SHMEM_MIN(3) Open MPI SHMEM_MIN(3)
NAME
shmem_double_min_to_all(3), shmem_float_min_to_all(3),
shmem_int_min_to_all(3), shmem_int4_min_to_all(3),
shmem_int8_min_to_all(3), shmem_long_min_to_all(3), shmem_longdou-
ble_min_to_all(3), shmem_longlong_min_to_all(3),
shmem_real4_min_to_all(3), shmem_real8_min_to_all(3),
shmem_real16_min_to_all(3), shmem_short_min_to_all(3) - Performs a min-
imum function reduction across a set of processing elements (PEs)
SYNOPSIS
C or C++:
#include <mpp/shmem.h>
void shmem_double_min_to_all(double *target, const double *source,
int nreduce, int PE_start, int logPE_stride, int PE_size,
double *pWrk, long *pSync);
void shmem_float_min_to_all(float *target, const float *source,
int nreduce, int PE_start, int logPE_stride, int PE_size,
float *pWrk, long *pSync);
void shmem_int_min_to_all(int *target, const int *source,
int nreduce, int PE_start, int logPE_stride, int PE_size,
int *pWrk, long *pSync);
void shmem_long_min_to_all(long *target, const long *source,
int nreduce, int PE_start, int logPE_stride, int PE_size,
long *pWrk, long *pSync);
void shmem_longdouble_min_to_all(long double *target,
const long double *source, int nreduce, int PE_start,
int logPE_stride, int PE_size, long double *pWrk,
long *pSync);
void shmem_longlong_min_to_all(long long *target,
const long long *source, int nreduce, int PE_start, int logPE_stride,
int PE_size, long long *pWrk, long *pSync);
void shmem_short_min_to_all(short *target, const short *source,
int nreduce, int PE_start, int logPE_stride, int PE_size,
short *pWrk, long *pSync);
Fortran:
INCLUDE "mpp/shmem.fh"
INTEGER pSync(SHMEM_REDUCE_SYNC_SIZE)
INTEGER nreduce, PE_start, logPE_stride, PE_size
CALL SHMEM_INT4_MIN_TO_ALL(target, source, nreduce, PE_start,
& logPE_stride, PE_size, pWrk, pSync)
CALL SHMEM_INT8_MIN_TO_ALL(target, source, nreduce, PE_start,
& logPE_stride, PE_size, pWrk, pSync)
CALL SHMEM_REAL4_MIN_TO_ALL(target, source, nreduce, PE_start,
& logPE_stride, PE_size, pWrk, pSync)
CALL SHMEM_REAL8_MIN_TO_ALL(target, source, nreduce, PE_start,
& logPE_stride, PE_size, pWrk, pSync)
CALL SHMEM_REAL16_MIN_TO_ALL(target, source, nreduce, PE_start,
& logPE_stride, PE_size, pWrk, pSync)
DESCRIPTION
The shared memory (SHMEM) reduction routines compute one or more reduc-
tions across symmetric arrays on multiple virtual PEs. A reduction per-
forms an associative binary operation across a set of values. For a
list of other SHMEM reduction routines, see intro_shmem(3).
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 nreduce argument determines the number of separate reductions to
perform. The source array on all PEs in the active set provides one el-
ement for each reduction. The results of the reductions are placed in
the target array on all PEs in the active set. The active set is de-
fined by the PE_start, logPE_stride, PE_size triplet.
The source and target arrays may be the same array, but they may not be
overlapping arrays.
The arguments are as follows:
target A symmetric array of length nreduce elements to receive the re-
sults of the reduction operations. The data type of target
varies with the version of the reduction routine being called.
When calling from C/C++, refer to the SYNOPSIS section for data
type information. When calling from Fortran, the target data
types are as follows:
shmem_int4_min_to_all: Integer, with an element size of 4 bytes
shmem_int8_min_to_all: Integer, with an element size of 8 bytes
shmem_real4_min_to_all: Real, with an element size of 4 bytes
shmem_real8_min_to_all: Real, with an element size of 8 bytes
shmem_real16_min_to_all: Real, with an element size of 16 bytes
source A symmetric array: of length nreduce elements, that con-
tains one
element for each separate reduction operation. The source
argument must have the same data type as target.
nreduce
The number of elements in the target and source arrays. nreduce
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 in-
teger. If you are using Fortran, it must be a default integer
value.
pWrk A symmetric work array. The pWrk argument must have the same
data type as target. In C/C++, this contains max(nreduce/2 + 1,
_SHMEM_REDUCE_MIN_WRKDATA_SIZE) elements. In Fortran, this con-
tains max(nreduce/2 + 1, SHMEM_REDUCE_MIN_WRKDATA_SIZE) ele-
ments.
pSync A symmetric work array. In C/C++, pSync is of type long and size
_SHMEM_REDUCE_SYNC_SIZE. In Fortran, pSync is of type integer
and size SHMEM_REDUCE_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 the reduction routine.
The values of arguments nreduce, 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 pWrk and pSync work arrays, must be passed to all
PEs in the active set.
Before any PE calls a reduction 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 pWrk and pSync arrays on
all PEs in the active set are not still in use from a prior call to a
collective SHMEM routine. The target array on all PEs in the active set
is ready to accept the results of the reduction.
Upon return from a reduction 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, symmetric, and cache aligned are defined in in-
tro_shmem(3). All SHMEM reduction routines reset the values in pSync
before they return, so a particular pSync buffer need only be initial-
ized 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 reduction routine. Be careful of the following 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 or pWrk array can be reused in a subsequent re-
duction routine call only if none of the PEs in the active set are
still processing a prior reduction routine call that used the same
pSync or pWrk arrays. In general, this can be assured only by doing
some type of synchronization. However, in the special case of reduction
routines being called with the same active set, you can allocate two
pSync and pWrk arrays and alternate between them on successive calls.
EXAMPLES
Example 1: This Fortran example statically initializes the pSync array
and finds the minimum value of real variable FOO across all the even
PEs.
INCLUDE "mpp/shmem.fh"
INTEGER PSYNC(SHMEM_REDUCE_SYNC_SIZE)
DATA PSYNC /SHMEM_REDUCE_SYNC_SIZE*SHMEM_SYNC_VALUE/
PARAMETER (NR=1)
REAL FOO, FOOMIN, PWRK(MAX(NR/2+1,SHMEM_REDUCE_MIN_WRKDATA_SIZE))
COMMON /COM/ FOO, FOOMIN, PWRK
INTRINSIC MY_PE
IF ( MOD(MY_PE(),2) .EQ. 0) THEN
CALL SHMEM_REAL8_MIN_TO_ALL(FOOMIN, FOO, NR, 0, 1, N$PES/2,
& PWRK, PSYNC)
PRINT *, 'Result on PE ', MY_PE(), ' is ', FOOMIN
ENDIF
Example 2: Consider the following C/C++ call:
shmem_int_min_to_all( target, source, 3, 0, 0, 8, pwrk, psync );
The preceding call is more efficient, but semantically equivalent to,
the combination of the following calls:
shmem_int_min_to_all(&(target[0]), &(source[0]), 1, 0, 0, 8,
pwrk1, psync1);
shmem_int_min_to_all(&(target[1]), &(source[1]), 1, 0, 0, 8,
pwrk2, psync2);
shmem_int_min_to_all(&(target[2]), &(source[2]), 1, 0, 0, 8,
pwrk1, psync1);
Note that two sets of pWrk and pSync arrays are used alternately be-
cause no synchronization is done between calls.
SEE ALSO
intro_shmem(3)
4.1.2 Nov 24, 2021 SHMEM_MIN(3)
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