MPI questions

[Lars Rindorf]

Dear all

I have some questions regarding MPI and PETSc. I have browsed the
documentation and the internet, and I have not found the answers to
them. But if they are trivial, then please accept my apologies :-)

First of all, I'm running petsc on a quad core xeon and I'm using MUMPS
direct solver (iterative solvers not an option for my purposes). I
compile PETSc with g++ and gfortran under redhat linux. My program using
petsc is compiled with mpiCC (a C++ program).

The questions are:

1. Does petsc need to be compiled with mpi (mpicc, mpiCC etc) to be
efficient? If not, is it faster when compiled with mpi compilers?

2. Using a CPU with four cores then does MPI provide any significant
speed-up? And if so, can it be optimized?

3. I'm having problem with a problem that should scale (approx.) linear
in time as function of DOF. But it only scales linearly for small number
of DOF for large number of DOF it scales quadraticly. This is before the
RAM is even 1/4 full. (Direct solvers use a lot of RAM.). I guess what
is limiting the speed is the communication between the CPU and the RAM,
but I don't know Does anybody has experience with this problem? Or maybe
a small test program that I can test?

Thanks!

KR, Lars