[Ukfreebsd] Disabling hyperthreading on Intel CPUs

[andy thomas]

On Wed, 26 Jan 2011, Mark Blackman wrote:

> I think you sent this to more lists than you meant to. :)
>
> Out of curiousity, why does one turn off HT?

Oops, sorry (still recovering from Burn's night). I want to disable HT 
because it can impact performance when used to execute code compiled for 
parallel execution across multiple PCs. It can be disabled in the BIOS but 
these machines run Windows during the daytime and some users complain if 
they suspect HT is switched off. Being able to disable it at runtime when 
these machine boot FBSD at night would be great.

Andy

> 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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