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  81.   <div class="section" id="using-openmp">
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  82. <span id="openmp"></span><h1>Using OpenMP<a class="headerlink" href="#using-openmp" title="Permalink to this headline"></a></h1>
    83. 

  83. <p>The Clawpack Fortran Classic 3d code, AMRClaw 2d and 3d code,
    84. 

  84. and GeoClaw codes include
    85. 

  85. OpenMP directives for making use of multicore shared memory machines.</p>
    86. 

  86. <p><strong>Note:</strong> Versions of gfortran before 4.6 are known to have OpenMP bugs.
    87. 

  87. You should use a recent version or a different compiler if you want to use
    88. 

  88. OpenMP.</p>
    89. 

  89. <p>To invoke OpenMP you need to compile the entire code with appropriate
    90. 

  90. compiler flags (see <a class="reference internal" href="fortran_compilers.html#fortran-compilers"><span class="std std-ref">Fortran Compilers</span></a>).  For example, with gfortran
    91. 

  91. and the bash shell you could do:</p>
    92. 

  92. <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">export</span> <span class="n">FFLAGS</span><span class="o">=</span><span class="s1">&#39;-O2 -fopenmp&#39;</span>  <span class="c1"># or hardwire FFLAGS in the Makefile</span>
    93. 

  93. <span class="n">make</span> <span class="n">new</span>
    94. 

  94. </pre></div>
    95. 

  95. </div>
    96. 

  96. <p>in an application directory, which should recompile all of the library
    97. 

  97. routines as well.</p>
    98. 

  98. <p>Then you may want to specify how many threads OpenMP should split the work
    99. 

  99. between, e.g.</p>
    100. 

  100. <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">export</span> <span class="n">OMP_NUM_THREADS</span><span class="o">=</span><span class="mi">2</span>
    101. 

  101. </pre></div>
    102. 

  102. </div>
    103. 

  103. <p>If you do not set this environment variable some default for your system
    104. 

  104. will be used.</p>
    105. 

  105. <p>You may also need to increase the stack size if the code bombs for no
    106. 

  106. apparent reason (and no useful error message):</p>
    107. 

  107. <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">export</span> <span class="n">OMP_STACKSIZE</span><span class="o">=</span><span class="mi">16</span><span class="n">M</span>
    108. 

  108. </pre></div>
    109. 

  109. </div>
    110. 

  110. <p>and also:</p>
    111. 

  111. <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">ulimit</span> <span class="o">-</span><span class="n">s</span> <span class="n">unlimited</span>
    112. 

  112. </pre></div>
    113. 

  113. </div>
    114. 

  114. <p>On a Mac this isn’t allowed and the best you can do is</p>
    115. 

  115. <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">ulimit</span> <span class="o">-</span><span class="n">s</span> <span class="n">hard</span>
    116. 

  116. </pre></div>
    117. 

  117. </div>
    118. 

  118. <p>To stop using OpenMP you could do:</p>
    119. 

  119. <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">export</span> <span class="n">FFLAGS</span><span class="o">=-</span><span class="n">O2</span>   <span class="c1"># or hardwire FFLAGS in the Makefile</span>
    120. 

  120. <span class="n">make</span> <span class="n">new</span>
    121. 

  121. </pre></div>
    122. 

  122. </div>
    123. 

  123. <div class="section" id="using-openmp-with-amr">
    124. 

  124. <span id="openmp-amr"></span><h2>Using OpenMP with AMR<a class="headerlink" href="#using-openmp-with-amr" title="Permalink to this headline"></a></h2>
    125. 

  125. <p>The code in AMRClaw and GeoClaw is parallelized by splitting the list of
    126. 

  126. patches that must be advanced in time between threads, and then each grid
    127. 

  127. patch is handled by a single thread.  For this reason good performance will
    128. 

  128. be seen only when there are a sufficiently large number of patches at each
    129. 

  129. level relative to the number of threads.  For this reason it is recommended
    130. 

  130. that the parameter <cite>max1d</cite> be set to 60 in the modules</p>
    131. 

  131. <ul class="simple">
    132. 

  132. <li><p><cite>$CLAW/amrclaw/src/2d/amr_module.f90</cite></p></li>
    133. 

  133. <li><p><cite>$CLAW/amrclaw/src/3d/amr_module.f90</cite></p></li>
    134. 

  134. </ul>
    135. 

  135. <p>when OpenMP is used.  This limits the size of any patch to have at most
    136. 

  136. <cite>max1d</cite> grid cells in each direction.  If OpenMP is not used, a larger value
    137. 

  137. of <cite>max1d</cite> might give somewhat better performance since there is less
    138. 

  138. overhead associated with passing boundary values in ghost cells and other
    139. 

  139. per-patch work.  However, this is generally negligible and <cite>max1d=60</cite> is the
    140. 

  140. default value set in the code.   If you do change this value, remember to
    141. 

  141. recompile everything via:</p>
    142. 

  142. <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">make</span> <span class="n">new</span>
    143. 

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    144. 

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    145. 

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    146. 

  146. <div class="section" id="fixed-grid-output-in-geoclaw">
    147. 

  147. <span id="openmp-fixedgrids"></span><h2>Fixed grid output in GeoClaw<a class="headerlink" href="#fixed-grid-output-in-geoclaw" title="Permalink to this headline"></a></h2>
    148. 

  148. <p>The original fixed grid output routines are not thread safe and so OpenMP
    149. 

  149. should not be used if you want to produce output on fixed grids.</p>
    150. 

  150. <p>The newer fgmax routines that keep track of maxima on fixed grids should be
    151. 

  151. thread safe, see <a class="reference internal" href="fgmax.html#fgmax"><span class="std std-ref">Fixed grid monitoring</span></a>.</p>
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  168. <li><a class="reference internal" href="#">Using OpenMP</a><ul>
    169. 

  169. <li><a class="reference internal" href="#using-openmp-with-amr">Using OpenMP with AMR</a></li>
    170. 

  170. <li><a class="reference internal" href="#fixed-grid-output-in-geoclaw">Fixed grid output in GeoClaw</a></li>
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