[J3] asynchronous coarray collectives?

[Jeff Hammond]

I was not aware that coarray collectives were pure. They have strong side effects. Are you sure they are pure?

If the do concurrent is execute in a random order on a single thread, which seems like a legal implementation, this will deadlock.

Finally, how does do concurrent inform the Fortran implementation to take advantage of network offloading?


On 6. May 2023, at 21.39, Van Snyder via J3 <j3 at mailman.j3-fortran.org> wrote:


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Try this:

subroutine stuff(A,B,C,D)
 implicit none
 double, intent(inout) :: A, B, C
 double, intent(in) :: D(:)
  integer :: I
do concurrent ( i = 1:3 )
select case ( i )
case ( 1 )
 call co_sum(A)
case ( 2 )
  call co_min(B)
case ( 3 )
 call co_max(C)
end select
end do

 print*,D
end subroutine stuff

Bill Long pointed out several years ago that a fork/join construct would be syntactic sugar for this.

On Sat, 2023-05-06 at 07:59 +0000, Jeff Hammond via J3 wrote:

How do I make a Fortran coarry program like this...


subroutine stuff(A,B,C,D)

 implicit none

 double, intent(inout) :: A, B, C

 double, intent(in) :: D(:)

 call co_sum(A)

 call co_min(B)

 call co_max(C)

 print*,D

end subroutine stuff


...behave like this...


subroutine stuff(A,B,C,D)

 use mpi_f08

 implicit none

 double, intent(inout) :: A, B, C

 double, intent(in) :: D(:)

 type(MPI_Request) :: R(3)

 call MPI_Iallreduce(MPI_IN_PLACE, A, 1, MPI_DOUBLE, MPI_SUM, MPI_COMM_WORLD, R(1))

 call MPI_Iallreduce(MPI_IN_PLACE, B, 1, MPI_DOUBLE, MPI_MIN, MPI_COMM_WORLD, R(2))

 call MPI_Iallreduce(MPI_IN_PLACE, C, 1, MPI_DOUBLE, MPI_MAX, MPI_COMM_WORLD, R(3))

 print*,D

 call MPI_Waitall(3,R,MPI_STATUSES_IGNORE)

end subroutine stuff


...in the sense that it is possible for the network to execute the communication operations asynchronously relative to the print statement?


Do any compilers, e.g. Cray’s, automatically convert coarry operations to asynchronous communication and push the completion of those operations as far down as possible?


Jeff
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