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

[Jiancang Zhuang]

Thanks a lot.

In fact, I have tried MPI_REAL16. The same thing happened.  mpi_gather does
not work for it either.
I have decided to rewrite a new subroutine to replace mpi_reduce.

Best regards,



On Fri, Aug 10, 2018 at 12:38 AM Jeff Hammond <jeff.science at gmail.com>
wrote:

> 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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>


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email: zhuangjc at ism.ac.jp
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