On 2007-04-05, Michael E. Thomadakis <miket@sc.tamu.edu> wrote:
>
> Jan thanks for your reply. We have a 40 node p5-575 cluster with 32GB
> DRAM/node. Nodes are attached to 2 planes of HPS. I was contending that
> SMT ON should benefit the cluster since all of the cluster control
> processes (RSCT, GPFS, HPS assistance, etc.) are heavily multi-threaded
> and by not being compute-intensive, they should benefit from SMT ON.

I believe these cluster control processes will have a negligble
impact (*). It's the applications you'll be running that will decide
if you should use SMT or not. We let the users decide on a job
by job basis if they want SMT by specifying this in their LoadLeveler
scripts, and then have LL pre/post scripts turn on/off SMT

>
> Are there any public benchmarks that I can run to demonstrate the
> benefits of SMT ?

Don't know, but it will probably be easy to demonstrate higher
troughput by running 32 separate cpu intensive tasks on a system
with SMT=on compared to SMT=off. And then you could probably also
experience the negative impact of SMT by running a 16 task
MPI-job on a system with SMT=on -- where you might not get all
tasks running on separate physical cpus...

* BTW: do make sure that there's nothing causing any load on a
system that's idle. An idle system should show a load of
0.00 in "uptime". We had an "xmtrend" system process that
was generating a load of about 0.05-0.10 on our nodes,
and that was killing the parallel benchmark performance
on fully loaded systems.

-jf