linux-kernel - Epic regression in throughput since v2.6.23

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Message-ID: <87hbvdiogq.fsf@depni.sinp.msu.ru>
Date:	Tue, 08 Sep 2009 12:57:09 +0000
From:	Serge Belyshev <belyshev@...ni.sinp.msu.ru>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Con Kolivas <kernel@...ivas.org>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>
Subject: Epic regression in throughput since v2.6.23

Hi. I've done measurments of time taken by make -j4 kernel build
on a quadcore box.  Results are interesting: mainline kernel
has regressed since v2.6.23 release by more than 10%.

The following graph is time taken by "make -j4" (median over 9 runs)
versus kernel version.  The huge (10%) regressions since v2.6.23 is
apparent.  Note that tip/master c26f010 is better than current mainline.
Also note that BFS is significantly better than both and shows the same
throughput as vanilla v2.6.23:

http://img403.imageshack.us/img403/7029/epicmakej4.png

The following plot is a detailed comparison of time taken versus number
of parallel jobs. Note that at "make -j4" (which equals number of hardware
threads), BFS has the minimum (best performance), 
and tip/master -- maximum (worst).  I've also tested mainline v2.6.31
(not shown on the graph) which produces similar, albeit a bit slower,
results as the tip/master.

http://img179.imageshack.us/img179/5335/epicbfstip.png

Conclusion are
1) mainline has severely regressed since v2.6.23
2) BFS shows optimal performance at make -jN where N equals number of
   h/w threads, while current mainline scheduler performance is far from
   optimal in this case.
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