From: Andi Kleen <andi@firstfloor.org>
To: Luciano Rocha <luciano@eurotux.com>
Cc: Michael Meyer <mike65134@yahoo.de>, linux-kernel@vger.kernel.org
Subject: Re: performance differences: "maxcpus=1" vs. "echo 0 > /sys/devices/system/cpu/cpu1/online"
Date: 25 Mar 2008 18:16:47 +0100 [thread overview]
Message-ID: <87abkmhgg0.fsf@basil.nowhere.org> (raw)
In-Reply-To: <20080325140825.GA19722@bit.office.eurotux.com>
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.
-Andi
next prev parent reply other threads:[~2008-03-25 17:17 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-03-25 13:47 performance differences: "maxcpus=1" vs. "echo 0 > /sys/devices/system/cpu/cpu1/online" Michael Meyer
2008-03-25 14:08 ` Luciano Rocha
2008-03-25 16:38 ` Michael Meyer
2008-03-25 17:16 ` Andi Kleen [this message]
2008-03-25 17:23 ` Michael Meyer
2008-03-25 17:49 ` Wander Winkelhorst
2008-03-25 17:56 ` Michael Meyer
2008-03-25 17:57 ` Andi Kleen
2008-03-25 23:27 ` Len Brown
2008-03-26 7:26 ` Michael Meyer
2008-03-28 14:00 ` Pavel Machek
2008-03-28 13:59 ` Pavel Machek
2008-03-29 20:55 ` Michael Meyer
2008-03-29 21:22 ` Pavel Machek
2008-03-29 22:16 ` Michael Meyer
2008-04-02 11:31 ` Pavel Machek
2008-03-29 23:22 ` Bernd Eckenfels
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