/* -*- Mode: C; c-basic-offset:4 ; indent-tabs-mode:nil ; -*- */
/*
* (C) 2017 by Argonne National Laboratory.
* See COPYRIGHT in top-level directory.
*/
#include "mpiimpl.h"
/* Algorithm: Pairwise Exchange
*
* This pairwise exchange algorithm takes p-1 steps.
*
* We calculate the pairs by using an exclusive-or algorithm:
* for (i=1; i<comm_size; i++)
* dest = rank ^ i;
* This algorithm doesn't work if the number of processes is not a power of
* two. For a non-power-of-two number of processes, we use an
* algorithm in which, in step i, each process receives from (rank-i)
* and sends to (rank+i).
*
* Cost = (p-1).alpha + n.beta
*
* where n is the total amount of data a process needs to send to all
* other processes.
*/
#undef FUNCNAME
#define FUNCNAME MPIR_Ialltoall_sched_intra_pairwise
#undef FCNAME
#define FCNAME MPL_QUOTE(FUNCNAME)
int MPIR_Ialltoall_sched_intra_pairwise(const void *sendbuf, int sendcount, MPI_Datatype sendtype,
void *recvbuf, int recvcount, MPI_Datatype recvtype,
MPIR_Comm * comm_ptr, MPIR_Sched_t s)
{
int mpi_errno = MPI_SUCCESS;
int i;
int src, dst, is_pof2;
int rank, comm_size;
MPI_Aint sendtype_extent, recvtype_extent;
#ifdef HAVE_ERROR_CHECKING
MPIR_Assert(sendbuf != MPI_IN_PLACE); /* we do not handle in-place */
#endif
comm_size = comm_ptr->local_size;
rank = comm_ptr->rank;
MPIR_Datatype_get_extent_macro(sendtype, sendtype_extent);
MPIR_Datatype_get_extent_macro(recvtype, recvtype_extent);
/* Make local copy first */
mpi_errno = MPIR_Sched_copy(((char *) sendbuf + rank * sendcount * sendtype_extent),
sendcount, sendtype,
((char *) recvbuf + rank * recvcount * recvtype_extent),
recvcount, recvtype, s);
if (mpi_errno)
MPIR_ERR_POP(mpi_errno);
is_pof2 = MPL_is_pof2(comm_size, NULL);
/* Do the pairwise exchanges */
for (i = 1; i < comm_size; i++) {
if (is_pof2 == 1) {
/* use exclusive-or algorithm */
src = dst = rank ^ i;
} else {
src = (rank - i + comm_size) % comm_size;
dst = (rank + i) % comm_size;
}
mpi_errno = MPIR_Sched_send(((char *) sendbuf + dst * sendcount * sendtype_extent),
sendcount, sendtype, dst, comm_ptr, s);
if (mpi_errno)
MPIR_ERR_POP(mpi_errno);
mpi_errno = MPIR_Sched_recv(((char *) recvbuf + src * recvcount * recvtype_extent),
recvcount, recvtype, src, comm_ptr, s);
if (mpi_errno)
MPIR_ERR_POP(mpi_errno);
MPIR_SCHED_BARRIER(s);
}
fn_exit:
return mpi_errno;
fn_fail:
goto fn_exit;
}