linux-kernel - Re: BFS vs. mainline scheduler benchmarks and measurements

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Message-ID: <4AB52667.6020608@openwrt.org>
Date:	Sat, 19 Sep 2009 20:43:51 +0200
From:	Felix Fietkau <nbd@...nwrt.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Michael Buesch <mb@...sch.de>, Con Kolivas <kernel@...ivas.org>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>
Subject: Re: BFS vs. mainline scheduler benchmarks and measurements

Ingo Molnar wrote:
> * Felix Fietkau <nbd@...nwrt.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.
The difference here cannot be flash related, as I ran a kernel image
with the whole userland contained in initramfs. No on-flash filesystem
was mounted or accessed.

- Felix
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