Hyper Threading on P4

[Steve Snyder]

On Wednesday 01 September 2004 12:29 pm, John McBride wrote:
> Matthias Bauw wrote:
> >hi,
> >
> >I was in fact wondering how to get the most out of my P4 3.0Ghz with
> >Hyper Threading.
>
> Watch out that the intel marketing monster doesn't kick you in the
> shorts. You still have only one CPU. Turning virtual CPUs on in our
> dual P4 boxes ("four"  CPUs) severely degraded the box performance. In
> many cases all turning on virtual CPUs does is make linux, under
> certain loads, to think "hey, I can shuffle things off to another
> CPU"--false in this case (it's the same CPU)  and there is overhead to
> swapping processes between CPUs, thus yielding "less than 0"
> performance increase.

Was this with a 2.6.x kernel?

I've read that the behavior you described was true with 2.4.x, which 
didn't distinguished between physical and virtual CPUs.  Supposedly the 
2.6 series of kernels is smarter about this.