[mpich-discuss] MPICH does not work for quad precision

[Jeff Hammond]

There is a challenge here because there is no standard type in C
corresponding to REAL*16.  Either the reduction operation needs to be
written in Fortran or MPICH needs to figure out the compiler-dependent
equivalent of REAL*16 that works in C.  While GCC __float128 and Intel
_Quad might be equivalent, this is not a rigorous assumption.

I recommend that you write your own user-defined reduction for REAL*16 with
the reduction operation callback in Fortran.

Jeff

On Thu, Aug 9, 2018 at 2:31 AM, Jiancang Zhuang <zhuangjc at ism.ac.jp> wrote:

> I have found that the fortran version mpi_reduce does not work for
> real*18. This can be shown by the following program. I have not test the C
> version of mpi_reduce.
>
>
> c-----------------------Fortran code begins -----------------------------
>       implicit real*8 (a-h, o-z)
>       include 'mpif.h'
>       real*16 h1, h
>
>
>       call mpi_init(ierr)
>       call mpi_comm_size(mpi_comm_world, nprocs, ierr)
>       call mpi_comm_rank(mpi_comm_world, myrank, ierr)
>
>
>       h1 = (myrank+4) *2.00000000000000
>       write(*,'(''before reduce --'', i4,2f12.8)')myrank, h1,h
>
>
>       call mpi_reduce(h1,h,1,mpi_long_double,mpi_sum,0,
>      &    mpi_comm_world,ierr)
>       write(*,'(''after reduce --'',i4,2f12.8)')myrank, h1,h
>
>       call mpi_bcast(h,1,mpi_long_double,0,
>      &    mpi_comm_world,ierr)
>
>       write(*,'(''bcastvalue -- '',i4,2f12.8)')myrank, h1,h
>
>       call mpi_finalize(ierr)
>        end
>
> c-----------------------Fortran code begins -----------------------------
>
>
> $ mpif77 a.f -o a.out
> $ mpirun -np 3 ./a.out
> before reduce --   1 10.00000000  0.00000000
> after reduce --   1 10.00000000  0.00000000
> before reduce --   2 12.00000000  0.00000000
> before reduce --   0  8.00000000  0.00000000
> after reduce --   2 12.00000000  0.00000000
> after reduce --   0  8.00000000  8.00000000
> bcastvalue --    0  8.00000000  8.00000000
> bcastvalue --    1 10.00000000  8.00000000
> bcastvalue --    2 12.00000000  8.00000000
>
>
>
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-- 
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/
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