SHMEM_COLLECT(3) Open MPI SHMEM_COLLECT(3)
NAME
shmem_collect4(3), shmem_collect8(3), shmem_collect32(3), shmem_col-
lect64(3), shmem_fcollect(3), shmem_fcollect4(3), shmem_fcollect8(3),
shmem_fcollect32(3), shmem_fcollect64(3) - Concatenates blocks of data
from multiple processing elements (PEs) to an array in every PE
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 tar-
get 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 par-
ticipating 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 re-
sults.
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 in-
teger 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 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 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 For-
tran, 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 synchro-
nization array. A pSync array can be reused on a subsequent SHMEM col-
lective 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 oper-
ate 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)
4.1.2 Nov 24, 2021 SHMEM_COLLECT(3)
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