Messages in this thread | | | Date | Wed, 16 Oct 2002 15:14:24 -0700 | From | Samuel Flory <> | 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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