linux-kernel - Re: [BUG nohz]: wrong user and system time accounting

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Message-ID: <1490880922.4702.66.camel@gmx.de>
Date:   Thu, 30 Mar 2017 15:35:22 +0200
From:   Mike Galbraith <efault@....de>
To:     Rik van Riel <riel@...hat.com>,
        Frederic Weisbecker <fweisbec@...il.com>
Cc:     Luiz Capitulino <lcapitulino@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [BUG nohz]: wrong user and system time accounting

On Thu, 2017-03-30 at 09:02 -0400, Rik van Riel wrote:
> On Thu, 2017-03-30 at 14:51 +0200, Frederic Weisbecker wrote:

> > Also, why does it raise power consumption issues?
> 
> On a system without either nohz_full or nohz idle
> mode, skewed ticks result in CPU cores waking up
> at different times, and keeping an idle system
> consuming power for more time than it would if all
> the ticks happened simultaneously.

And if your server farm is mostly idle, that power savings may delay
your bankruptcy proceedings by a whole microsecond ;-)

Or more seriously, what skew does do on boxen of size X today is
something for perf to say.  At the time, removal was very bad for my 8
socket box, and allegedly caused huge SGI beasts in horrific pain.

	-Mike