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Date: Sat, 4 Dec 2010 12:39:48 -0500
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Subject: Re: [PATCH v4] sched: automated per session task groups
From: Colin Walters <walters@verbum.org>
To: Mike Galbraith <efault@gmx.de>
Cc: Ingo Molnar <mingo@elte.hu>, Oleg Nesterov <oleg@redhat.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
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On Sat, Nov 20, 2010 at 2:35 PM, Mike Galbraith <efault@gmx.de> wrote:

> A recurring complaint from CFS users is that parallel kbuild has a negative
> impact on desktop interactivity.  This patch implements an idea from Linus,
> to automatically create task groups.  This patch only per session autogroups,
> but leaves the way open for enhancement.

Resurrecting this thread a bit, one question I didn't see discussed is simply:

Why doesn't "nice" work for this?  On my Fedora 14 system, "ps alxf"
shows almost everything in my session is running at the default nice
0.  The only exceptions are "/usr/libexec/tracker-miner-fs" at 19, and
pulseaudio at -11.

I don't know What would happen if say the scheduler effectively
group-scheduled each nice value?  Then, what we tell people to do is
run "nice make".  Which in fact, has been documented as a thing to do
for decades.  Actually I tend to use "ionice" too, which is also
useful if any of your desktop applications happen to make the mistake
of doing I/O in the mainloop (emacs fsync()ing in UI thread, I'm
looking at you).

Quickly testing kernel-2.6.35.6-48.fc14.x86_64 on a "Intel(R)
Core(TM)2 Quad CPU    Q9400  @ 2.66GHz", the difference between "make
-j 128" and "nice make -j 128" is quite noticeable.  As you'd expect.
The CFS docs already say:

"The CFS scheduler has a much stronger handling of nice levels and SCHED_BATCH
than the previous vanilla scheduler: both types of workloads are isolated much
more aggressively"

Does it just need to be even more aggressive, and people use "nice"?