linux-kernel - Re: performance differences: "maxcpus=1" vs. "echo 0

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Date:	Sat, 29 Mar 2008 21:55:19 +0100 (CET)
From:	Michael Meyer <mike65134@...oo.de>
To:	Pavel Machek <pavel@....cz>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: performance differences: "maxcpus=1" vs. "echo 0 > /sys/devices/system/cpu/cpu1/online"


--- Pavel Machek <pavel@....cz> schrieb:

> On Tue 2008-03-25 14:47:50, Michael Meyer wrote:
> > Hi, 
> > 
> > what is the difference between booting a dual core
> > machine with "maxcpus=1" or by deactivating the second
> > core at run time with "echo 0 >
> > /sys/devices/system/cpu/cpu1/online"?
> 
> maxcpus=1 : core stays powered off
> 0 > online : core enters halt.
> 
> > I observed a funny behaviour of apache ant: although
> > it uses javac which is single threaded, a compile run
> > with "maxcpus=1" is actually faster than a compile run
> > with both cores activated. But with the second core
> > deactivated using "echo 0 >
> > /sys/devices/system/cpu/cpu1/online" it is even slower
> > than with both cores.
> 
> Thermal fun? Check cooling.

I have an extremely well cooled case and a Thermaltake Typhoon sitting
on the E6600 (Intel TDP 65 Watt). At a room temperature of 20°C both
cores idle at 21°C and after one hour of two instances of prime95
running they are both at 28-33°C. As the E6600 is specified until 60°C
(I believe), this should prevent any thermal throttling?
 
> > Is here any method to get the exact same behaviour of
> > "maxcpus=1" with disabling the second core only
> > temporarily? So that the second core could be disabled
> > before the ant execution and enabled after the ant
> > execution?
> 
> Patch pushing 0 > online cores into C4 would be nice.
> -- 
> (english) http://www.livejournal.com/~pavelmachek
> (cesky, pictures)
> http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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