[Wrf-users] wrfv3 & mpi

[Rodrigo Delgado Urzúa]

Cesar Hernandez Baño wrote:
> Hi
>
> I'm using wrfv3 , with mpich 1.2.7p1. The problem is when I use more 
> cpus/nodes in the cluster, the more slower runs the test.
>
> I have 4 pcs, Pentium 4 , four cpu each server. I run 
> test/em_real/wrf.exe with mpirun
>
> Network cards are Gigabit; using 2 processes network traffic flows at 
> 70 MBits aprox. Using 12 processes uses 150 MBit.
> When I use 2 processes, each CPU uses 80-90% aprox. Using 3 processes, 
> each CPU uses 50-60%.
>
> What's the problem?
>
> Cheers
>

Hi Cesar,

I think that's a normal thing. The explanation is simple: more processes 
in different machines implies more communication to perform computing. 
This happens with relatively slow interconnects (as FastE or GigE) as 
with the faster ones (e.g., Infiniband), the difference is the fast 
interconnects have room to allow more traffic and low latencies than the 
slow ones, so you can add CPUs to the simulation without compromise 
performance. But even the fastest interconnects can be completely busy 
when you add several CPUs to the simulation, resulting in lack of 
performance.

Regards,

Rodrigo