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 own part of the matrix in the correct order. The output file is in "unformatted" form. But when I tried to read the file in a serial code (I have the correct size of the big matrix allocated), I got an error which I don't understand.
My question is: in an MPI program, how do you get a binary file as the serial version output for a big matrix which is stored by different processes?
Here is my attempt:
if(ThisProcs == RootProcs) then
open(unit = file_restart%unit, file = file_restart%file, form = 'unformatted')
write(file_restart%unit)psi
close(file_restart%unit)
endif
#ifdef USEMPI
call mpi_barrier(mpi_comm_world,MPIerr)
#endif
do i = 1, NProcs - 1
if(ThisProcs == i) then
open(unit = file_restart%unit, file = file_restart%file, form = 'unformatted', status = 'old', position = 'append')
write(file_restart%unit)psi
close(file_restart%unit)
endif
#ifdef USEMPI
call mpi_barrier(mpi_comm_world,MPIerr)
#endif
enddo
Psi is the big matrix, it is allocated as:
Psi(N_lattice_points, NPsiStart:NPsiEnd)
But when I tried to load the file in a serial code:
open(2,file=File1,form="unformatted")
read(2)psi
forrtl: severe (67): input statement requires too much data, unit 2 (I am using MSVS 2012+intel fortran 2013)
How can I fix the parallel part to make the binary file readable for the serial code? Of course one can combine them into one big matrix in the MPI program, but is there an easier way?
Edit 1
The two answers are really nice. I'll use access = "stream"
to solve my problem. And I just figured I can use inquire to check whether the file is "sequential" or "stream".
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
Alternatively, read with a similar loop structure:
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.