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

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Message-ID: <87abkmhgg0.fsf@basil.nowhere.org>
Date:	25 Mar 2008 18:16:47 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Luciano Rocha <luciano@...otux.com>
Cc:	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"

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.

-Andi
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