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

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Date:	Tue, 25 Mar 2008 18:49:01 +0100
From:	"Wander Winkelhorst" <w.winkelhorst@...il.com>
To:	"Michael Meyer" <mike65134@...oo.de>
Cc:	"Andi Kleen" <andi@...stfloor.org>, linux-kernel@...r.kernel.org
Subject: Re: performance differences: "maxcpus=1" vs. "echo 0 > /sys/devices/system/cpu/cpu1/online"

On Tue, Mar 25, 2008 at 6:23 PM, Michael Meyer <mike65134@...oo.de> wrote:
>
>  --- Andi Kleen <andi@...stfloor.org> schrieb:
>
>
>  > 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.
>
>  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
>
>

What kind of CPU are you using? Some Intel CPU's do "funny stuff",
like dynamically overclocking itself when working on a single thread,
or using all of the 2nd level cache instead of sharing it with the
second core.

Regards,
Wander Winkelhorst.
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