SHMEM_LONG_SUM_TO_ALL(3)

📦

version 5.0.10 · 4 versions

💖 SHMEM_LONG_SUM_TO_ALL(3) 💖

okay so bestie, welcome to Open MPI's corner of the SHMEM universe, where this whole page is one big family reunion of reduction routines 🌈 — 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, and shmem_short_sum_to_all — and they ALL do the same cute little thing:

Performs a sum reduction across a set of processing elements (PEs)

that's it. that's the whole vibe. she adds numbers together across PEs and she ate 💅

✨ SYNOPSIS ✨

🐍 C or C++

#include <mpp/shmem.h>

void shmem_complexd_sum_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_sum_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_sum_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_sum_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_sum_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_sum_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_sum_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_sum_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_sum_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_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 🌸

so the 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. wanna see the other SHMEM reduction routines? they're all hanging out in intro_shmem(3) 💜

like literally every SHMEM collective routine ever, each of these assumes only PEs in the active set call the routine. if a PE that is NOT in the active set calls a SHMEM collective routine, you get undefined behavior. periodt. don't do it 🙈

the nreduce argument decides how many separate reductions are happening. the source array on all PEs in the active set hands over one element for each reduction. the results land in the target array on all PEs in the active set. and the active set is defined by the PE_start, logPE_stride, PE_size triplet ✨

oh and — the source and target arrays may be the same array, but they may not be overlapping arrays. no cap 🫶

the arguments are as follows:

target — the remotely accessible integer data object to be updated on the remote PE. if you're in C/C++, the type of target should match what's implied in the SYNOPSIS section. if you're using the Fortran compiler, it must be of type integer with an element size of 4 bytes for SHMEM_INT4_ADD and 8 bytes for SHMEM_INT8_ADD.
value — the value to be atomically added to target. if you're in C/C++, the type of value should match what's implied in the SYNOPSIS section. if you're using Fortran, it must be of type integer with an element size of target.
pe — an integer that indicates the PE number upon which target is to be updated. if you're using Fortran, it must be a default integer value.

👀 okay bestie we know, those last three looked like they wandered in from a totally different page. they're in the original, so they're in here too. moving on 💅

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 info. when calling from Fortran, the target data types are:
- shmem_comp4_sum_to_all: COMPLEX(KIND=4).
- shmem_comp8_sum_to_all: Complex. if you're 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're 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're using Fortran, it must be a default real value.
- shmem_real16_sum_to_all: Real. if you're using Fortran, it must be a default real value.
source — a symmetric array, of length nreduce elements, containing one element for each separate reduction operation. source must have the same data type as target. matching outfits, iconic 💖
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. yes bestie, log base TWO, that's the whole point 🌈
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. 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. do not skip this step, it's giving "I read the docs" energy 👑

the values of the arguments nreduce, PE_start, logPE_stride, and PE_size must be equal on all PEs in the active set. and the same target and source arrays, plus the same pWrk and pSync work arrays, must be passed to all PEs in the active set. no solo missions 🦋

before any PE calls a reduction routine, you must make sure these conditions exist (synchronization via a barrier or some other method is often needed to ensure this — fr fr):

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). go peek if you need the lore 💜

all SHMEM reduction routines reset the values in pSync before they return, so a particular pSync buffer only needs to be initialized the very first time it's used. lowkey a glow-up 🎀

you must ensure that the pSync array is not being updated on any PE in the active set while any of the PEs are participating in the processing of a SHMEM reduction routine. be careful of 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 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. the classic two-outfit rotation, obsessed 👑

✨ EXAMPLES ✨

Okay bestie, real talk — this is where it gets fun. Two whole examples, both iconic 💅

Example 1 🎀

This one's a Fortran moment fr fr. It statically initializes the pSync array and finds the sum of the real variable FOO across all even PEs. Slay.

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

See how PSYNC gets loaded up with SHMEM_REDUCE_SYNC_SIZE copies of SHMEM_SYNC_VALUE? That's the setup, and it's giving prepared 🫶

Example 2 🦋

Now clock this 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. 👀 Not me stressing — just actually important, bestie. Two sets, alternating, because nobody's syncing between them. No cap.

Copyright 💜

2003-2026, The Open MPI Community

That's a wrap on the whole page, gorgeous 💖 This one's from the Open MPI community, dated Sep 10, 2026, and it lives in the manual as SHMEM_LONG_SUM_TO_ALL(3). Sum it up, sync it up, and go be the main character of your own parallel program. Periodt. 👑🌸

📦 where this page comes from

ubuntu stonking: openmpi-doc 5.0.10-6 · other
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-11-03: a language model's rewrite of the original, under its terms

SHMEM_LONG_SUM_TO_ALL(3)