[mpich-discuss] attribute value of MPI_TAG_UB

[Wei-keng Liao]

According to MPI 3.1, Section 8.1.2, the attribute value of
MPI_TAG_UB attached to MPI_COMM_WORLD can be inquired by function
MPI_Comm_get_attr(). It has the same value on all processes of
MPI_COMM_WORLD.

My first question is should this value be the same across different runs?
A small test program using MPICH 3.3 shows they are not the same across
runs and across processes. But when compiled with the latest master branch,
they are the same. So, can I assume the answer to my question is YES and
MPICH has fixed this in the master branch?

My second question is that I notice MPI_TAG_UB is defined as a constant
of 0x64400001 (=1681915905) in mpi.h. That value is not the one returned
by MPI_Comm_get_attr(), which is 10345120. Is this intended?

I also tested OpenMPI 4.0.0. The inconsistency occurs across different
runs, as well as different processes. I can see OpenMPI defines
MPI_TAG_UB as an enum type, rather than a constant.

Wei-keng