Problems.

MPICH does not implement MPI_Cancel according to the MPI specification (it can't always cancel sends). As described above, this ``bug'' has been repeated in many MPICH-derived implementations but is not too important. An earlier version of MPICH used Fortran common blocks to implement some Fortran MPI constants. This is fixed (with a lot of work) in the current MPICH release (1.1), but remains in many MPICH-derived implementations. While MPICH is generally carefully written, there are many MPICH routines that are not implemented as efficiently as they could be. Sometimes these are fixed in MPICH-derived implementations, but often not. An example is the MPI_Alltoall routine, which is implemented in a way that is likely to cause hot spots in any network.