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Date: Wed, 16 Oct 2002 15:14:24 -0700
From: Samuel Flory <sflory@rackable.com>
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To: Mark Mielke <mark@mark.mielke.cc>
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Subject: Re: Kernel reports 4 CPUS instead of 2...
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Mark Mielke wrote:

>On Wed, Oct 16, 2002 at 01:47:32PM -0700, Samuel Flory wrote:
>  
>
>>  Try shutting off hyperthreading in the bios.  Keep in mind 
>>hyperthreading is net loss if you are running a single nonthreaded app. 
>>Also you might want to check if there aren't io speed issues.  
>>    
>>
>
>Is this true? It seems to me that the 'on-demand execution units' would
>simply be devoted to the one task, resulting in zero loss.
>  
>

  In perfect world yes, but in reality there is overhead.  I've tested 
this on a quad xeon.  A "make bzImage" is a bit faster with 
hyperthreading off.  Of course a make -j 8 bzImage is faster with 
hyperthreading on.  I haven't tried this on a dual xeon.  (It could be a 
scaling issue 4 vs 8 processors.)

>I see hyperthreading becoming a problem if two threads are scheduled to
>execute at the same time before the operating system, and if they each
>need access to the same execution units at the same time.
>  
>
And if both threads need different items in L(whatever) cache it gets 
even worse.

There are a few good overviews on the subject:
http://www.intel.com/technology/hyperthread/
http://arstechnica.com/paedia/h/hyperthreading/hyperthreading-1.html
http://www.anandtech.com/cpu/showdoc.html?i=1576&p=2

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