Hyperthreading in P4

Daniel Kidger Daniel.Kidger at quadrics.com
Mon Apr 29 13:54:35 EDT 2002

Ole W. Saastad [mailto:ole at scali.no] wrote:

>New Pentium 4 processors has hyper threading capabilities
>and when setting this the linux sees 4 cpus on each dual node.

>I have done some testing with OpenMP programs and found that
>for OpenMP threaded programs there is no performance gain in using 
>the hypertheading. Using a number of threads that equal the number 
>of real processors seems to be optimal.

Having a multi-threaded processor should help codes which are limited by
memory *latency*.
I doubt if memory-bandwidth limited codes would benefit much, since memory
bandwidth is a limited resource which is already oversubscribed on many
dual-P4 nodes.

Also there are no more floating-point units that a standard P4, so CPU
limited codes wont see any improvement either.

Perhaps the interesting area though is where the CPU can issue instructions
to the FPU *AND* to the integer execution units concurrently but for
different threads. This would perhaps allow general Linux system services to
not impact the performance on application codes?


Dr. Dan Kidger, Quadrics Ltd.      daniel.kidger at quadrics.com
One Bridewell St., Bristol, BS1 2AA, UK         0117 915 5505
----------------------- www.quadrics.com --------------------

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