[Ukfreebsd] Disabling hyperthreading on Intel CPUs

Daniel Austin daniel at kewlio.net
Wed Jan 26 09:31:35 GMT 2011


Hi,

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.

As they appear as cores, you could disable the cores via sysctl - but as 
Mark said, why would you want to?


Thanks,

Daniel.

On 26/01/2011 09:19, Mark Blackman wrote:
> I think you sent this to more lists than you meant to. :)
>
> Out of curiousity, why does one turn off HT?
>
>
> On Wed, 26 Jan 2011 08:34:17 -0000, andy thomas <andy at time-domain.co.uk>
> wrote:
>
>> Dos anyone know what happned to the machdep.hyperthreading_allowed
>> tunable? It was definitely there in (I think) 5.x but in 8.1 it's not:
>>
>> andy at dell380:~ $ sysctl machdep.hyperthreading_allowed=0
>> sysctl: unknown oid 'machdep.hyperthreading_allowed'
>>
>> There are some new CPU sysctls in 8.x but they seem to disable cores
>> rather than hyperthreading, which is enabled by default in the kernel.
>> I would like to be able to turn off hyperthreading at runtime on a
>> large number of HP PCs that are fitted with single quad-core CPUs
>> without touching the BIO settings.
>>
>> Andy
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>
>



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