.. _shmem_short_min_to_all:


shmem_short_min_to_all
======================

.. include_body

:ref:`shmem_double_min_to_all`\ (3), :ref:`shmem_float_min_to_all`\ (3),
:ref:`shmem_int_min_to_all`\ (3), shmem_int4_min_to_all\ (3),
shmem_int8_min_to_all\ (3), :ref:`shmem_long_min_to_all`\ (3),
:ref:`shmem_longdouble_min_to_all`\ (3), :ref:`shmem_longlong_min_to_all`\ (3),
shmem_real4_min_to_all\ (3), shmem_real8_min_to_all\ (3),
shmem_real16_min_to_all\ (3), :ref:`shmem_short_min_to_all`\ (3) - Performs
a minimum function reduction across a set of processing elements (PEs)


SYNOPSIS
--------

C or C++:

.. code-block:: 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:

.. code-block:: 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
-----------

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. For a
list of other SHMEM reduction routines, see *intro_shmem*\ (3).

As with all SHMEM collective routines, each of these routines assumes
that only PEs in the active set call the routine. If a PE not in the
active set calls a SHMEM collective routine, undefined behavior results.

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 are 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.

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

The arguments are as follows:

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. 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_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. 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. If you are using Fortran, 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.

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.

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). 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.

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.


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:

.. code-block:: c++

   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.


.. seealso::
   *intro_shmem*\ (3)