Write several distributed arrays with MPI IO - fortran

Question

I am rewriting a numerical simulation code that is parallelized using MPI in one direction.
So far, the arrays containing the data were saved by the master MPI process, which implied transferring the data from all MPI processes to one and allocate huge arrays to store the whole thing. It is not very efficient nor classy, and is a problem for large resolutions.
I am therefore trying to use MPI-IO to write directly the file from the distributed arrays. One of the constraint I have is that the written file needs to respect the fortran "unformatted" format (i.e. 4 bytes integer before and after each field indicating its size).
I wrote a simple test program, that works when I write only one distributed array to the file. However, when I write several arrays, the total size of the file is wrong and when comparing to the equivalent fortran 'unformatted' file, the files are different.
Here is the sample code :
module arrays_dim
implicit none
INTEGER, PARAMETER :: dp = kind(0.d0)
integer, parameter :: imax = 500
integer, parameter :: jmax = 50
integer, parameter :: kmax = 10
end module arrays_dim
module mpi_vars
use mpi
implicit none
integer, save :: ierr, myID, numprocs
integer, save :: i_start, i_end, i_mean, i_loc
integer, save :: subArray, fileH
integer(MPI_OFFSET_KIND), save :: offset, currPos
end module mpi_vars
program test
use mpi
use arrays_dim
use mpi_vars
real(dp), dimension(0:imax,0:jmax+1,0:kmax+1) :: v, w
real(dp), dimension(:,:,:), allocatable :: v_loc, w_loc
integer :: i, j, k
call MPI_INIT(ierr)
call MPI_COMM_RANK(MPI_COMM_WORLD, myID, ierr)
call MPI_COMM_SIZE(MPI_COMM_WORLD, numprocs, ierr)
i_mean = (imax+1)/numprocs
i_start = myID*i_mean
i_end = i_start+i_mean-1
if(i_mean*numprocs<imax+1) then
if(myID == numprocs-1) i_end = imax
endif
i_loc = i_end - i_start + 1
allocate(v_loc(i_start:i_end,0:jmax+1,0:kmax+1))
allocate(w_loc(i_start:i_end,0:jmax+1,0:kmax+1))
print*, 'I am:', myID, i_start, i_end, i_loc
do k=0,kmax+1
do j=0,jmax+1
do i=0,imax
v(i,j,k) = i+j+k
w(i,j,k) = i*j*k
enddo
enddo
enddo
if(myID==0) then
open(10,form='unformatted')
write(10) v
!write(10) w
close(10)
endif
do k=0,kmax+1
do j=0,jmax+1
do i=i_start,i_end
v_loc(i,j,k) = i+j+k
w_loc(i,j,k) = i*j*k
enddo
enddo
enddo
call MPI_Type_create_subarray (3, [imax+1, jmax+2, kmax+2], [i_loc, jmax+2, kmax+2], &
[i_start, 0, 0], &
MPI_ORDER_FORTRAN, MPI_DOUBLE_PRECISION, subArray, ierr)
call MPI_Type_commit(subArray, ierr)
call MPI_File_open(MPI_COMM_WORLD, 'mpi.dat', &
MPI_MODE_WRONLY + MPI_MODE_CREATE + MPI_MODE_APPEND, &
MPI_INFO_NULL, fileH, ierr )
call saveMPI(v_loc, (i_loc)*(jmax+2)*(kmax+2))
!call saveMPI(w_loc, (i_loc)*(jmax+2)*(kmax+2))
call MPI_File_close(fileH, ierr)
deallocate(v_loc,w_loc)
call MPI_FINALIZE(ierr)
end program test
!
subroutine saveMPI(array, n)
use mpi
use arrays_dim
use mpi_vars
implicit none
real(dp), dimension(n) :: array
integer :: n
offset = (imax+1)*(jmax+2)*(kmax+2)*8
if(myID==0) then
call MPI_File_seek(fileH, int(0,MPI_OFFSET_KIND), MPI_SEEK_CUR, ierr)
call MPI_File_write(fileH, [(imax+1)*(jmax+2)*(kmax+2)*8], 1, MPI_INTEGER, MPI_STATUS_IGNORE, ierr)
call MPI_File_seek(fileH, offset, MPI_SEEK_CUR, ierr)
call MPI_File_write(fileH, [(imax+1)*(jmax+2)*(kmax+2)*8], 1, MPI_INTEGER, MPI_STATUS_IGNORE, ierr)
endif
call MPI_File_set_view(fileH, int(4,MPI_OFFSET_KIND), MPI_DOUBLE_PRECISION, subArray, 'native', MPI_INFO_NULL, ierr)
call MPI_File_write_all(fileH, array, (i_loc)*(jmax+2)*(kmax+2), MPI_DOUBLE_PRECISION, MPI_STATUS_IGNORE, ierr)
end subroutine saveMPI
when the lines !write(10) w and !call saveMPI(w_loc, (i_loc)*(jmax+2)*(kmax+2)) are commented (i.e. I only write the v array), the code is working fine :
mpif90.openmpi -O3 -o prog main.f90
mpirun.openmpi -np 4 ./prog
cmp mpi.dat fort.10
cmp does not generate an output, so the files are identical.
If however I uncomment these lines, then the resulting files (mpi.dat and fort.10) are different. I am sure that the problem lies in the way I define the offset I use to write the data at the right position on the file, but I do not know how to indicate to the second call of saveMPI that the initial position should be the end of the file. What am I missing ?

Answer 1

Only the first call to saveMPI is working as you expect it to. Everything get messed up from the second call up. Here are few indications of what is happening:
MPI_File_set_view resets the independent file pointers and the shared file pointer to zero. See MPI_File_set_view for more details. So you are actually overwriting v data with w data when you call MPI_File_set_view in saveMPI.
with MPI_File_write, the data is written into those parts of the file specified by the current view. This mean that the way you are adding the size information into the file, is not really compatible with the view previously set for v.
calling MPI_File_seek with MPI_SEEK_CUR set the position relative to the current position of the individual pointer. So, for the second call, it is relative to the individual pointer of process 0
I do not use parallel IO that much, so I can not help more that this unless I step into the docs, which I do not have time to. The hint I can give is to:
add an additional parameter to saveMPI that will contain the absolute displacement of the data to write; this can be an [in out] arg. For the first call, it will be zero and for subsequent calls, it will be the size of all data already written to file, including the size information. It can be updated in saveMPI.
before writing the size information (by process 0) call MPI_File_set_view to reset the view to linear byte stream as originally given by MPI_File_open. This can be done by setting the etype and filetype to both MPI_BYTE in calling MPI_File_set_view. look into the doc of MPI_File_open for more information. You will then have to calls to MPI_File_set_view in saveMPI.
Your saveMPI subroutine could look like
subroutine saveMPI(array, n, disp)
use mpi
use arrays_dim
use mpi_vars
implicit none
real(dp), dimension(n) :: array
integer :: n, disp
offset = (imax+1)*(jmax+2)*(kmax+2)*8
call MPI_File_set_view(fileH, int(disp,MPI_OFFSET_KIND), MPI_BYTE, MPI_BYTE, 'native', MPI_INFO_NULL, ierr)
if(myID==0) then
call MPI_File_seek(fileH, int(0,MPI_OFFSET_KIND), MPI_SEEK_END, ierr)
call MPI_File_write(fileH, [(imax+1)*(jmax+2)*(kmax+2)*8], 1, MPI_INTEGER, MPI_STATUS_IGNORE, ierr)
call MPI_File_seek(fileH, int(offset,MPI_OFFSET_KIND), MPI_SEEK_CUR, ierr)
call MPI_File_write(fileH, [(imax+1)*(jmax+2)*(kmax+2)*8], 1, MPI_INTEGER, MPI_STATUS_IGNORE, ierr)
endif
call MPI_File_set_view(fileH, int(disp+4,MPI_OFFSET_KIND), MPI_DOUBLE_PRECISION, subArray, 'native', MPI_INFO_NULL, ierr)
call MPI_File_write_all(fileH, array, (i_loc)*(jmax+2)*(kmax+2), MPI_DOUBLE_PRECISION, MPI_STATUS_IGNORE, ierr)
disp = disp+offset+8
end subroutine saveMPI
and called like:
disp = 0
call saveMPI(v_loc, (i_loc)*(jmax+2)*(kmax+2), disp)
call saveMPI(w_loc, (i_loc)*(jmax+2)*(kmax+2), disp)
Finally, make sure that you delete the file between two calls because you are using MPI_MODE_APPEND.