SHMEM_ALLTOALL(3) Open MPI SHMEM_ALLTOALL(3)
NAME
shmem_alltoall32(3), shmem_alltoall64(3), shmem_alltoalls32(3),
shmem_alltoalls64(3) - collective routine where each PE exchanges a
fixed amount of data with all other PEs in the Active set
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 argu-
ments 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 ob-
jects, 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 fol-
lowing 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 to-
tal 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 ob-
ject. The stride is scaled by the element size. A value of 1 in-
dicates 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 inte-
ger. 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 in-
teger. 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 fol-
lowing 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 rou-
tine. 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 synchro-
nization 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 syn-
chronization. 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 %d0,
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)
4.1.2 Nov 24, 2021 SHMEM_ALLTOALL(3)
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