Re: Kernel reports 4 CPUS instead of 2...

From: Samuel Flory (
Date: Wed Oct 16 2002 - 17:14:24 EST

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:

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