[TriLUG] Hyperthreading and Xeons

[Aaron S. Joyner]

Jeremy Portzer wrote:

>So, I got my first Xeon server recently, and although it's really dual
>2.6 GHz, it shows as four processors to Linux.
>
>Does this mean that I should treat it as a four-processor computer for
>the purposes of starting processes?  For example, I have a program that
>runs as many copies of itself as there are processors (but you have to
>tell it how many processors there are).  Should I let it run four copies
>or stick with two?
>
>Any other consequences to hyperthreading I should know about?
>
>Thanks,
>Jeremy
>  
>
For all practical purposes, in my humble opinion, you should treat it as 
four processors.  The advantage of hyper-threading is if you feed it 
more threads to run at once, it can schedule the threads for more 
efficient execution.  If you're only sending it two threads, it won't be 
able to schedule as efficiently as if it had 4 threads to try to stuff 
into the available processing pipelines at once.  For a particularly 
enlightening diagram, check out the images on this page: 
http://arstechnica.com/articles/paedia/cpu/hyperthreading.ars/2  For a 
full explanation, read that whole article.  :)

Aaron S. Joyner