[Ukfreebsd] Disabling hyperthreading on Intel CPUs

Robert Watson rwatson at FreeBSD.org
Wed Jan 26 09:44:12 GMT 2011

On Wed, 26 Jan 2011, Bruce Cran wrote:

> On Wed, 26 Jan 2011 09:31:35 +0000 Daniel Austin <daniel at kewlio.net> wrote:
>> In days of old, sometimes HT could slow things down in some situations. But 
>> with newer FreeBSD's handling of HT, I understood that this was no longer 
>> the case - so no reason to disable it anymore.
> I think it might be more that while HT often slowed things down in NetBurst, 
> with Nehalem Intel redesigned it and as a result it often improves 
> performance (e.g. 
> http://sqlblog.com/blogs/joe_chang/archive/2010/03/23/hyper-threading-comments.aspx).

Yes, this is my impression as well--synchronisation in P4 Xeon, etc, was 
incredibly expensive, and individual threads very weak.  The result was that 
the added synchronisation overhead compared to the potential throughput 
benefits of additional threads were not a winning combination.  Recent 
threaded hardware is much, much better.  Synchronisation costs are, as a 
result of abandoning the earlier cache coherency model, down, improving the 
efficiency of SMP (etc) dramatically on Intel hardware.  These days, I'd leave 
HTT on for performance.


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