Hypertherading

[Boris Kochergin]

Barney Cordoba wrote:
>
>
> --- On Wed, 5/6/09, pluknet <pluknet at gmail.com> wrote:
>
>   
>> From: pluknet <pluknet at gmail.com>
>> Subject: Re: Hypertherading
>> To: "Barney Cordoba" <barney_cordoba at yahoo.com>
>> Cc: "Current at freebsd.org" <Current at freebsd.org>
>> Date: Wednesday, May 6, 2009, 10:55 PM
>> 2009/5/7 Barney Cordoba <barney_cordoba at yahoo.com>:
>>     
>>> I just got a shiny new nehalem box and it comes up
>>>       
>> with 16 processors with dual quads installed. Is there any
>> benefit or should hyperthreading be disabled?
>>     
>> Hi. There is a measurable win if hyperthreading is enabled
>> [1].
>> You can switch it off via machdep.hyperthreading_enabled
>> loader tunable.
>>
>> [1]
>> http://lists.freebsd.org/pipermail/freebsd-stable/2009-January/047460.html
>>
>>
>> -- 
>> wbr,
>> pluknet
>>     
>
> I assume you mean hyperthreading-allowed?
>
> I set 
>
> sysctl -a | grep hyper
>
> machdep.hyperthreading_allowed: 0
>
>
> but it still launches 16 cpus. Is that expected? It doesn't seem correct.
>
> Barney
>
>
>       
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>   
If I recall correctly, that sysctl only prevents processes from being 
scheduled on the "virtual" hyper-threaded CPUs, and does not affect 
their discovery by the kernel.

-Boris