Twin dual core XEON reports 8 CPUs on 64bit kernel

Carsten Aulbert carsten at
Sat Mar 24 06:09:55 UTC 2007

Paul Oppenheim wrote:

> You do NOT want to disable this in BIOS, you will see a performance 
> penalty if you do. Linux knows what to do with SMT since kernel ... 
> 2.6.9 IIRC?

The performance penalty *might* happen, but there are certain codes
which are running much much slower with HT enabled. I'm working in
scientific computing and I've seen codes which are heavy on the memory
interface and the results are simply abysmal with HT (IIRC single thread
finishes in 200 seconds, two HT threads take much more than 2500 seconds
mostly due to memory interface congestion/cache line thrashing).

So, I guess in the end it comes down to: YMMV ;)


PS: I guess with standard desktop applications you'll see a performance

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