How can I disable hyperthreads, but NOT smp ?

Richard Collyer richard at
Tue Sep 20 21:28:21 UTC 2011

On 20/09/2011 21:34, Jason Usher wrote:
> FreeBSD 6.4-RELEASE system with two physical CPUs, each of which are HT capable.  From dmesg:
> cpu0 (BSP): APIC ID: 0
> cpu1 (AP): APIC ID: 1
> cpu2 (AP): APIC ID: 6
> cpu3 (AP): APIC ID: 7
> I also see this:
> machdep.hyperthreading_allowed: 0
> The problem is, I have reason to believe that my workload benefits greatly from SMP, but is not playing nicely with hyperthreading.  That is, it is trying to farm out 4 things at a time to a cpu setup that really is only able to do two things at a time.
> How can I disable HT completely, but still retain ALL the benefits of SMP ?
> Thanks.

It's usually a setting in the BIOS to disable Hyperthreading then 
reconfigure the kernel to only use 2 cores (which it might do 

Hyperthreading is outside the world of FreeBSD. As far as it knows there 
is still 4 "logical processors". Disabling hyperthreading it should see 
2 LPs.


Richard Collyer
Blue Apex Ltd
07900 997 380

Registered as a company in England and Wales (Reg Number: 05982514).
Registered Address: 42 King Richards Hill, Earl Shilton, Leicester, LE9 7EY

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