Re: SMP question

[Artemio <artemio@artemio.net>]

Hello!

Thanks for your reply!

> > Hmmm... So, you mean uni-processor Linux kernel can't see two processors
> > as one "big" processor?
>
> No, but it will work as if it does...explain below.
>
> You have 2 processor packages, each one is HyperThreading capable. This
> means you have two 'CPUs' inside each package, so that sums up your 4 CPUs.
> But there is a flaw. The 2 'CPUs' inside each processor package are not
> full real CPUs, just two register sets that share cache, FP units, integer
> units and so on. So let's say your Xeon has 8 FP units, and you want to
> run a FPU intensive task with low or null disk IO. If you activate
> hyperthreading each of the 2 'cpus' has 4 FP units, so half the computation
> power. If you deactivate HT, you have 1 CPU with 8 FP units.
>
> In short, for FP intensive tasks, hyperthreading is a big lie...
> You can't run 2 computations in parallel.

Thanks for such in-depth explanation! 

As I understood, with HT enabled, Linux-SMP sees four CPUs with 5000 bogo mips 
each (of course I've already seen this in /proc/cpuinfo).

So, if I deactivate HT, will a UP Linux see one CPU with 4x5000=20000 bogo 
mips?

Anyway, I will try what I mentioned on that machine today.


Thank you again and good luck!


Artemio.