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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