & Rafael & Quentin doesn't hate the whole thermal load
> > > tracking approach.
> >
> > I seem to remember competing proposals, and have forgotten everything
> > about them; the cover letter also didn't have references to them or
> > mention them in any way.
> >
> > As to the averaging and period, I personally prefer a PELT signal with
> > the windows lined up, if that really is too short a window, then a PELT
> > like signal with a natural multiple of the PELT period would make sense,
> > such that the windows still line up nicely.
> >
> > Mixing different averaging methods and non-aligned windows just makes me
> > uncomfortable.
>
> Yeah, so the problem with PELT is that while it nicely approximates
> variable-period decay calculations with plain additions, shifts and table
> lookups (i.e. accelerates pow()), AFAICS the most important decay
> parameter is fixed: the speed of decay, the dampening factor, which is
> fixed at 32:
>
> Documentation/scheduler/sched-pelt.c
>
> #define HALFLIFE 32
>
> Right?
>
> Thara's numbers suggest that there's high sensitivity to the speed of
> decay. By using PELT we'd be using whatever averaging speed there is
> within PELT.
>
> Now we could make that parametric of course, but that would both
> complicate the PELT lookup code (one more dimension) and would negatively
> affect code generation in a number of places.

I missed the other solution, which is what you suggested: by
increasing/reducing the PELT window size we can effectively shift decay
speed and use just a single lookup table.

I.e. instead of the fixed period size of 1024 in accumulate_sum(), use
decay_load() directly but use a different (longer) window size from 1024
usecs to calculate 'periods', and make it a multiple of 1024.

This might just work out right: with a half-life of 32 the fastest decay
speed should be around ~20 msecs (?) - and Thara's numbers so far suggest
that the sweet spot averaging is significantly longer, at a couple of
hundred millisecs.

Thanks,

Ingo