linux-kernel - Re: [PATCH] sched: remove sched_find_first

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Date:   Thu, 18 May 2017 09:14:11 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Yury Norov <ynorov@...iumnetworks.com>
Cc:     linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        Akinobu Mita <mita@...aclelinux.com>,
        Mike Galbraith <efault@....de>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] sched: remove sched_find_first_bit()


* Yury Norov <ynorov@...iumnetworks.com> wrote:

> On Tue, May 16, 2017 at 10:30:42AM +0200, Ingo Molnar wrote:
> > 
> > * Yury Norov <ynorov@...iumnetworks.com> wrote:
> > 
> > > I collected about 700 results in dmesg, and took 600 fastest.
> > > For the vanilla kernel, the average value is 368, and for patched
> > > kernel it is 388. It's 5% slower. But the standard deviation is 
> > > really big for both series' - 131 and 106 cycles respectively, which
> > > is ~ 30%. And so, my conclusion is: there's no benefit in using
> > > sched_find_first_bit() comparing to find_first_bit().
> > 
> > Erm, so you in essence claim:
> > 
> > 	"according to measurements the new code is 5% slower, with a high, 30% 
> > 	 stddev, hence the new code is better!"
> > 
> > Basic logic fail...
> > 
> > Thanks,
> > 
> > 	Ingo
> 
> No, in essence I claim that scatter is so big (in both cases, and in
> case of vanilla kernel even bigger) that 5% is not a meaningful
> difference. To be specific - new measured value is inside the
> confidence interval of previous one.

Firstly, the high spread is due to the poor measurement method: by increasing the 
number of measurements the standard deviation can be reduced.

Secondly, and most importantly, the claim you made based on the numbers is simply 
false:

> > > And so, my conclusion is: there's no benefit in using
> > > sched_find_first_bit() comparing to find_first_bit().

you _measured no benefit_, and in fact the result you got is leaning towards it 
being a benefit.

When doing a proper measurement it might strengthen, vanish or turn around - we 
simply don't know.

Thanks,

	Ingo