linux-kernel - Re: [patch] CFS scheduler, -v10

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Date:	Tue, 08 May 2007 09:51:38 +0200
From:	Mike Galbraith <efault@....de>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Con Kolivas <kernel@...ivas.org>,
	Nick Piggin <npiggin@...e.de>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Williams <pwil3058@...pond.net.au>,
	Thomas Gleixner <tglx@...utronix.de>, caglar@...dus.org.tr,
	Willy Tarreau <w@....eu>,
	Gene Heskett <gene.heskett@...il.com>, Mark Lord <lkml@....ca>
Subject: Re: [patch] CFS scheduler, -v10

On Mon, 2007-05-07 at 18:33 +0200, Ingo Molnar wrote:
>  - complete and finetune the 'smooth load calculation' mechanism
>    (Mike Galbraith) 

Houston, we have alien artifacts.

Artifact 1:

root@...er: taskset -c 1 ./thud 5
starting 5 children
running...

root@...er: taskset -c 1 ./chew
pid 7551, prio   0, interval of 99984800 nsec
pid 7551, prio   0, out for    7 ms, ran for 1690 ms, load  99%
pid 7551, prio   0, out for    2 ms, ran for 1794 ms, load  99%
pid 7551, prio   0, out for    4 ms, ran for 9665 ms, load  99%
pid 7551, prio   0, out for  775 ms, ran for  379 ms, load  32% <== hurt pain ouch!
pid 7551, prio   0, out for   32 ms, ran for    6 ms, load  17%
pid 7551, prio   0, out for   32 ms, ran for    4 ms, load  13%
pid 7551, prio   0, out for   32 ms, ran for    4 ms, load  13%
pid 7551, prio   0, out for   32 ms, ran for    4 ms, load  13%

When thud starts, chew takes a size 14EEE latency hit if bits 1, 2 and 3
are set in sysctl_sched_load_smoothing.  Good for burst load, rotten for
interactivity if that burst load ain't X.  Start a thud 5 without the
taskset, and you'll feel the lurch.

Artifact 2:

If both bits 1 and 2 are set, and bit 3 is _not_ set, chew's out drops
to ~6ms and run drops to < 1ms.  Excessive context switching.

Computing delta_fair with smoothed load in update_curr() fixes some
things, but still breaks others.  The cost/benefit ratio isn't adding up
very favorably.  Tinkering.

Values 0x7 and 0xf should be avoided the like plague if your test load
includes bursty multiple hogs-from-hell.  Values 0x3 and 0xb are merely
context switch happy.

	-Mike

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