[Ukfreebsd] Disabling hyperthreading on Intel CPUs

Mark Blackman mark at blackmans.org
Wed Jan 26 09:19:40 GMT 2011


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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