Re: [PATCH] doc: add note on usleep_range range

Pavel Machek
2017-01-10T21:25:35Z

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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.

> > > + less than 50 microseconds probably is only preventing
> > > + timer subsystem optimization but providing no benefit.
> >
> > And I don't trust you here. _If_ it prevents timer optimalization,
> > _then_ it provides benefit, at least in the average case.
> >
> here is the data:
>
> System: Intel Core i7 CPU 920 @ 2.67GHz Ocotocore
> OS: Debian 8.1 (but thats quite irrelevant)
> Kernel: 4.10-rc2 (localversion-next next-20170106)
> config: x86_64_defconfig (Voluntary | Preempt)
>
> Test-setup - poped this into akernel module and just
> brute force load/unload it in a loop - not very elegant
> but it does the job.
>
> static int __init usleep_test_init(void)
> {
> ktime_t now,last;
> unsigned long min,max;
> min = 200;
> max = 250;
> last = ktime_get();
> usleep_range(min, max);
> now = ktime_get();
> printk("%llu\n", ktime_to_ns(now)-ktime_to_ns(last));
> return 0;
> }
>
> Results:
>
> 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).

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

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