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<li class="toctree-l1 current"><a class="reference internal" href="index.html">8. Process placement</a><ul class="current">
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<li class="breadcrumb-item"><a href="index.html"><span class="section-number">8. </span>Process placement</a></li>
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.wy-table-responsive table td,.wy-table-responsive table th{white-space:normal}
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<h1><span class="section-number">8.5. </span>Fundamentals<a class="headerlink" href="#fundamentals" title="Permalink to this heading"></a></h1>
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<p>The mapping of processes to nodes can be defined not just with general
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policies but also, if necessary, using arbitrary mappings that cannot
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be described by a simple policy. Supported directives, given on the
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command line via the <code class="docutils literal notranslate"><span class="pre">--map-by</span></code> option, include:</p>
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<ul class="simple">
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<li><p><code class="docutils literal notranslate"><span class="pre">SEQ</span></code>: (often accompanied by the <code class="docutils literal notranslate"><span class="pre">file=<path></span></code> qualifier)
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assigns one process to each node specified in the file. The
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sequential file is to contain an entry for each desired process, one
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per line of the file.</p></li>
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<li><p><code class="docutils literal notranslate"><span class="pre">RANKFILE</span></code>: (often accompanied by the <code class="docutils literal notranslate"><span class="pre">file=<path></span></code> qualifier)
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assigns one process to the node/resource specified in each entry of
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the file, one per line of the file.</p></li>
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</ul>
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<p>For example, using the hostfile below:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ cat myhostfile
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aa slots=4
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bb slots=4
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cc slots=4
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</pre></div>
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</div>
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<p>The command below will launch three processes, one on each of nodes
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<code class="docutils literal notranslate"><span class="pre">aa</span></code>, <code class="docutils literal notranslate"><span class="pre">bb</span></code>, and <code class="docutils literal notranslate"><span class="pre">cc</span></code>, respectively. The slot counts don’t
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matter; one process is launched per line on whatever node is listed on
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the line.</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ prun --hostfile myhostfile --map-by seq ./a.out
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</pre></div>
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</div>
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<p>Impact of the ranking option is best illustrated by considering the
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following hostfile and test cases where each node contains two
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packages (each package with two cores). Using the <code class="docutils literal notranslate"><span class="pre">--map-by</span>
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<span class="pre">ppr:2:package</span></code> option, we map two processes onto each package and
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utilize the <code class="docutils literal notranslate"><span class="pre">--rank-by</span></code> option as show below:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ cat myhostfile
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aa
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bb
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</pre></div>
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</div>
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<table class="docutils align-default">
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<colgroup>
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<col style="width: 33%" />
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<col style="width: 33%" />
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<col style="width: 33%" />
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</colgroup>
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<thead>
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<tr class="row-odd"><th class="head"><p>Command</p></th>
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<th class="head"><p>Ranks on <code class="docutils literal notranslate"><span class="pre">aa</span></code></p></th>
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<th class="head"><p>Ranks on <code class="docutils literal notranslate"><span class="pre">bb</span></code></p></th>
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</tr>
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</thead>
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<tbody>
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<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">--rank-by</span> <span class="pre">core</span></code></p></td>
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<td><p>0 1 ! 2 3</p></td>
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<td><p>4 5 ! 6 7</p></td>
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</tr>
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<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">--rank-by</span> <span class="pre">package</span></code></p></td>
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<td><p>0 2 ! 1 3</p></td>
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<td><p>4 6 ! 5 7</p></td>
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</tr>
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<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">--rank-by</span> <span class="pre">package:SPAN</span></code></p></td>
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<td><p>0 4 ! 1 5</p></td>
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<td><p>2 6 ! 3 7</p></td>
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</tr>
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</tbody>
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</table>
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<p>Ranking by slot provides the identical result as ranking by core in
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this case — a simple progression of ranks across each
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node. Ranking by package does a round-robin ranking across packages
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within each node until all processes have been assigned a rank, and
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then progresses to the next node. Adding the <code class="docutils literal notranslate"><span class="pre">:SPAN</span></code> qualifier to
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the ranking directive causes the ranking algorithm to treat the entire
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allocation as a single entity — thus, the process ranks are
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assigned across all packages before circling back around to the
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beginning.</p>
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<p>The binding operation restricts the process to a subset of the CPU
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resources on the node.</p>
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<p>The processors to be used for binding can be identified in terms of
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topological groupings — e.g., binding to an l3cache will bind
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each process to all processors within the scope of a single L3 cache
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within their assigned location. Thus, if a process is assigned by the
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mapper to a certain package, then a <code class="docutils literal notranslate"><span class="pre">--bind-to</span> <span class="pre">l3cache</span></code> directive
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will cause the process to be bound to the processors that share a
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single L3 cache within that package.</p>
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<p>To help balance loads, the binding directive uses a round-robin method,
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binding a process to the first available specified object type within
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the object where the process was mapped. For example, consider the case
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where a job is mapped to the package level, and then bound to core. Each
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package will have multiple cores, so if multiple processes are mapped to
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a given package, the binding algorithm will assign each process located
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to a package to a unique core in a round-robin manner.</p>
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<p>Binding can only be done to the mapped object or to a resource located
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within that object.</p>
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<p>An object is considered completely consumed when the number of
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processes bound to it equals the number of CPUs within it. Unbound
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processes are not considered in this computation. Additional
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processes cannot be mapped to consumed objects unless the
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OVERLOAD qualifier is provided via the “–bind-to” command
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line option.</p>
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<p>Default process mapping/ranking/binding policies can also be set with MCA
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parameters, overridden by the command line options when provided. MCA
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parameters can be set on the <code class="docutils literal notranslate"><span class="pre">prte</span></code> command line when starting the
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DVM (or in the <code class="docutils literal notranslate"><span class="pre">prterun</span></code> command line for a single-execution job), but
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also in a system or user <code class="docutils literal notranslate"><span class="pre">mca-params.conf</span></code> file or as environment
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variables, as described in the MCA section below. Some examples include:</p>
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<table class="docutils align-default">
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<colgroup>
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<col style="width: 33%" />
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<col style="width: 33%" />
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<col style="width: 33%" />
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</colgroup>
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<thead>
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<tr class="row-odd"><th class="head"><p><code class="docutils literal notranslate"><span class="pre">prun</span></code> option</p></th>
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<th class="head"><p>MCA parameter key</p></th>
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<th class="head"><p>Value</p></th>
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</tr>
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</thead>
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<tbody>
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<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">--map-by</span> <span class="pre">core</span></code></p></td>
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<td><p><code class="docutils literal notranslate"><span class="pre">rmaps_default_mapping_policy</span></code></p></td>
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<td><p><code class="docutils literal notranslate"><span class="pre">core</span></code></p></td>
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</tr>
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<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">--map-by</span> <span class="pre">package</span></code></p></td>
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<td><p><code class="docutils literal notranslate"><span class="pre">rmaps_default_mapping_policy</span></code></p></td>
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<td><p><code class="docutils literal notranslate"><span class="pre">package</span></code></p></td>
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</tr>
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<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">--rank-by</span> <span class="pre">core</span></code></p></td>
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<td><p><code class="docutils literal notranslate"><span class="pre">rmaps_default_ranking_policy</span></code></p></td>
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<td><p><code class="docutils literal notranslate"><span class="pre">core</span></code></p></td>
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</tr>
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<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">--bind-to</span> <span class="pre">core</span></code></p></td>
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<td><p><code class="docutils literal notranslate"><span class="pre">hwloc_default_binding_policy</span></code></p></td>
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<td><p><code class="docutils literal notranslate"><span class="pre">core`</span></code></p></td>
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</tr>
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<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">--bind-to</span> <span class="pre">package</span></code></p></td>
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<td><p><code class="docutils literal notranslate"><span class="pre">hwloc_default_binding_policy</span></code></p></td>
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<td><p><code class="docutils literal notranslate"><span class="pre">package</span></code></p></td>
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</tr>
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<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">--bind-to</span> <span class="pre">none</span></code></p></td>
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<td><p><code class="docutils literal notranslate"><span class="pre">hwloc_default_binding_policy</span></code></p></td>
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<td><p><code class="docutils literal notranslate"><span class="pre">none</span></code></p></td>
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</tr>
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</tbody>
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</table>
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