linux-kernel - Re: [ANNOUNCE] BFS CPU scheduler version 0.420 AKA "Smoking" for linux kernel 3.3.0

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Message-ID: <CABqErrGaBLisO4YK5dP2O9Pv0QonZ+q9G43jm=Nf12yWVGv6tw@mail.gmail.com>
Date:	Tue, 27 Mar 2012 09:30:21 +1100
From:	Con Kolivas <kernel@...ivas.org>
To:	Mike Galbraith <efault@....de>
Cc:	Valdis.Kletnieks@...edu, Gene Heskett <gene.heskett@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [ANNOUNCE] BFS CPU scheduler version 0.420 AKA "Smoking" for
 linux kernel 3.3.0

On 26 March 2012 00:37, Mike Galbraith <efault@....de> wrote:
> Yeah.  In all the interactivity testing I've ever done, it's really hard
> to not see what you expect and/or hope to see.  For normal desktop use,
> I don't see any real difference with BFS vs CFS unless I load test of
> course, and that can go either way, depending on the load.
>
> Example:
>
> 3.3.0-bfs vs 3.3.0-cfs - identical config
>
> Q6600 desktop box doing a measured interactivity test.
>
> time mplayer BigBuckBunny-DivXPlusHD.mkv, with massive_intr 8 as competition
>
> no bg load real    9m56.627s              1.000
> CFS        real    9m59.199s              1.004
> BFS        real    12m8.166s              1.220
>
> As you can see, neither scheduler can run that perfectly on my box, as
> the load needs a tad more than its fair share.  However, the Interactive
> Experience was far better in CFS in this case due to it being more fair.
> In BFS, the interactive tasks (mplayer/Xorg) could not get their fair
> share, causing interactivity to measurably suffer.

massive_intr runs a number of threads that each run for 8ms and then
sleep for 1ms. That means they are 89% cpu bound. Run 8 of them and
your CPU load is 88.8 * 8 = 7.1. So now you're testing a difficult
mplayer benchmark in the presence of a load of 7.1 on a CPU with 4
cores. I don't know how much CPU the playback of your particular video
is but I suspect it does require a fair amount of CPU based on the CPU
it got back in your test. I can virtually guarantee that the amount of
CPU BFS is giving to mplayer is proportional to how much CPU is left.
Ergo as far as I can see, BFS is likely being absolutely perfectly
fair. This sort of fairness equation has been already elucidated in
the pHD that I linked to in my original post and he has done a much
more thorough analysis than this kind of drive-by test that you're
doing and misinterpreting has already shown that BFS is fair to a
fault.

snip the rest

'top' snapshots are uninteresting because CFS and BFS report cpu time
completely differently and a single snapshot tells us nothing.

Snip uninteresting-to-desktop-user throughput benchmarks.
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