对于16字节实数,MPI_AllReduce的奇怪结果

时间:2015-10-13 17:33:37

标签: fortran mpi openmpi quadruple-precision

编译器:gfortran-4.8.5

MPI库:OpenMPI-1.7.2(预装OpenSuSE 13.2)

这个程序:

  use mpi
  implicit none

  real*16 :: x
  integer :: ierr, irank, type16

  call MPI_Init(ierr)

  call MPI_Comm_Rank(MPI_Comm_World, irank, ierr)

  if (irank+1==1) x = 2.1
  if (irank+1==8) x = 2.8
  if (irank+1==7) x = 5.2
  if (irank+1==4) x = 6.7
  if (irank+1==6) x = 6.5
  if (irank+1==3) x = 5.7
  if (irank+1==2) x = 4.0
  if (irank+1==5) x = 6.8

  print '(a,i0,a,f3.1)', "rank+1: ",irank+1," x: ",x

  call MPI_AllReduce(MPI_IN_PLACE, x, 1, MPI_REAL16, MPI_MAX, MPI_Comm_World, ierr)

  if (irank==0) print '(i0,a,f3.1)', irank+1," max x: ", x

  call MPI_Finalize(ierr)
end

我还尝试了real(16)real(kind(1.q0))。对于此编译器,real(real128)实际上等同于real*10

结果是:

> mpif90 reduce16.f90 
> mpirun -n 8 ./a.out 
rank+1: 1 x: 2.1
rank+1: 2 x: 4.0
rank+1: 3 x: 5.7
rank+1: 4 x: 6.7
rank+1: 5 x: 6.8
rank+1: 6 x: 6.5
rank+1: 7 x: 5.2
rank+1: 8 x: 2.8
1 max x: 2.8

程序会找到保留real*10的{​​{1}}的真实最大值。如果MPI_REAL16对应MPI_REAL16real*16,如果它们不同,则MPI规范(3.1,第628和674页)不是很清楚。

另外,假设real(real128)实际上是MPI_REAL16并且尝试在程序中使用它会导致另一个问题:

real(real128)

Error: There is no specific subroutine for the generic 'mpi_recv' at (1) Error: There is no specific subroutine for the generic 'mpi_send' at (1) 不会发生这种情况。 (无论那个人应该能够传递任何位模式,所以这个检查是多余的)

使用16字节实数的正确方法是什么? OpenMPI库是错误的吗?

1 个答案:

答案 0 :(得分:1)

虽然这应该在每个MPI实现中都能正常工作,但是一个简单的解决方法是为Fortran中编写的这种类型实现用户定义的缩减,因此在C中实现它没有问题(这就是MPICH和OpenMPI尝试做所有事情,因此当C无法重现Fortran的行为时会出现问题。)

以下是尝试实现此目的。这是Fortran中用户定义的缩减。我确信经验丰富的现代Fortran程序员可以做得更好。

  subroutine sum_real16(iv,iov,n)
    implicit none
    integer, intent(in) ::  n
    real*16, intent(in) :: iv(:)
    real*16, intent(inout) :: iov(:)
    integer :: i
    do i = 1,n
      iov(i) = iov(i) + iv(i)
    enddo
  end subroutine sum_real16
  subroutine reduce_sum_real16(iv, iov, n, dt)
    use, intrinsic ::  iso_c_binding, only : c_ptr
    use mpi_f08
    implicit none
    type(c_ptr), value ::  iv, iov
    integer ::  n
    type(MPI_Datatype) ::  dt
    if ( dt .eq. MPI_REAL16 ) then
        call sum_real16(iv,iov,n)
    endif
  end subroutine reduce_sum_real16
  program test_reduce_sum_real16
    use, intrinsic ::  iso_c_binding
    use mpi_f08
    implicit none
    integer, parameter ::  n = 10
    real*16 :: output(n)
    real*16 :: input(n)
    real*16 :: error
    integer :: me, np
    procedure(MPI_User_function) :: reduce_sum_real16
    type(MPI_Op) :: mysum
    integer :: i
    call MPI_Init()
    call MPI_Comm_rank(MPI_COMM_WORLD,me)
    call MPI_Comm_size(MPI_COMM_WORLD,np)
    output = 0.0
    input  = 1.0*me
    call MPI_Op_create(reduce_sum_real16,.true.,mysum)
    call MPI_Allreduce(input,output,n,MPI_REAL16,mysum,MPI_COMM_WORLD)
    error = 0.0
    do i = 1,n
      error = error + (output(i)-1.0*np)
    enddo
    if (error.gt.0.0) then
        print*,'SAD PANDA = ',error
        call MPI_Abort(MPI_COMM_SELF,1)
    endif
    call MPI_Op_free(mysum)
    call MPI_Finalize()
  end program test_reduce_sum_real16

此程序在使用Intel 16 Fortran编译器和MPICH 3.2+时无错误地返回。显然我并没有正确使用I / O,所以我对这个程序正确性的信心并不像我能把所有结果都写到stdout那样高。