Re: SMP question

[J.A. Magallon]

On 06.11, Artemio wrote:
> Hello!
> 
> > > How much performance will I loose this way? Is SMP *THAT* critical?
> >
> > 	You will lose about half your CPU power.
> 
> 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.

-- 
J.A. Magallon <[e-mail]>      \                 Software is like sex:
werewolf.able.es                         \           It's better when it's free
Mandrake Linux release 9.2 (Cooker) for i586
Linux 2.4.21-rc7-jam1 (gcc 3.3 (Mandrake Linux 9.2 3.3-1mdk))