linux-kernel - Re: [Bugme-new] [Bug 12562] New: High overhead while switching or synchronizing threads on different cores

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Date:	Thu, 29 Jan 2009 15:05:34 +0100
From:	Thomas Pilarski <thomas.pi@...or.de>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Galbraith <efault@....de>,
	Gregory Haskins <ghaskins@...ell.com>,
	bugme-daemon@...zilla.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [Bugme-new] [Bug 12562] New: High overhead while switching or
 synchronizing threads on different cores


> In short this program is carefully crafted to defeat all our affinity
> tests - and I'm not sure what to do.

I am sorry, although it is not carefully crafted. The function random()
is causing my problem. I currently have no real data, so I tried to make
some random utilization and data.

Without the random() function it works even with 80MB of data and I get
great results.

./ThreadSchedulingIssue 1 10485760 8 312
All threads finished: 309 messages in 29.369 seconds / 10.521 msg/s

schedtool -a 1 -e ./ThreadSchedulingIssue 1 10485760 8 312
All threads finished: 312 messages in 44.284 seconds / 7.045 msg/s

It does not even regress with more then two threads. 

./ThreadSchedulingIssue 2 10485760 8 312
All threads finished: 311 messages in 28.040 seconds / 11.091 msg/s

./ThreadSchedulingIssue 4 10485760 8 312
All threads finished: 309 messages in 28.021 seconds / 11.027 msg/s

With small amounts of data the speed on two core is even doubled. 

schedtool -a 1 -e ./ThreadSchedulingIssue 1 1048 8 312000
All threads finished: 311992 messages in 19.437 seconds / 16051.247
msg/s

./ThreadSchedulingIssue 3 1048 8 312000
All threads finished: 311998 messages in 9.652 seconds / 32324.411 msg/s

./ThreadSchedulingIssue 8 1048 8 312000
All threads finished: 311997 messages in 9.339 seconds / 33406.370 msg/s

--------------
Perhaps it is as it should be, but when I run the test (without
random()) with 2*8 threads, it uses ~186 of the cpu, while an instance
of "bzip2 -9 -c /dev/urandom >/dev/null" gets only 12%.

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