OT? hypertread on or off for SMP kernel

[michael <linux@networkingnewsletter.org.uk>]

Hi - I've had a quick look on the WWW but can't find much of help. I've
a dual Xeon box running testing and kernel from backports:
	m@r:~$ uname -a
	Linux r 2.4.27-1-686-smp #1 SMP Fri Sep 3 06:34:36 UTC 2004 i686
	GNU/Linux

The SMP kernel does indeed make use of the processors available: 2 if
hyperthreading (HT) is off and 4 if it is on.

My questions are:
a) for MPI codes (mpich v1.2.6 compiled with Intel 8.1 compilers), under
which circumstances is better performance achieved with HT on or off?
I'm looking for quite a detailed analysis which, if hasn't already been
done, I can kick off if anybody's interested

b) for general day to day running do people leave HT on or turn it off?

Thanks in advance,
Michael, Atmospheric Research Group (Arg), Univ of Manchester