SHMEM_INT_SUM_TO_ALL(3)

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💅 SHMEM_INT_SUM_TO_ALL(3) — Open MPI, bestie edition

Okay so this is page SHMEM_INT_SUM_TO_ALL(3) straight out of the Open MPI manual, and it's a whole squad of reduction routines. Girl. There are sixteen of them. Grab a snack. 🍓

We're in part 1 of 2, so buckle up, hydrate, and let's get into it.

✨ NAME ✨

Every single one of these does the same vibe: performs a sum reduction across a set of processing elements (PEs). The only thing that changes is the data type and the language you're yelling at it in.

Not me listing sixteen functions like it's a group chat roll call. 😭 Anyway. Same page, same energy, same arguments. Periodt.

🌸 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)

See how target is a plain pointer but source is const? Iconic. We're reading from source and writing into target, no cap.

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)

Fortran besties, notice the & continuation characters. Those are load-bearing. Do not remove them. 🙈

🦋 DESCRIPTION 🦋

Okay 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. And if you want the whole guest list of other SHMEM reduction routines, go look at intro_shmem(3).

Like every SHMEM collective routine, each of these 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. Like... don't. It's giving chaos and not the cute kind.

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. The active set is defined by the PE_start, logPE_stride, PE_size triplet. That trio? They're a package deal. ✨

The source and target arrays may be the same array, but they may not be overlapping arrays. Same = fine, overlapping = absolutely not.

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.
  • 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.
Yes bestie, target shows up twice in that list. We're keeping both. The manual said it, so it stays, no editing, no notes. 💀
  • 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. That's not a suggestion, that's the rule. 🌈

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. Everybody matching, everybody agreeing, it's giving coordinated. 🫶

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). Go read it, it's the lore. 📖

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. Cute, right? One-time setup 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 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.

Two arrays, alternating, like outfits on rotation. That's the trick. 💅

to be continued… part 2 is on its way, don't go anywhere ✨

🌸 EXAMPLES 🌸

okay bestie, real talk, this is where it all comes together ✨ we got two examples, one Fortran and one C/C++, and they're both serving.

💖 Example 1: the Fortran one 💖

So this Fortran example statically initializes the pSync array (yes bestie, statically, she's committed) and then 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

like, look at her. the INCLUDE "mpp/shmem.fh", the DATA PSYNC /SHMEM_REDUCE_SYNC_SIZE*SHMEM_SYNC_VALUE/, the whole IF ( MOD(MY_PE(),2) .EQ. 0) THEN vibe check — she's only doing the work on the even PEs, main character energy only 💅

✨ Example 2: the C/C++ one ✨

okay so consider the following C/C++ call:

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

and here's the tea 🍓 — that single call up there is more efficient, but semantically equivalent to, this whole combination of calls down here:

          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. fr fr, do not skip this part, it's lowkey the whole point 🙈

like, she does the same thing, she just does it better when you let her do all 3 in one go. queen behavior 👑

🌈 See also: 🌈

2003-2026, The Open MPI Community

okay that's the whole page bestie!! 🎀 this edition was from Sep 10, 2026 and it's page SHMEM_INT_SUM_TO_ALL(3) in the manual, no cap 💖 go forth and sum, you absolute icon 🦋✨

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