Hyperthreading in P4

[Daniel Kidger]

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?

Yours,
Daniel.

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