[petsc-users] MatAssemblyBegin freezes during MPI communication

[袁煕]

Dear PETSc Experts,

My FEM program works well generally, but in some specific cases with
multiple CPUs are used, it freezes when calling MatAssemblyBegin where
PMPI_Allreduce is called (see attached file).

After some investigation, I found that it is most probably due to

・ MatSetValue is not called from all CPUs before MatAssemblyBegin

For example, when 4 CPUs are used, if there are elements in CPU 0,1,2 but
no elements in CPU 3, then all CPUs other than CPU 3 would call
MatSetValue  function. I want to know

1. If my conjecture could be right? And If so
2. Are there any convenient means to avoid this problem?

Thanks,
Xi YUAN, PhD Solid Mechanics
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