SHMEM_FLOAT_SUM_TO_ALL(3)

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✨ SHMEM_FLOAT_SUM_TO_ALL ✨ (Open MPI)

okay so bestie, buckle up 💖 this is the Open MPI manual page for shmem_float_sum_to_all(3) and its entire glittery entourage. It's giving "reduction routine," and it's giving it hard.

Here's the whole squad that lives on this page, no cap 🫶:

— and what do they all do? They perform a sum reduction across a set of processing elements (PEs). 🎀 That's the whole vibe. That's the assignment, and they 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 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. Want the full roster of other SHMEM reduction routines? Go peek at intro_shmem(3). 👑

And like every SHMEM collective routine, the rule is simple and absolutely non-negotiable: each of these routines assumes that only PEs in the active set call the routine. If a PE that is not in the active set calls a SHMEM collective routine, undefined behavior results. 💀 Sorry bestie, that's the law.

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 get placed in the target array on all PEs in the active set. And the active set? That's defined by the PE_start, logPE_stride, PE_size triplet. ✨

The source and target arrays may be the same array — cute, love that for them — but they may not be overlapping arrays. Not the same vibe. 🙈

The arguments are as follows:

  • target — The remotely accessible integer data object to be updated on the remote PE. If you are using C/C++, the type of target should match that implied in the SYNOPSIS section. If you are 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.
just so you know bestie, this one and the next one literally appear on the page as written — not me making it up 😭 — so here they are, kept 100% intact.
  • value — The value to be atomically added to target. If you are using C/C++, the type of value should match that implied in the SYNOPSIS section. If you are 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 are using Fortran, it must be a default integer value.
  • 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:
    • 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.

okay so 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. Everybody matching, everybody synced, that's the group-chat rule 🫶

Before any PE calls a reduction routine, you must make sure 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.

And upon 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.

🎀 NOTES 🎀

The terms collective, symmetric, and cache aligned are defined in intro_shmem(3). iykyk 💅

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. That's the good news.

Now the careful part, bestie, read this twice 😭: 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. 👑 iconic workaround, honestly.

part 2 is loading, don't go anywhere bestie 🦋

🍓 EXAMPLES 🍓

okay so like, here's where it gets real. two examples, both iconic, both doing The Most. 💅

💖 Example 1 💖

This Fortran one statically initializes the pSync array and finds the sum of the real variable FOO across all even PEs. even PEs only, bestie, we don't do odd numbers in this house 🫶

          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 🦋

okay and now consider this C/C++ call, like literally look at it:

          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 — i.e. it's the glow-up version of doing it three separate times 🧚

          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. bestie this is not optional, this is the whole plot twist 😭

👀 SEE ALSO 👀

2003-2026, The Open MPI Community

and that's a wrap on this page, fr fr 💜 this was the SHMEM_FLOAT_SUM_TO_ALL(3) manual page, from The Open MPI Community, and the source/version footer says this baby was last touched Sep 10, 2026 — slay, marking it in your calendar is optional but iconic. bye bestie, go sum something 🎀✨

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