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

[Jeff Hammond]

The specific issue here is that MPI_LONG_DOUBLE does not map to REAL*16.
You should try MPI_REAL16.  I suspect it generates an error telling you
that it is not supported.

MPI_LONG_DOUBLE corresponds to C "long double", which is an awful type that
means a bunch of different things depending on your hardware and compiler.
If you are using x86 Linux, it should be the x87 80-bit type.  See
https://en.wikipedia.org/wiki/Long_double for details.

Jeff

On Thu, Aug 9, 2018 at 8:33 AM, Jeff Hammond <jeff.science at gmail.com> wrote:

> 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/
>



-- 
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/
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