Provided by: openmpi-doc_5.0.7-1_all 
SYNOPSIS
C or C++:
#include <mpp/shmem.h>
void shmem_collect32(void *target, const void *source,
size_t nelems, int PE_start, int logPE_stride, int PE_size,
long *pSync)
void shmem_collect64(void *target, const void *source,
size_t nelems, int PE_start, int logPE_stride, int PE_size,
long *pSync)
void shmem_fcollect32(void *target, const void *source,
size_t nelems, int PE_start, int logPE_stride, int PE_size,
long *pSync)
void shmem_fcollect64(void *target, const void *source,
size_t nelems, int PE_start, int logPE_stride, int PE_size,
long *pSync)
Fortran:
INCLUDE "mpp/shmem.fh"
INTEGER nelems
INTEGER PE_start, logPE_stride, PE_size
INTEGER pSync(SHMEM_COLLECT_SYNC_SIZE)
CALL SHMEM_COLLECT4(target, source, nelems, PE_start,
& logPE_stride, PE_size, pSync)
CALL SHMEM_COLLECT8(target, source, nelems, PE_start,
& logPE_stride, PE_size, pSync)
CALL SHMEM_FCOLLECT4(target, source, nelems, PE_start,
& logPE_stride, PE_size, pSync)
CALL SHMEM_FCOLLECT8(target, source, nelems, PE_start,
& logPE_stride, PE_size, pSync)
DESCRIPTION
The shared memory (SHMEM) collect and fcollect routines concatenate nelems 64-bit or 32-bit data items
from the source array into the target array, over the set of PEs defined by PE_start, log2PE_stride, and
PE_size, in processor number order. The resultant target array contains the contribution from PE PE_start
first, then the contribution from PE PE_start + PE_stride second, and so on. The collected result is
written to the target array for all PEs in the active set.
The fcollect routines require that nelems be the same value in all participating PEs, while the collect
routines allow nelems to vary from PE to PE.
The resulting target array is as follows:
----------------------------------------------------------
source(1..nelems)
from PE (PE_start + 0 * (2**logPE_stride))
----------------------------------------------------------
source(1..nelems)
from PE (PE_start + 1 * (2**logPE_stride))
----------------------------------------------------------
...
----------------------------------------------------------
source(1..nelems) from
PE (PE_start + (PE_size - 1) * (2**logPE_stride))
----------------------------------------------------------
As with all SHMEM collective routines, each of these routines assumes that only PEs in the active set
call the routine. If a PE not in the active set calls a SHMEM collective routine, undefined behavior
results.
The arguments are as follows:
target A symmetric array. The target argument must be large enough to accept the concatenation of the
source arrays on all PEs. The data types are as follows:
[shmem_collect8, shmem_collect64, shmem_fcollect8, and
shmem_fcollect64] any data type with an element size of 64 bits. Fortran derived types,
Fortran character type, and C/C++ structures are not permitted.
[shmem_collect4, shmem_collect32, shmem_fcollect4, and
shmem_fcollect32] any data type with an element size of 32 bits. Fortran derived types,
Fortran character type, and C/C++ structures are not permitted.
source A symmetric data object that can be of any type permissible for the target argument.
nelems The number of elements in the source array. nelems must be of type integer. 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.
logPE_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 int and size _SHMEM_COLLECT_SYNC_SIZE. In
Fortran, pSync must be of type integer and size SHMEM_COLLECT_SYNC_SIZE. If you are using Fortran,
it must be a default integer value. 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().
The values of arguments PE_start, logPE_stride, and PE_size must be equal on all PEs in the active set.
The same target and source arrays and the same pSync work array must be passed to all PEs in the active
set.
Upon return from a collective routine, the following are true for the local PE: The target array is
updated. The values in the pSync array are restored to the original values.
NOTES
The terms collective and symmetric are defined in intro_shmem(3). All SHMEM collective routines reset
the values in pSync before they return, so a particular pSync buffer need only be initialized the first
time it is used.
You must ensure that the pSync array is not being updated on any PE in the active set while any of the
PEs participate in processing of a SHMEM collective 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 can be reused on a subsequent SHMEM collective routine only if none
of the PEs in the active set are still processing a prior SHMEM collective routine call that used the
same pSync array. In general, this may 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.
The collective routines operate on active PE sets that have a non-power-of-two PE_size with some
performance degradation. They operate with no performance degradation when nelems is a non-power-of-two
value.
EXAMPLES
C/C++:
for (i=0; i < _SHMEM_COLLECT_SYNC_SIZE; i++) {
pSync[i] = _SHMEM_SYNC_VALUE;
}
shmem_barrier_all(); /* Wait for all PEs to initialize pSync */
shmem_collect32(target, source, 64, pe_start, logPE_stride,
pe_size, pSync);
Fortran:
INTEGER PSYNC(SHMEM_COLLECT_SYNC_SIZE)
DATA PSYNC /SHMEM_COLLECT_SYNC_SIZE*SHMEM_SYNC_VALUE/
CALL SHMEM_COLLECT4(TARGET, SOURCE, 64, PE_START,
& LOGPE_STRIDE, PE_SIZE, PSYNC)
SEE ALSO:
intro_shmem(3)
COPYRIGHT
2003-2025, The Open MPI Community
Feb 17, 2025 SHMEM_FCOLLECT32(3)