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

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Message-ID: <20080328140056.GC4404@ucw.cz>
Date:	Fri, 28 Mar 2008 15:00:57 +0100
From:	Pavel Machek <pavel@....cz>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Luciano Rocha <luciano@...otux.com>,
	Michael Meyer <mike65134@...oo.de>,
	linux-kernel@...r.kernel.org
Subject: Re: performance differences: "maxcpus=1" vs. "echo 0 > /sys/devices/system/cpu/cpu1/online"

On Tue 2008-03-25 18:16:47, Andi Kleen wrote:
> Luciano Rocha <luciano@...otux.com> writes:
> 
> > On Tue, Mar 25, 2008 at 02:47:50PM +0100, 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 should turn off the SMP alternative and switch to UP only,
> > optimising some locks and instructions.
> 
> CPU hot unplug will do the same. But it is unlikely it accounts
> for that much performance difference.
> 
> If he used maxcpus=0 it would make sense. maxcpus=0 disables
> the IO-APIC which likely makes a large difference. But it should
> be actually slower.
> 
> There should be actually no difference in theory between max_cpus=1
> and hot unplug to one CPU. Might be some bug.

Or thermal effect.  I'd expect that behaviour with bad cooling.

							Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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