CPU utilization
Julian Elischer
julian at elischer.org
Thu Apr 12 18:05:15 UTC 2007
Robert Watson wrote:
>
> On Thu, 12 Apr 2007, Dag-Erling Smørgrav wrote:
>
>> Randall Stewart <rrs at cisco.com> writes:
>>> machdep.hyperthreading_allowed: 0
>>
>> Note that enabling hyperthreading is more likely to harm performance
>> than to help it. You should just disable it in the BIOS, and run a UP
>> kernel.
>
> Historically this has been true, but some more recent results I've seen
> suggest that both hyperthreading hardware has improved, and the
> efficiency of our SMP implementation and scheduler has lead to it being
> more effective used. I would reevaluate this on more modern hardware and
> using a more recent kernel before assuming this remains true for your
> application.
In addition to this, to answer the original question, I remember a
commit so that if you disable a cpu (or HT cpu) it doesn't get counted
in the CPU % so if you have 2 cpus and disable one hten prior to that
commit it was not possible to get > 50% busy but after that commit
you could get 100% "of the available CPUs". That fix is not (I believe)
in 6.2.
I have had applications where HT was useful. They had a mix of
integer and floating point work, and were long running, using all
of their quanta. Usually with normal work it was a was or a loss.
>
> Robert N M Watson
> Computer Laboratory
> University of Cambridge
>
>
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