Currently, binary viewers using POSIX file descriptors with READ mode open the file in ALL processes in the communicator. For WRITE mode, only process zero opens the file. The current PetscViewerBynaryXXX APIs make it really unnecessary to open the file in all processes for READ. I would like to get rid of that and always open on rank 0 for both READ or WROTE. Such change may impact third-party code using low-level IO. However, note that after MR 2594 https://gitlab.com/petsc/petsc/-/merge_requests/2594 gets merged, third party code should hardly need to access the file descriptors/MPI_File to perform IO (this would require some minor and relatively easy improvements to subviewers that I'm planning). In short, I would like to change binary viewers in READ mode using POSIX file descriptors to open files ONLY in rank 0 rather than ALL ranks as it is now. Any objections? PS: After all the enhancements to binary viewers, we should consider whether we should turn MPI-IO on by default. -- Lisandro Dalcin ============ Research Scientist Extreme Computing Research Center (ECRC) King Abdullah University of Science and Technology (KAUST) http://ecrc.kaust.edu.sa/