MPI_Allreduce mix elements in the sum - arrays

Question

I am parallelising a fortran code which works with no problem in a no-MPI version. Below is an excerpt of the code.
Every processor does the following:
For a certain number of particles it evolves certain quantities in the loop "do 203"; in a given interval which is divided in Nint subintervals (j=1,Nint), every processor produces an element of the vectors Nx1(j), Nx2(j).
Then, the vectors Nx1(j), Nx2(j) are sent to the root (mype =0) which in every subinterval (j=1,Nint) sums all the contributions for every processor: Nx1(j) from processor 1 + Nx1(j) from processor 2.... The root sums for every value of j (every subinterval), and produces Nx5(j), Nx6(j).
Another issue is that if I deallocate the variables the code remains in standby after the end of the calculation without completing the execution; but I don't know if this is related to the MPI_Allreduce issue.
include "mpif.h"
...
integer*4 ....
...
real*8
...
call MPI_INIT(mpierr)
call MPI_COMM_SIZE(MPI_COMM_WORLD, npe, mpierr)
call MPI_COMM_RANK(MPI_COMM_WORLD, mype, mpierr)
! Allocate variables
allocate(Nx1(Nint),Nx5(Nint))
...
! Parameters
...
call MPI_Barrier (MPI_COMM_WORLD, mpierr)
! Loop on particles
do 100 npartj=1,npart_local
call init_random_seed()
call random_number (rand)
...
Initial condition
...
do 203 i=1,1000000 ! loop for time evolution of single particle
if(ufinp.gt.p1.and.ufinp.le.p2)then
do j=1,Nint ! spatial position at any momentum
ls(j) = lb+(j-1)*Delta/Nint !Left side of sub-interval across shock
rs(j) = ls(j)+Delta/Nint
if(y(1).gt.ls(j).and.y(1).lt.rs(j))then !position-ordered
Nx1(j)=Nx1(j)+1
endif
enddo
endif
if(ufinp.gt.p2.and.ufinp.le.p3)then
do j=1,Nint ! spatial position at any momentum
ls(j) = lb+(j-1)*Delta/Nint !Left side of sub-interval across shock
rs(j) = ls(j)+Delta/Nint
if(y(1).gt.ls(j).and.y(1).lt.rs(j))then !position-ordered
Nx2(j)=Nx2(j)+1
endif
enddo
endif
203 continue
100 continue
call MPI_Barrier (MPI_COMM_WORLD, mpierr)
print*,"To be summed"
do j=1,Nint
call MPI_ALLREDUCE (Nx1(j),Nx5(j),npe,mpi_integer,mpi_sum,
& MPI_COMM_WORLD, mpierr)
call MPI_ALLREDUCE (Nx2(j),Nx6(j),npe,mpi_integer,mpi_sum,
& MPI_COMM_WORLD, mpierr)
enddo
if(mype.eq.0)then
do j=1,Nint
write(1,107)ls(j),Nx5(j),Nx6(j)
enddo
107 format(3(F13.2,2X,i6,2X,i6))
endif
call MPI_Barrier (MPI_COMM_WORLD, mpierr)
print*,"Now deallocate"
! deallocate(Nx1) !inserting the de-allocate
! deallocate(Nx2)
close(1)
call MPI_Finalize(mpierr)
end
! Subroutines
...

Answer 1

Then, the vectors Nx1(j), Nx2(j) are sent to the root (mype =0) which in every subinterval (j=1,Nint) sums all the contributions for every processor: Nx1(j) from processor 1 + Nx1(j) from processor 2.... The root sums for every value of j (every subinterval), and produces Nx5(j), Nx6(j).
This is not what an allreduce does. Reduction means the summation is done in parallel across all processes. allreduce means all processes will get the result of the summing.
Your MPI_Allreduces:
call MPI_ALLREDUCE (Nx1(j),Nx5(j),npe,mpi_integer,mpi_sum, &
& MPI_COMM_WORLD, mpierr)
call MPI_ALLREDUCE (Nx2(j),Nx6(j),npe,mpi_integer,mpi_sum, &
& MPI_COMM_WORLD, mpierr)
Actually look like the count should be 1 here. This is because count just states how many elements you are to receive from each process, not how many there will be in total.
However, you actually do not need that loop, because the allreduce luckily is capable of handling multiple elements all at once. Thus, I believe instead of the loop with your allreduces, you actually want something like:
integer :: Nx1(nint)
integer :: Nx2(nint)
integer :: Nx5(nint)
integer :: Nx6(nint)
call MPI_ALLREDUCE (Nx1, Nx5, nint, mpi_integer, mpi_sum, &
& MPI_COMM_WORLD, mpierr)
call MPI_ALLREDUCE (Nx2, Nx6, nint, mpi_integer, mpi_sum, &
& MPI_COMM_WORLD, mpierr)
Nx5 will contain the sum of Nx1 across all partitions, and Nx6 the sum across Nx2.
The information in your question is a little bit lacking, so I am not quite sure, if this is what you are looking for.