Date: Sat, 19 Sep 2009 21:39:56 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Felix Fietkau <nbd@openwrt.org>
Cc: Michael Buesch <mb@bu3sch.de>, Con Kolivas <kernel@kolivas.org>,
       linux-kernel@vger.kernel.org, Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Mike Galbraith <efault@gmx.de>
Subject: Re: BFS vs. mainline scheduler benchmarks and measurements
Message-ID: <20090919193956.GA21719@elte.hu>
References: <20090906205952.GA6516@elte.hu> <20090907182629.GA3484@elte.hu> <20090908074825.GA11413@elte.hu> <200909081645.18505.mb@bu3sch.de> <20090918112454.GE9930@elte.hu> <4AB39D3A.3000204@openwrt.org> <20090919180124.GK5366@elte.hu> <4AB52667.6020608@openwrt.org>
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* Felix Fietkau <nbd@openwrt.org> wrote:

> Ingo Molnar wrote:
> > * Felix Fietkau <nbd@openwrt.org> wrote:
> > 
> >> Ingo Molnar wrote:
> >> > Well that's a really memory constrained MIPS device with like 16 MB of 
> >> > RAM or so? So having effects from small things like changing details in 
> >> > a kernel image is entirely plausible.
> >>
> >> Normally changing small details doesn't have much of an effect. While 
> >> 16 MB is indeed not that much, we do usually have around 8 MB free 
> >> with a full user space running. Changes to other subsystems normally 
> >> produce consistent and repeatable differences that seem entirely 
> >> unrelated to memory use, so any measurable difference related to 
> >> scheduler changes is unlikely to be related to the low amount of RAM. 
> >> By the way, we do frequently also test the same software with devices 
> >> that have more RAM, e.g. 32 or 64 MB and it usually behaves in a very 
> >> similar way.
> > 
> > Well, Michael Buesch posted vmstat results, and they show what i have 
> > found with my x86 simulated reproducer as well (these are Michael's 
> > numbers):
> > 
> > procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
> >  r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
> >  1  0	   0  15892   1684   5868    0    0     0     0  268    6 31 69  0  0
> >  1  0	   0  15892   1684   5868    0    0     0     0  266    2 34 66  0  0
> >  1  0	   0  15892   1684   5868    0    0     0     0  266    6 33 67  0  0
> >  1  0	   0  15892   1684   5868    0    0     0     0  267    4 37 63  0  0
> >  1  0	   0  15892   1684   5868    0    0     0     0  267    6 34 66  0  0
> > 
> > on average 4 context switches _per second_. The scheduler is not a 
> > factor on this box.
> > 
> > Furthermore:
> > 
> >  | I'm currently unable to test BFS, because the device throws strange 
> >  | flash errors. Maybe the flash is broken :(
> > 
> > So maybe those flash errors somehow impacted the measurements as well?
> I did some tests with BFS v230 vs CFS on Linux 2.6.30 on a different
> MIPS device (Atheros AR2317) with 180 MHz and 16 MB RAM. When running
> iperf tests, I consistently get the following results when running the
> transfer from the device to my laptop:
> 
> CFS: [  5]  0.0-60.0 sec    107 MBytes  15.0 Mbits/sec
> BFS: [  5]  0.0-60.0 sec    119 MBytes  16.6 Mbits/sec
> 
> The transfer speed from my laptop to the device are the same with BFS
> and CFS. I repeated the tests a few times just to be sure, and I will
> check vmstat later.

Which exact mainline kernel have you tried? For anything performance 
related running latest upstream -git (currently at 202c467) would be 
recommended.

	Ingo
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