[TriLUG] Hyperthreading and Xeons

[John Franklin]

On Oct 13, 2004, at 1:06 PM, 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?

One of the big issues with HT is that it looks like SMP to HT-unaware 
kernels.  The difference between SMP and HT is the shared resources on 
the chip.  When moving a process to a second CPU (core), an HT-unaware 
kernel may blow the CPU cache, move the process to the second core 
thinking it's a physically separate CPU, and refill the CPU cache.  An 
HT-aware kernel will move the process w/o blowing the cache.

I don't know which Linux kernels (if any) are fully HT aware.

jf
-- 
John Franklin
franklin at elfie.org
ICBM: 38º 56' 32.6"N 77º 24' 47.7"W Z+62m
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