linux-kernel - Re: [PATCH] doc: add note on usleep

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Message-ID: <20170112103207.GG29366@amd>
Date:   Thu, 12 Jan 2017 11:32:07 +0100
From:   Pavel Machek <pavel@....cz>
To:     Nicholas Mc Guire <der.herr@...r.at>
Cc:     Nicholas Mc Guire <hofrat@...dl.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH] doc: add note on usleep_range range

On Wed 2017-01-11 08:50:07, Nicholas Mc Guire wrote:
> On Tue, Jan 10, 2017 at 10:25:29PM +0100, Pavel Machek wrote:
> > Hi!
> > 
> > > > "to have zero jitter" at least. I believe it is "does not".
> > > > 
> > > > I don't see how atomic vs. non-atomic context makes difference. There
> > > > are sources of jitter that affect atomic context...
> > > 
> > > The relevance is that while there is jitter in atomic context it can
> > > be quite small (depending on your hardware and the specifics of system
> > > config) but in non-atomic context the jitter is so large that it
> > > makes no relevant difference if you give usleep_range slack of a few
> > > microseconds.
> > 
> > I disagree here. Even in non-atomic code, you'll get _no_ jitter most
> > of the time. If you care about average case, small slack may still
> > make sense.
> 
> yes - thats what the results say - the mean does not differe significantly
> so if you care about average case - it makes no difference.

You did not demonstrate that.

> > > usleep_range() 5000 samples - idle system 
> > >  100,100         200,200         190,200
> > >  Min.   :188481  Min.   :201917  Min.   :197793
> > >  1st Qu.:207062  1st Qu.:207057  1st Qu.:207051
> > >  Median :207139  Median :207133  Median :207133
> > >  Mean   :207254  Mean   :207233  Mean   :207244
> > >  3rd Qu.:207341  erd Qu.:207262  3rd Qu.:207610
> > >  Max.   :225340  Max.   :214222  Max.   :214885
> > > 
> > > 100,200 to 200,200 is maybe relevant impact for
> > > some systems with respect to the outliers, but
> > > mean and median are almost the same, for
> > > 190,200 to 200,200 there is statistically no
> > > significant difference with respect to performance
> > > Note that the timestamp before and after also has
> > > jitter - so only part of the jitter can be attributed
> > > to usleep_range() it self. But idle system optimization
> > > is not that interesting for most systems.
> > 
> > I disagree here. Most of systems are idle, most of the time. You say
> > that basically everyone should provide 50 usec of slack... So I guess
> > I'd like to see comparisons for 200,200 and 200,250 (and perhaps also
> > 200,500 or something).
> >
> I did not say that everyone should use 50us of slack - rather the statement 
> was "makes no relevant difference if you give usleep_range slack of a few
> microseconds." and that min==max makes *no* sense and that providing 
> even just small slack (in 10s of us range) makes a relevant difference 
> at system level.

You did not demonstrate any "relevant difference at system level".

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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