Provided by: openmpi-doc_5.0.7-1_all 
SYNOPSIS
C or C++:
#include <mpp/shmem.h>
void shmem_alltoall32(void *target, const void *source,
size_t nelems, int PE_start, int logPE_stride,
int PE_size, long *pSync)
void shmem_alltoall64(void *target, const void *source,
size_t nelems, int PE_start, int logPE_stride,
int PE_size, long *pSync)
void shmem_alltoalls32(void *target, const void *source,
ptrdiff_t dst, ptrdiff_t sst,
size_t nelems, int PE_start, int logPE_stride,
int PE_size, long *pSync)
void shmem_alltoalls64(void *target, const void *source,
ptrdiff_t dst, ptrdiff_t sst,
size_t nelems, int PE_start, int logPE_stride,
int PE_size, long *pSync)
Fortran:
INCLUDE "mpp/shmem.fh"
INTEGER dst, sst, nelems, PE_root, PE_start, logPE_stride, PE_size
INTEGER pSync(SHMEM_ALLTOALL_SYNC_SIZE)
CALL SHMEM_ALLTOALL32(target, source, nelems,
& PE_start, logPE_stride, PE_size, fIpSync)
CALL SHMEM_ALLTOALL64(target, source, nelems,
& PE_start, logPE_stride, PE_size, pSync)
CALL SHMEM_ALLTOALLS32(target, source, dst, sst, nelems,
& PE_start, logPE_stride, PE_size, pSync)
CALL SHMEM_ALLTOALLS64(target, source, dst, sst, nelems,
& PE_start, logPE_stride, PE_size, pSync)
DESCRIPTION
The shmem_alltoalls routines are collective routines. Each PE in the Active set exchanges nelems strided
data elements of size 32 bits (for shmem_alltoalls32) or 64 bits (for shmem_alltoalls64) with all other
PEs in the set. Both strides, dst and sst, must be greater than or equal to 1. The sst*jth block sent
from PE i to PE j is placed in the dst*ith block of the dest data object on PE j. As with all OpenSHMEM
collective routines, these routines assume that only PEs in the Active set call the routine. If a PE not
in the Active set calls an OpenSHMEM collective routine, undefined behavior results. The values of
arguments dst, sst, nelems, PE_start, logPE_stride, and PE_size must be equal on all PEs in the Active
set. The same dest and source data objects, and the same pSync work array must be passed to all PEs in
the Active set. Before any PE calls to a shmem_alltoalls routine, the following conditions must exist
(synchronization via a barrier or some other method is often needed to ensure this): The pSync array on
all PEs in the Active set is not still in use from a prior call to a shmem_alltoalls routine. The dest
data object on all PEs in the Active set is ready to accept the shmem_alltoalls data. Upon return from a
shmem_alltoalls routine, the following is true for the local PE: Its dest symmetric data object is
completely updated and the data has been copied out of the source data object. The values in the pSync
array are restored to the original values.
The arguments are as follows:
A symmetric data object with one of the following data types:
shmem_alltoall32: Any noncharacter type that
has an element size of 32 bits. No Fortran derived types or C/C++ structures are allowed.
shmem_alltoall64: Any noncharacter type that has an element size
of 64 bits. No Fortran derived types or C/C++ structures are allowed.
target A symmetric data object large enough to receive the combined total of nelems elements from each PE
in the Active set.
source A symmetric data object that contains nelems elements of data for each PE in the Active set,
ordered according to destination PE.
dst The stride between consecutive elements of the dest data object. The stride is scaled by the
element size. A value of 1 indicates contiguous data. dst must be of type ptrdiff_t. If you are
using Fortran, it must be a default integer value.
sst The stride between consecutive elements of the source data object. The stride is scaled by the
element size. A value of 1 indicates contiguous data. sst must be of type ptrdiff_t. If you are
using Fortran, it must be a default integer value.
nelems The number of elements to exchange for each PE. nelems must be of type size_t for C/C++. If you
are using Fortran, it must be a default integer value
PE_start
The lowest virtual PE number of the active set of PEs. PE_start must be of type integer. If you
are using Fortran, it must be a default integer value.
logPE_stride
The log (base 2) of the stride between consecutive virtual PE numbers in the active set.
log_PE_stride must be of type integer. If you are using Fortran, it must be a default integer
value.
PE_size
The number of PEs in the active set. PE_size must be of type integer. If you are using Fortran,
it must be a default integer value.
pSync A symmetric work array. In C/C++, pSync must be of type long and size _SHMEM_ALLTOALL_SYNC_SIZE.
In Fortran, pSync must be of type integer and size SHMEM_ALLTOALL_SYNC_SIZE. Every element of this
array must be initialized with the value _SHMEM_SYNC_VALUE (in C/C++) or SHMEM_SYNC_VALUE (in
Fortran) before any of the PEs in the active set enter shmem_barrier().
Upon return from a shmem_alltoalls routine, the following is true for the local PE: Its dest symmetric
data object is completely updated and the data has been copied out of the source data object. The values
in the pSync array are restored to the original values.
The values of arguments PE_root, PE_start, logPE_stride, and PE_size must be equal on all PEs in the
active set. The same target and source data objects and the same pSync work array must be passed to all
PEs in the active set.
Before any PE calls a alltoall routine, you must ensure that the following conditions exist
(synchronization via a barrier or some other method is often needed to ensure this): The pSync array on
all PEs in the active set is not still in use from a prior call to a alltoall routine. The target array
on all PEs in the active set is ready to accept the alltoall data.
NOTES
The terms collective and symmetric are defined in intro_shmem(3).
All SHMEM alltoall routines restore pSync to its original contents. Multiple calls to SHMEM routines
that use the same pSync array do not require that pSync be reinitialized after the first call.
You must ensure the that the pSync array is not being updated by any PE in the active set while any of
the PEs participates in processing of a SHMEM broadcast routine. Be careful to avoid these situations: If
the pSync array is initialized at run time, some type of synchronization is needed to ensure that all PEs
in the working set have initialized pSync before any of them enter a SHMEM routine called with the pSync
synchronization array. A pSync array may be reused on a subsequent SHMEM broadcast routine only if none
of the PEs in the active set are still processing a prior SHMEM alltoall routine call that used the same
pSync array. In general, this can be ensured only by doing some type of synchronization. However, in the
special case of SHMEM routines being called with the same active set, you can allocate two pSync arrays
and alternate between them on successive calls.
EXAMPLES
C/C++ example:
#include <shmem.h>
#include <stdio.h>
long pSync[SHMEM_ALLTOALL_SYNC_SIZE];
int main(void)
{
int64_t *source, *dest;
int i, count, pe;
shmem_init();
count = 2;
dest = (int64_t*) shmem_malloc(count * shmem_n_pes() * sizeof(int64_t));
source = (int64_t*) shmem_malloc(count * shmem_n_pes() * sizeof(int64_t));
/* assign source values */
for (pe=0; pe <shmem_n_pes(); pe++){
for (i=0; i<count; i++){
source[(pe*count)+i] = shmem_my_pe() + pe;
dest[(pe*count)+i] = 9999;
}
}
for (i=0; i< SHMEM_ALLTOALLS_SYNC_SIZE; i++) {
pSync[i] = SHMEM_SYNC_VALUE;
}
/* wait for all PEs to initialize pSync */
shmem_barrier_all();
/* alltoalls on all PES */
shmem_alltoalls64(dest, source, 1, 1, count, 0, 0, shmem_n_pes(), pSync);
/* verify results */
for (pe=0; pe<shmem_n_pes(); pe++) {
for (i=0; i<count; i++){
if (dest[(pe*count)+i] != shmem_my_pe() + pe) {
printf("[%d] ERROR: dest[%d]=%ld, should be %d,
shmem_my_pe(),(pe*count)+i,dest[(pe*count)+i],
shmem_n_pes() + pe);
}
}
}
shmem_barrier_all();
shmem_free(dest);
shmem_free(source);
shmem_finalize();
return 0;
}
SEE ALSO:
intro_shmem(3)
COPYRIGHT
2003-2025, The Open MPI Community
Feb 17, 2025 SHMEM_ALLTOALLS32(3)