SHMEM_INT_MIN_TO_ALL(3)

📦
version 5.0.10 · 4 versions
, in ubuntu stonking

🌸 NAME 🌸

okay so bestie, welcome to the minimum-reduction corner of Open MPI 💖 this whole page is about the shmem_*_min_to_all family, and yeah, it's a big family, we love a big family.

Here's everybody invited to the party:

and what do they all do? they do a minimum function reduction across a set of processing elements (PEs). that's it. that's the vibe. 🔥

✨ SYNOPSIS ✨

C or C++:

#include <mpp/shmem.h>

void shmem_double_min_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_min_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_min_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_min_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_min_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_min_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_min_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_INT4_MIN_TO_ALL(target, source, nreduce, PE_start,
& logPE_stride, PE_size, pWrk, pSync)

CALL SHMEM_INT8_MIN_TO_ALL(target, source, nreduce, PE_start,
& logPE_stride, PE_size, pWrk, pSync)

CALL SHMEM_REAL4_MIN_TO_ALL(target, source, nreduce, PE_start,
& logPE_stride, PE_size, pWrk, pSync)

CALL SHMEM_REAL8_MIN_TO_ALL(target, source, nreduce, PE_start,
& logPE_stride, PE_size, pWrk, pSync)

CALL SHMEM_REAL16_MIN_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. wanna see the other SHMEM reduction routines too? go look at intro_shmem(3), she has the whole list. 👑

ngl this part is important so listen up: like all SHMEM collective routines, every one of these routines assumes that only the PEs in the active set call it. if a PE that is not in the active set calls a SHMEM collective routine, you get undefined behavior. not a warning, not a cute little error, just chaos. 💀

the nreduce argument decides how many separate reductions we're doing. the source array on every PE in the active set hands over one element for each reduction. the results land in the target array on every PE in the active set. and the active set itself? defined by the PE_start, logPE_stride, PE_size triplet. that trio is iconic, they go everywhere together fr fr.

oh and the source and target arrays may be the same array, but they may not be overlapping arrays. read that twice bestie. 🙈

here come the arguments:

  • target — a symmetric array of length nreduce elements that receives the results of the reduction operations. the data type of target changes depending on which version of the reduction routine you're calling. calling from C/C++? check the SYNOPSIS section for data type info. calling from Fortran? the target data types are:
    • shmem_int4_min_to_all: Integer, with an element size of 4 bytes
    • shmem_int8_min_to_all: Integer, with an element size of 8 bytes
    • shmem_real4_min_to_all: Real, with an element size of 4 bytes
    • shmem_real8_min_to_all: Real, with an element size of 8 bytes
    • shmem_real16_min_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.
  • 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. every single 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 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, no solo acts. 🫶

before any PE calls a reduction routine, you have to make sure these conditions exist (synchronization via a barrier or some other method is often needed to guarantee it):

  • 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 once a reduction routine returns, here's what's 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 only needs to be initialized the first time it's used. cute, right? ⭐

bestie you MUST make sure the pSync array is not being updated on any PE in the active set while any of the PEs are participating in processing of a SHMEM reduction routine. watch out for these situations: if the pSync array is initialized at run time, some type of synchronization is needed to make sure 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 only be reused in a subsequent reduction routine call 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. see, there's always a loophole. iykyk. 😌

🎀 EXAMPLES 🎀

Example 1: this Fortran example statically initializes the pSync array and finds the minimum value of 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, FOOMIN, PWRK(MAX(NR/2+1,SHMEM_REDUCE_MIN_WRKDATA_SIZE))
COMMON /COM/ FOO, FOOMIN, PWRK
INTRINSIC MY_PE

IF ( MOD(MY_PE(),2) .EQ. 0) THEN
  CALL SHMEM_REAL8_MIN_TO_ALL(FOOMIN, FOO, NR, 0, 1, N$PES/2,
  & PWRK, PSYNC)
  PRINT *, 'Result on PE ', MY_PE(), ' is ', FOOMIN
ENDIF

Example 2: consider the following C/C++ call:

shmem_int_min_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_min_to_all(&(target[0]), &(source[0]), 1, 0, 0, 8,
  pwrk1, psync1);
shmem_int_min_to_all(&(target[1]), &(source[1]), 1, 0, 0, 8,
  pwrk2, psync2);
shmem_int_min_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. alternating buffers? that's main character energy. 👑

👀 SEE ALSO 👀

2003-2026, The Open MPI Community

and that's the whole page bestie, you ate it 💅✨ this was the Open MPI SHMEM_INT_MIN_TO_ALL(3) page, last touched Sep 10, 2026 — go forth and reduce your minimums, no cap. 🦋🌸

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