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Date: Tue, 25 Mar 2008 18:23:41 +0100 (CET) From: Michael Meyer <mike65134@...oo.de> To: Andi Kleen <andi@...stfloor.org> Cc: linux-kernel@...r.kernel.org Subject: Re: performance differences: "maxcpus=1" vs. "echo 0 > /sys/devices/system/cpu/cpu1/online" --- 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 Lesen Sie Ihre E-Mails jetzt einfach von unterwegs. www.yahoo.de/go -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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