Közben úgy gondoltam mégis keresek… nem teljesen az a kérdés, de a válaszban benne van ez is:
http://www.linuxquestions.org/questions/sh…threadid=348539

Azért ezt beidézem:

I do have a P4 w/ hyperthreading if that factors into the SMP or not.If you have a Hyperthreading CPU (and you enable Hyperthreading in the BIOS), then your single CPU will present itself as two CPUs to the Operating System (provided that the OS understands and supports SMP). In fact, you could consider Hyperthreading as a kind of „poor man’s SMP.” (BTW, for those unfamiliar with the acronym, „SMP” stands for „Symmetrical MultiProcessing”).
Now, if you run a non-SMP kernel (e.g., your „Red Hat Linux (2.4.20-31.9)” entry), then Linux will behave as a uniprocessor Operating System, and ignore the second CPU instance that Hyperthreading makes available.
If, on the other hand, you run an SMP kernel, then Linux will become a Multi-Processor system, and exploit both CPU instances provided by the Hyperthreading feature.

Note that both kernel versions (SMP vs. non-SMP) will support both operating modes (Hyperthreading enabled vs. disabled);
…
By the way, it’s not a bad idea to keep the non-SMP entry available, even if you have an SMP (or Hyperthreading) system, just in case you run into trouble with SMP in any way.