Fortran unformatted output with each MPI process writing part of an array

Question

In my parallel program, there was a big matrix. Each process computed and stored a part of it. Then the program wrote the matrix to a file by letting each process wrote its

Answer 1

This isn't a problem specific to MPI, but would also happen in a serial program which took the same approach of writing out chunks piecemeal.

Ignore the opening and closing for each process and look at the overall connection and transfer statements. Your connection is an unformatted file using sequential access. It's unformatted because you explicitly asked for that, and sequential because you didn't ask for anything else.

Sequential file access is based on records. Each of your write statements transfers out a record consisting of a chunk of the matrix. Conversely, your input statement attempts to read from a single record.

Your problem is that while you try to read the entire matrix from the first record of the file that record doesn't contain the whole matrix. It doesn't contain anything like the correct amount of data. End result: "input statement requires too much data".

So, you need to either read in the data based on the same record structure, or move away from record files.

The latter is simple, use stream access

open(unit = file_restart%unit, file = file_restart%file,  &
     form = 'unformatted', access='stream')

Alternatively, read with a similar loop structure:

do i=1, NPROCS
  ! read statement with a slice
end do

This of course requires understanding the correct slicing.

Alternatively, one can consider using MPI-IO for output, which is very similar to using stream output. Read this back in with stream access. You can find about this concept elsewhere on SO.