OS X and many Linux distributions now come with a system MPI. If you install by downloading MPICH and then use 'mpirun', your code will never run in parallel, at best, or do something completely unexpected, at worst.
The current default instructions are: export PETSC_DIR=$PWD ./config/configure.py --with-cc=gcc --with-fc=gfortran --download-f-blas-lapack=1 --download-mpich=1 make all test I think OS X-specific instructions that don't install MPICH or that explicitly suggest appending $PETSC_DIR/$PETSC_ARCH/bin to your path when running parallel jobs after installing mpich will help prevent users from falling into this trap. A -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-dev/attachments/20110605/ba62f261/attachment.html>