[petsc-dev] Question about Binary-IO in READ mode with POSIX APIs

[Lisandro Dalcin]

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/
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