[OpenMP][doc] Add documents for barrier related environment variables

[t-msn]

Summary

Update for KMP_FORKJOIN_BARRIER{_PATTERN}, KMP_PLAIN_BARRIER{_PATTERN}
and KMP_REDUCTION_BARRIER{_PATTERN}.

[t-msn]

I couldn't find any documents on the web. Is there any existing materials?

[jlpeyton]

There are no existing documentation for these. Thanks for creating this!

openmp/docs/design/Runtimes.rst

446

This pattern is used for the parallel and teams constructs.

633

Also add that this pattern is used for implicit barriers at end of worksharing constructs.

[t-msn]

Follow suggestions by jlpeyton. Thanks a lot.

[t-msn]

openmp/docs/design/Runtimes.rst

446

updated.

633

updated.

[jlpeyton]

LGTM

openmp/docs/design/Runtimes.rst

 Selects the method used to determine the number of threads to use for a parallel
 region when ``OMP_DYNAMIC=true``. Possible values: (``load_balance`` | ``thread_limit``), where,
 
 * ``load_balance``: tries to avoid using more threads than available execution units on the machine;
 * ``thread_limit``: tries to avoid using more threads than total execution units on the machine.
 
 **Default:** ``load_balance`` (on all supported platforms)
 
+KMP_FORKJOIN_BARRIER
+""""""""""""""""""""
+Specifies log2 of branching-factor of forkjoin barrier.
+This only has a meaning if tree or hyper barrier pattern is used.
+
+| **Syntax:** ``KMP_FORKJOIN_BARRIER=<factor-for-gather>,<factor-for-release>``
+| **Default:** 2,2
+
+KMP_FORKJOIN_BARRIER_PATTERN
+""""""""""""""""""""""""""""
+Specifies barrier pattern of forkjoin barrier which is used for the parallel and teams constructs.
+As libomp barrier adopts centralized barrier, barrier pattern can be specified for gather
+and release phase respectively.
+
+**Currently supported patterns:**
+
+* ``linear``: Linear centralized barrier
+* ``tree``: Static tree barrier. Branching-factor is set by ``KMP_FORKJOIN_BARRIER``
+* ``hyper``: Like tree but use hyper-cube. Branching-factor is set by ``KMP_FORKJOIN_BARRIER``
+* ``hier``: Like tree but use system's hierarchical information of caches
+* ``dist``: Two-level distributed barrier. To use this, all barrier patterns must be
+  specified as dist
+
+| **Syntax:** ``KMP_FORKJOIN_BARRIER_PATTERN=<gather-pattern>,<release-pattern>``
+| **Default:** hyper,hyper
+| **Related environment variable:** ``KMP_PLAIN_BARRIER_PATTERN``, ``KMP_REDUCTION_BARRIER_PATTERN``
+
 KMP_HOT_TEAMS_MAX_LEVEL
 """""""""""""""""""""""
 Sets the maximum nested level to which teams of threads will be hot.
 
 .. note::
     A hot team is a team of threads optimized for faster reuse by subsequent
     parallel regions. In a hot team, threads are kept ready for execution of
     the next parallel region, in contrast to the cold team, which is freed

 """""""""""
 
 Selects the OpenMP run-time library execution mode. The values for this variable
 are ``serial``, ``turnaround``, or ``throughput``.
 
 | **Default:** ``throughput``
 | **Related environment variable:** ``KMP_BLOCKTIME`` and ``OMP_WAIT_POLICY``
 
+KMP_PLAIN_BARRIER
+"""""""""""""""""
+Specifies log2 of branching-factor of plain barrier.
+This only has a meaning if tree or hyper barrier pattern is used.
+
+| **Syntax:** ``KMP_PLAIN_BARRIER=<factor-for-gather>,<factor-for-release>``
+| **Default:** 2,2
+
+KMP_PLAIN_BARRIER_PATTERN
+"""""""""""""""""""""""""
+Specifies barrier pattern of plain barrier which is used for ``#pragma omp barrier`` and
+implicit barriers at the end of worksharing constructs.
+See ``KMP_FORKJOIN_BARRIER_PATTERN`` for currently supported patterns.
+
+| **Syntax:** ``KMP_PLAIN_BARRIER_PATTERN=<gather-pattern>,<release-patern>``
+| **Default:** hyper,hyper
+| **Related environment variable:** ``KMP_FORKJOIN_BARRIER_PATTERN``, ``KMP_REDUCTION_BARRIER_PATTERN``
+
+KMP_REDUCTION_BARRIER
+"""""""""""""""""""""
+Specifies log2 of branching-factor of reduction barrier.
+This only has a meaning if tree or hyper barrier pattern is used.
+
+| **Syntax:** ``KMP_REDUCTION_BARRIER=<factor-for-gather>,<factor-for-release>``
+| **Default:** 1,1
+
+KMP_REDUCTION_BARRIER_PATTERN
+"""""""""""""""""""""""""""""
+Specifies barrier pattern of reduction barrier which is used in reduction.
+See ``KMP_FORKJOIN_BARRIER_PATTERN`` for currently supported patterns.
+
+| **Syntax:** ``KMP_REDUCTION_BARRIER_PATTERN=<gather-pattern>,<release-pattern>``
+| **Default:** hyper,hyper
+| **Related environment variable:** ``KMP_FORKJOIN_BARRIER_PATTERN``, ``KMP_PLAIN_BARRIER_PATTERN``
+
 KMP_SETTINGS
 """"""""""""
 
 Enables (``true``) or disables (``false``) the printing of OpenMP run-time library
 environment variables during program execution. Two lists of variables are printed:
 user-defined environment variables settings and effective values of variables used
 by OpenMP run-time library.