linux-kernel - nohz and strange sleep latencies

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Message-ID: <20071119203152.GA1772@elf.ucw.cz>
Date:	Mon, 19 Nov 2007 21:31:52 +0100
From:	Pavel Machek <pavel@....cz>
To:	kernel list <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, tglx@...utronix.de
Subject: nohz and strange sleep latencies

Hi!

On unloaded x60 system, 2.6.24-rc3 (tainted-pavel-so if someone can
reproduce it, it would be helpful):

pavel@amd:~$ while true; do time sleep 0.01; done
0.00user 0.00system 0.01 (0m0.013s) elapsed 30.71%CPU
0.00user 0.00system 0.02 (0m0.024s) elapsed 8.36%CPU
0.00user 0.00system 0.01 (0m0.013s) elapsed 15.53%CPU
0.00user 0.00system 0.02 (0m0.026s) elapsed 7.75%CPU
0.00user 0.00system 0.01 (0m0.013s) elapsed 23.34%CPU
0.00user 0.00system 0.20 (0m0.207s) elapsed 1.45%CPU
0.00user 0.00system 0.01 (0m0.013s) elapsed 15.59%CPU
0.00user 0.00system 0.23 (0m0.230s) elapsed 1.30%CPU
0.00user 0.00system 0.01 (0m0.013s) elapsed 15.61%CPU
0.00user 0.00system 0.20 (0m0.201s) elapsed 1.48%CPU
0.00user 0.00system 0.01 (0m0.013s) elapsed 15.50%CPU
0.00user 0.00system 0.19 (0m0.190s) elapsed 1.05%CPU
0.00user 0.00system 0.01 (0m0.013s) elapsed 23.37%CPU
0.00user 0.00system 0.04 (0m0.049s) elapsed 4.09%CPU
0.00user 0.00system 0.01 (0m0.013s) elapsed 23.35%CPU
0.00user 0.00system 0.01 (0m0.017s) elapsed 11.89%CPU
0.00user 0.00system 0.01 (0m0.013s) elapsed 23.38%CPU
0.00user 0.00system 0.08 (0m0.081s) elapsed 3.71%CPU
0.00user 0.00system 0.01 (0m0.013s) elapsed 23.33%CPU
0.00user 0.00system 0.01 (0m0.019s) elapsed 15.92%CPU

nohz=off helps a lot. while true; do time sleep 0.0; done does not
have unexpected latencies.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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