Re: CPU utilization

From: Julian Elischer <julian_at_elischer.org>
Date: Thu, 12 Apr 2007 11:05:14 -0700
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
> 
> 
> ------------------------------------------------------------------------
> 
> _______________________________________________
> freebsd-current_at_freebsd.org mailing list
> http://lists.freebsd.org/mailman/listinfo/freebsd-current
> To unsubscribe, send any mail to "freebsd-current-unsubscribe_at_freebsd.org"
Received on Thu Apr 12 2007 - 16:05:15 UTC

This archive was generated by hypermail 2.4.0 : Wed May 19 2021 - 11:39:08 UTC