SHMEM_SUM(3)SHMEM_SUM(3)NAME
shmem_comp4_sum_to_all, shmem_comp8_sum_to_all,
shmem_complexd_sum_to_all, shmem_complexf_sum_to_all,
shmem_double_sum_to_all, shmem_float_sum_to_all, shmem_int_sum_to_all,
shmem_int4_sum_to_all, shmem_int8_sum_to_all, shmem_long_sum_to_all,
shmem_longdouble_sum_to_all, shmem_longlong_sum_to_all,
shmem_real4_sum_to_all, shmem_real8_sum_to_all,
shmem_real16_sum_to_all, shmem_short_sum_to_all - Performs a sum
reduction across a set of processing elements (PEs)
SYNOPSIS
C or C++:
#include <mpp/shmem.h>
void shmem_complexd_sum_to_all(double complex *target, double
complex *source, int nreduce, int PE_start, int logPE_stride, int
PE_size, double complex *pWrk, long *pSync);
void shmem_complexf_sum_to_all(float complex *target, float complex
*source, int nreduce, int PE_start, int logPE_stride, int PE_size,
float complex *pWrk, long *pSync);
void shmem_double_sum_to_all(double *target, double *source, int
nreduce, int PE_start, int logPE_stride, int PE_size, double *pWrk,
long *pSync);
void shmem_float_sum_to_all(float *target, float *source, int
nreduce, int PE_start, int logPE_stride, int PE_size, float *pWrk,
long *pSync);
void shmem_int_sum_to_all(int *target, int *source, int nreduce,
int PE_start, int logPE_stride, int PE_size, int *pWrk, long
*pSync);
void shmem_long_sum_to_all(long *target, long *source, int nreduce,
int PE_start, int logPE_stride, int PE_size, long *pWrk, long
*pSync);
void shmem_longdouble_sum_to_all(long double *target, long double
*source, int nreduce, int PE_start, int logPE_stride, int PE_size,
long double *pWrk, long *pSync);
void shmem_longlong_sum_to_all(long long *target, long long
*source, int nreduce, int PE_start, int logPE_stride, int PE_size,
long long *pWrk, long *pSync);
void shmem_short_sum_to_all(short *target, 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_COMP4_SUM_TO_ALL(target, source, nreduce, PE_start,
logPE_stride, PE_size, pWrk, pSync)
CALL SHMEM_COMP8_SUM_TO_ALL(target, source, nreduce, PE_start,
logPE_stride, PE_size, pWrk, pSync)
CALL SHMEM_INT4_SUM_TO_ALL(target, source, nreduce, PE_start,
logPE_stride, PE_size, pWrk, pSync)
CALL SHMEM_INT8_SUM_TO_ALL(target, source, nreduce, PE_start,
logPE_stride, PE_size, pWrk, pSync)
CALL SHMEM_REAL4_SUM_TO_ALL(target, source, nreduce, PE_start,
logPE_stride, PE_size, pWrk, pSync)
CALL SHMEM_REAL8_SUM_TO_ALL(target, source, nreduce, PE_start,
logPE_stride, PE_size, pWrk, pSync)
CALL SHMEM_REAL16_SUM_TO_ALL(target, source, nreduce, PE_start,
logPE_stride, PE_size, pWrk, pSync)
DESCRIPTION
The shared memory (SHMEM) reduction routines compute one or more
reductions across symmetric arrays on multiple virtual PEs. A
reduction performs 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
results.
The nreduce argument determines the number of separate reductions to
perform. The source array on all PEs in the active set provides one
element 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
defined 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 results of the reduction operations.
The data type of target varies with the version of the
reduction routine being called and the language used.
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:
Routine Data Type
shmem_comp4_sum_to_all COMPLEX(KIND=4).
shmem_comp8_sum_to_all Complex. If you are using
Fortran, it must be a
default complex value.
shmem_int4_sum_to_all INTEGER(KIND=4).
shmem_int8_sum_to_all Integer. If you are using
Fortran, it must be a
default integer value.
shmem_real4_sum_to_all REAL(KIND=4).
shmem_real8_sum_to_all Real. If you are using
Fortran, it must be a
default real value.
shmem_real16_sum_to_all Real. If you are using
Fortran, it must be a
default real value.
source A symmetric array, of length nreduce elements, that
contains 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 integer. 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 contains
max(nreduce/2 + 1, SHMEM_REDUCE_MIN_WRKDATA_SIZE)
elements.
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. 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
following 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
intro_shmem(3).
All SHMEM reduction 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 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 synchronization array.
* A pSync or pWrk array can be reused in a subsequent reduction
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 sum of the real variable FOO across all 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, FOOSUM, PWRK(MAX(NR/2+1,SHMEM_REDUCE_MIN_WRKDATA_SIZE))
COMMON /COM/ FOO, FOOSUM, PWRK
INTRINSIC MY_PE
IF ( MOD(MY_PE(),2) .EQ. 0) THEN
CALL SHMEM_INT4_SUM_TO_ALL(FOOSUM, FOO, NR, 0, 1, N$PES/2,
& PWRK, PSYNC)
PRINT*,'Result on PE ',MY_PE(),' is ',FOOSUM
ENDIF
Example 2: Consider the following C/C++ call:
shmem_int_sum_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_sum_to_all(&(target[0]), &(source[0]), 1, 0, 0, 8,
pwrk1, psync1);
shmem_int_sum_to_all(&(target[1]), &(source[1]), 1, 0, 0, 8,
pwrk2, psync2);
shmem_int_sum_to_all(&(target[2]), &(source[2]), 1, 0, 0, 8,
pwrk1, psync1);
Note that two sets of pWrk and pSync arrays are used alternately
because no synchronization is done between calls.
SEE ALSOintro_shmem(3)
Message Passing Toolkit: MPI Programmer's Manual
