Messages in this thread | | | Date | Tue, 25 Mar 2008 18:49:01 +0100 | From | "Wander Winkelhorst" <> | Subject | Re: performance differences: "maxcpus=1" vs. "echo 0 > /sys/devices/system/cpu/cpu1/online" |
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On Tue, Mar 25, 2008 at 6:23 PM, Michael Meyer <mike65134@yahoo.de> wrote: > > --- 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 > >
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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