SHMEM_SHORT_MAX_TO_ALL(3)

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version 5.0.10 · 7 versions

🎀 shmem_short_max_to_all 🎀

hiii bestie 💖 welcome to the Open MPI manual, section 3 — this is the page for the whole shmem_*_max_to_all family, aka the ✨max reduction✨ girlies. If you've ever needed "hey, what's the biggest value of this thing across a whole squad of PEs", this is your page. let's get into it 🦋

💖 NAME 💖

the entire friend group shows up here (yes, one page, many icons 👑):

shmem_double_max_to_all
shmem_float_max_to_all
shmem_int_max_to_all
shmem_int4_max_to_all
shmem_int8_max_to_all
shmem_long_max_to_all
shmem_longdouble_max_to_all
shmem_longlong_max_to_all
shmem_real4_max_to_all
shmem_real8_max_to_all
shmem_real16_max_to_all
shmem_short_max_to_all

and what is the vibe? Performs a maximum function reduction across a set of processing elements (PEs). periodt. 💅

🌸 SYNOPSIS 🌸

C or C++

#include <mpp/shmem.h>

void shmem_double_max_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_max_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_max_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_max_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_max_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_max_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_max_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_MAX_TO_ALL(target, source, nreduce,
& PE_start, logPE_stride, PE_size, pWrk, pSync)

CALL SHMEM_INT8_MAX_TO_ALL(target, source, nreduce,
& PE_start, logPE_stride, PE_size, pWrk, pSync)

CALL SHMEM_REAL4_MAX_TO_ALL(target, source, nreduce,
& PE_start, logPE_stride, PE_size, pWrk, pSync)

CALL SHMEM_REAL8_MAX_TO_ALL(target, source, nreduce,
& PE_start, logPE_stride, PE_size, pWrk, pSync)

CALL SHMEM_REAL16_MAX_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 is just an associative binary operation being run across a set of values — think "fold but make it parallel" 🧚. want the full roster of other SHMEM reduction routines? go peek at intro_shmem(3).

like all SHMEM collective routines, each of these assumes that only PEs in the active set call the routine. and bestie, if a PE that is not in the active set calls a SHMEM collective routine — undefined behavior results. 😭 not a vibe. don't do it.

nreduce tells you how many separate reductions to perform. the source array on all PEs in the active set hands over one element for each reduction. results land in the target array on all PEs in the active set. the active set itself is defined by the PE_start, logPE_stride, PE_size triplet. 💫

oh and important: the source and target arrays may be the same array, but they may not be overlapping arrays. okay? okay. 🫶

🌈 the arguments 🌈

target — a symmetric array of length nreduce elements that receives the results of the reduction operations. the data type of target varies with the version of the reduction routine you're calling. calling from C? check the SYNOPSIS section for the data type tea ☕.

when you're calling from Fortran, the target data types are as follows:

shmem_comp8_max_to_all: Complex, with an element size equal to two 8-byte real values.
shmem_int4_max_to_all: Integer, with an element size of 4 bytes.
shmem_int8_max_to_all: Integer, with an element size of 8 bytes.
shmem_real4_max_to_all: Real, with an element size of 4 bytes.
shmem_real16_max_to_all: Real, with an element size of 16 bytes.

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. no mixing, no matching, no chaos 💀
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. don't skip this step bestie 🙈

💜 the rules of the collective 💜

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. everyone on the same page or it's not happening. 💅

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):

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, these things 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). 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? 🎀

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 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 👑

🌈 EXAMPLES 🌈

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

IF ( MOD(MY_PE(),2) .EQ. 0) THEN
  CALL SHMEM_REAL8_MAX_TO_ALL(FOOMAX, FOO, NR, 0, 1, N$PES/2,
  & PWRK, PSYNC)
  PRINT *, 'Result on PE ', MY_PE(), ' is ', FOOMAX
ENDIF

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

shmem_int_max_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_max_to_all(&(target[0]), &(source[0]), 1, 0, 0, 8,
  pwrk1, psync1);
shmem_int_max_to_all(&(target[1]), &(source[1]), 1, 0, 0, 8,
  pwrk2, psync2);
shmem_int_max_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. see what they did there? alternating buffers, chef's kiss 😭🍓

👑 SEE ALSO 👑

intro_shmem(3)

📜 Copyright 📜

2003-2026, The Open MPI Community

and that's the whole page bestie!! 🫶 go forth and max-reduce responsibly, initialize your pSync before anyone enters anything, and never let a PE outside the active set crash the collective party 💀✨ this was SHMEM_SHORT_MAX_TO_ALL(3) from Open MPI, dated Sep 10, 2026 — mwah, love u, bye 💖💅

📦 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