I can't give you a complete answer, but I think this is less an MPI
question and more of a Fortran question. The question is if you have a
Fortran derived type, one of whose components is a POINTER, what does
the data structure look like in linear memory? I could imagine the
answer is implementation dependent. Anyhow, here is a sample, non-MPI,
Fortran program that illustrates the question:
% cat b.f90
type :: small
integer, pointer :: array(:)
end type small
type(small) :: lala
integer, pointer :: array(:)
n = 20
allocate( lala%array(n) )
allocate( array(n) )
lala%array = (/ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
15, 16, 17, 18, 19, 20 /)
array = (/ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
15, 16, 17, 18, 19, 20 /)
call sub(lala)
call sub(lala%array)
call sub( array)
end
subroutine sub(x)
integer x(20)
write(6,*) x
end
% f90 b.f90
% a.out
599376 20 4 599372 1 20 -4197508 1 2561 0 33 0 0 0 0 0 0 0 0 0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
%
So, your model of 20 consecutive words does not work if you pass the
derived type. It does work if you pass the POINTER component. This is
with Oracle (Sun) Studio Fortran. Again, I can imagine the behavior
depends on the Fortran compiler.
I suspect what's going on is that a POINTER is a complicated data
structure that has all sorts of metadata in it, but if you pass a
POINTER the compiler knows to pass the thing you're pointing to rather
than the metadata itself.
Jeremy Roberts wrote:
I'm trying to parallelize a Fortran
code with rather complicated derived types full of pointer arrays.
When I build the MPI type for sending, all the static components are
sent, but the pointer arrays are not (and retain initial values). I
imagine this has to do with memory addresses when creating the MPI
struct, but I have no idea how to fix it.
I've included a simple code illustrating my issue below. Any
suggestions?
program mpi_struct_example
use mpi
implicit none
! declarations
type :: small
real, pointer :: array(:)
end type small
type(small) :: lala
integer :: stat, counts(1), types(1), ierr, iam, n=0, MPI_SMALL
integer (kind=MPI_ADDRESS_KIND) :: displs(1)
! initialize MPI and get my rank
call MPI_INIT( ierr )
call MPI_COMM_RANK( MPI_COMM_WORLD, iam, ierr )
n = 20
allocate( lala%array(n) )
lala%array = 2.0
! build block counts, displacements, and oldtypes
counts = (/n/)
displs = (/0/)
types = (/MPI_REAL/)
! make and commit new type
call MPI_TYPE_CREATE_STRUCT( 1, counts, displs, types, MPI_SMALL,
ierr )
call MPI_TYPE_COMMIT( MPI_SMALL, ierr )
if (iam .eq. 0) then
! reset the value of the array
lala%array = 1.0
call MPI_SEND( lala, 1, MPI_SMALL, 1, 1, MPI_COMM_WORLD,
ierr) ! this doesn't work
!call MPI_SEND( lala%array, n, MPI_REAL, 1, 1,
MPI_COMM_WORLD, ierr) ! this does work
write (*,*) "iam ",iam," and lala%array(1) = ",
lala%array(1)
else
call MPI_RECV( lala, 1, MPI_SMALL, 0, 1, MPI_COMM_WORLD,
stat, ierr ) ! this doesn't work
!call MPI_RECV( lala%array, n, MPI_REAL, 0, 1,
MPI_COMM_WORLD, stat, ierr ) ! this does work
write (*,*) "iam ",iam," and lala%array(1) = ",
lala%array(1), " ( should be 1.0)"
end if
call MPI_FINALIZE(ierr)
end program mpi_struct_example
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