lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  linux-cve-announce  PHC 
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:   Thu, 25 Apr 2019 19:44:25 +0200
From:   Ingo Molnar <>
To:     Peter Zijlstra <>
Cc:     Thara Gopinath <>,,,,,,,,,,,,, Quentin Perret <>,
        "Rafael J. Wysocki" <>
Subject: Re: [PATCH V2 0/3] Introduce Thermal Pressure

* Ingo Molnar <> wrote:

> * Peter Zijlstra <> 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.



Powered by blists - more mailing lists