Messages in this thread | | | Subject | Re: performance differences: "maxcpus=1" vs. "echo 0 > /sys/devices/system/cpu/cpu1/online" | From | Andi Kleen <> | Date | 25 Mar 2008 18:16:47 +0100 |
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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
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