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Date: Tue, 25 Mar 2008 18:23:41 +0100 (CET)
From: Michael Meyer <mike65134@yahoo.de>
Subject: Re: performance differences: "maxcpus=1" vs. "echo 0 > /sys/devices/system/cpu/cpu1/online"
To: Andi Kleen <andi@firstfloor.org>
Cc: linux-kernel@vger.kernel.org
In-Reply-To: <87abkmhgg0.fsf@basil.nowhere.org>
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--- Andi Kleen <andi@firstfloor.org> schrieb:

> Luciano Rocha <luciano@eurotux.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.

I had the following time values:

maxcpus=1:
real	0m1.642s
user	0m1.528s
sys	0m0.068s

maxcpus=2 and 
echo 1 > /sys/devices/system/cpu/cpu1/online:
real	0m2.579s
user	0m4.096s
sys	0m0.160s

maxcpus=2 and
echo 0 > /sys/devices/system/cpu/cpu1/online:
real	0m3.757s
user	0m3.632s
sys	0m0.112s


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