Re: OT? hypertread on or off for SMP kernel
On Thu, 2004-12-09 at 12:53, Ron Johnson wrote:
> On Thu, 2004-12-09 at 11:48 +0000, michael wrote:
> > 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?
>
> How threaded are your apps? Or, maybe, how many apps do you run
> at once?
>
> On Windows benchmarks, at least, performance of non-threaded apps
> drops when HT is turned on.
The apps in question are MPI Fortran/C codes and I can say at run time
how many processors (threads) I require. Thus for my dual Xeon I could
have HT off and go up to 2 processors. But is it worth switching HT back
on and going to 4 (logical) processors, that is my main question. I
believe the answer is "depends on the nature of your codes" and I'm
interested in hearing if anybody has studied this (eg with known memory
intensive, compute intensive, comms intensive code kernels (not linux
kernels!)). If not, I'm happy to kick this off as a low priority
project. I've also posed this question to comp.parallel.mpi so if people
are interested I can summarize any results/answers here.
Thanks, Michael
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