SHMEM_DOUBLE_PROD_TO_ALL(3)

💖 SHMEM_DOUBLE_PROD_TO_ALL 💖

okay bestie, so this whole page is about the SHMEM product-reduction girlies — a whole squad of them, actually. We're talking:

• shmem_comp4_prod_to_all
• shmem_comp8_prod_to_all
• shmem_complexd_prod_to_all
• shmem_complexf_prod_to_all
• shmem_double_prod_to_all
• shmem_float_prod_to_all
• shmem_int_prod_to_all
• shmem_int4_prod_to_all
• shmem_int8_prod_to_all
• shmem_long_prod_to_all
• shmem_longdouble_prod_to_all
• shmem_longlong_prod_to_all
• shmem_real8_prod_to_all
• shmem_real16_prod_to_all
• shmem_real4_prod_to_all
• shmem_short_prod_to_all

and what do they do?? They perform a product reduction across a set of processing elements (PEs). that's the whole vibe. 🫶

(this page ships with Open MPI, btw, iykyk.)

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✨ SYNOPSIS ✨

C or C++ 🎀

#include <mpp/shmem.h>

void shmem_complexd_prod_to_all(double complex *target,
  const double complex *source, int nreduce, int PE_start,
  int logPE_stride, int PE_size, double complex *pWrk,
  long *pSync)

void shmem_complexf_prod_to_all(float complex *target,
  const float complex *source, int nreduce, int PE_start,
  int logPE_stride, int PE_size, float complex *pWrk,
  long *pSync)

void shmem_double_prod_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_prod_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_prod_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_prod_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_prod_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_prod_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_prod_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_COMP4_PROD_TO_ALL(target, source, nreduce, PE_start,
& logPE_stride, PE_size, pWrk, pSync)

CALL SHMEM_COMP8_PROD_TO_ALL(target, source, nreduce, PE_start,
& logPE_stride, PE_size, pWrk, pSync)

CALL SHMEM_INT4_PROD_TO_ALL(target, source, nreduce, PE_start,
& logPE_stride, PE_size, pWrk, pSync)

CALL SHMEM_INT8_PROD_TO_ALL(target, source, nreduce, PE_start,
& logPE_stride, PE_size, pWrk, pSync)

CALL SHMEM_REAL4_PROD_TO_ALL(target, source, nreduce, PE_start,
& logPE_stride, PE_size, pWrk, pSync)

CALL SHMEM_REAL8_PROD_TO_ALL(target, source, nreduce, PE_start,
& logPE_stride, PE_size, pWrk, pSync)

CALL SHMEM_REAL16_PROD_TO_ALL(target, source, nreduce, PE_start,
& logPE_stride, PE_size, pWrk, pSync)

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🌈 DESCRIPTION 🌈

so. the shared memory (SHMEM) reduction routines compute one or more reductions across symmetric arrays on multiple virtual PEs. a reduction does an associative binary operation across a set of values. want a list of the other SHMEM reduction routines? that's in intro_shmem(3), go look bestie 💅

and like every SHMEM collective routine, each of these 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. we do not do that. no cap. 💀

nreduce decides how many 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 land in the target array on all PEs in the active set. and the active set itself is defined by the PE_start, logPE_stride, PE_size triplet — iconic trio, honestly.

source and target can be the same array. they may not be overlapping arrays. that's the rule 🌸

the arguments, one by one 🦋

• 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. calling from C/C++? the data types are in the SYNOPSIS section above. calling from Fortran? the target data types are:
  • shmem_comp4_prod_to_all: Complex, with an element size equal to two 4-byte real values.
  • shmem_comp8_prod_to_all: Complex, with an element size equal to two 8-byte real values.
  • shmem_int4_prod_to_all: Integer, with an element size of 4 bytes
  • shmem_int8_prod_to_all: Integer, with an element size of 8 bytes
  • shmem_real4_prod_to_all: Real, with an element size of 4 bytes
  • shmem_real8_prod_to_all: Real, with an element size of 8 bytes
  • shmem_real16_prod_to_all: Real, with an element size of 16 bytes
• source — a symmetric array, of length nreduce elements, that contains one element for each separate reduction operation. source must have the same data type as target. matching outfits, periodt.
• nreduce — the number of elements in the target and source arrays. nreduce must be of type integer. if you're 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're 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're 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're using Fortran, it must be a default integer value.
• pWrk — a symmetric work array. pWrk 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. if you're using Fortran, it must be a default integer value. and before any of the PEs in the active set enter the reduction routine, every element of this array must be initialized with the value _SHMEM_SYNC_VALUE (in C/C++) or SHMEM_SYNC_VALUE (in Fortran). don't skip this step, bestie, it's not optional 😭

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okay so the values of 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 too. and before any PE calls a reduction routine, you gotta make sure these conditions exist (synchronization via a barrier or some other method is often needed to make that happen):

• 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.

once you return from a reduction routine, these are true for the local PE:

• the target array is updated ✨
• the values in the pSync array are restored to the original values 🎀

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💅 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 only needs to be initialized the first time it's used. slay.

⚠️ 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 these situations, fr fr: - 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, that second one can be assured only by doing some type of synchronization. however — special case, bestie — if reduction routines are being called with the same active set, you can allocate two pSync and pWrk arrays and alternate between them on successive calls. two-buffer rotation? that's the move 👑

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🧚 EXAMPLES 🧚

Example 1

this Fortran example statically initializes the pSync array and finds the product of the 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, FOOPROD, PWRK(MAX(NR/2+1,SHMEM_REDUCE_MIN_WRKDATA_SIZE))
COMMON /COM/ FOO, FOOPROD, PWRK
INTRINSIC MY_PE

IF ( MOD(MY_PE(),2) .EQ. 0) THEN
  CALL SHMEM_COMP8_PROD_TO_ALL(FOOPROD, FOO, NR, 0, 1, N$PES/2,
  & PWRK, PSYNC)
  PRINT *, 'Result on PE ', MY_PE(), ' is ', FOOPROD
ENDIF

Example 2

now consider this C/C++ call:

shmem_short_prod_to_all(target, source, 3, 0, 0, 8, pwrk, psync);

that one call is more efficient, but semantically equivalent to, the combination of these calls:

shmem_short_prod_to_all(&(target[0]), &(source[0]), 1, 0, 0, 8,
  pwrk1, psync1);
shmem_short_prod_to_all(&(target[1]), &(source[1]), 1, 0, 0, 8,
  pwrk2, psync2);
shmem_short_prod_to_all(&(target[2]), &(source[2]), 1, 0, 0, 8,
  pwrk1, psync1);

and notice: two sets of pWrk and pSync arrays are used alternately, because no synchronization is done between calls. alternating buffers, she's booked and busy 💖

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👀 See also:

• intro_shmem(3)

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📜 Copyright 📜

2003-2026, The Open MPI Community

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and that's the whole thing, bestie!! you just read the SHMEM_DOUBLE_PROD_TO_ALL(3) page from Open MPI, dated Sep 10, 2026, copyright 2003-2026, The Open MPI Community. go forth and multiply — literally, that's what a product reduction is 🍓✨ periodt 💅