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<h1><span class="section-number">7.4. </span>Relative host indexing<a class="headerlink" href="#relative-host-indexing" title="Permalink to this heading"></a></h1>
<p>Hostfile and <code class="docutils literal notranslate"><span class="pre">--host</span></code> specifications can also be made using relative
indexing. This allows a user to stipulate which hosts are to be used
for a given app context without specifying the particular host name,
but rather its relative position in the allocation.</p>
<p>This can probably best be understood through consideration of a few
examples. Consider the case where a DVM is comprised of a set of nodes
named <code class="docutils literal notranslate"><span class="pre">foo1</span></code>, <code class="docutils literal notranslate"><span class="pre">foo2</span></code>, <code class="docutils literal notranslate"><span class="pre">foo3</span></code>, <code class="docutils literal notranslate"><span class="pre">foo4</span></code>. The user wants the first
app context to have exclusive use of the first two nodes, and a second
app context to use the last two nodes. Of course, the user could
printout the allocation to find the names of the nodes allocated to
them and then use <code class="docutils literal notranslate"><span class="pre">--host</span></code> to specify this layout, but this is
cumbersome and would require hand-manipulation for every invocation.</p>
<p>A simpler method is to utilize PRRTE’s relative indexing capability to
specify the desired layout. In this case, a command line containing:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">--</span><span class="n">host</span> <span class="o">+</span><span class="n">n1</span><span class="p">,</span><span class="o">+</span><span class="n">n2</span> <span class="o">./</span><span class="n">app1</span> <span class="p">:</span> <span class="o">--</span><span class="n">host</span> <span class="o">+</span><span class="n">n3</span><span class="p">,</span><span class="o">+</span><span class="n">n4</span> <span class="o">./</span><span class="n">app2</span>
</pre></div>
</div>
<p>would provide the desired pattern. The <code class="docutils literal notranslate"><span class="pre">+</span></code> syntax indicates that the
information is being provided as a relative index into the existing
allocation. Two methods of relative indexing are supported:</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">+n#</span></code>: A relative index into the allocation referencing the <code class="docutils literal notranslate"><span class="pre">#</span></code>
node. PRRTE will substitute the <code class="docutils literal notranslate"><span class="pre">#</span></code> node in the allocation</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">+e[:#]</span></code>: A request for <code class="docutils literal notranslate"><span class="pre">#</span></code> empty nodes — i.e., PRRTE is
to substitute this reference with nodes that have not yet been used
by any other app_context. If the <code class="docutils literal notranslate"><span class="pre">:#</span></code> is not provided, PRRTE will
substitute the reference with all empty nodes. Note that PRRTE does
track the empty nodes that have been assigned in this manner, so
multiple uses of this option will result in assignment of unique
nodes up to the limit of the available empty nodes. Requests for
more empty nodes than are available will generate an error.</p></li>
</ul>
<p>Relative indexing can be combined with absolute naming of hosts in any
arbitrary manner, and can be used in hostfiles as well as with the
<code class="docutils literal notranslate"><span class="pre">--host</span></code> command line option. In addition, any slot specification
provided in hostfiles will be respected — thus, a user can
specify that only a certain number of slots from a relative indexed
host are to be used for a given app context.</p>
<p>Another example may help illustrate this point. Consider the case
where the user has a hostfile containing:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">dummy1</span> <span class="n">slots</span><span class="o">=</span><span class="mi">4</span>
<span class="n">dummy2</span> <span class="n">slots</span><span class="o">=</span><span class="mi">4</span>
<span class="n">dummy3</span> <span class="n">slots</span><span class="o">=</span><span class="mi">4</span>
<span class="n">dummy4</span> <span class="n">slots</span><span class="o">=</span><span class="mi">4</span>
<span class="n">dummy5</span> <span class="n">slots</span><span class="o">=</span><span class="mi">4</span>
</pre></div>
</div>
<p>This may, for example, be a hostfile that describes a set of
commonly-used resources that the user wishes to execute applications
against. For this particular application, the user plans to map
byslot, and wants the first two ranks to be on the second node of any
allocation, the next ranks to land on an empty node, have one rank
specifically on <code class="docutils literal notranslate"><span class="pre">dummy4</span></code>, the next rank to be on the second node of the
allocation again, and finally any remaining ranks to be on whatever
empty nodes are left. To accomplish this, the user provides a hostfile
of:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">+</span><span class="n">n2</span> <span class="n">slots</span><span class="o">=</span><span class="mi">2</span>
<span class="o">+</span><span class="n">e</span><span class="p">:</span><span class="mi">1</span>
<span class="n">dummy4</span> <span class="n">slots</span><span class="o">=</span><span class="mi">1</span>
<span class="o">+</span><span class="n">n2</span>
<span class="o">+</span><span class="n">e</span>
</pre></div>
</div>
<p>The user can now use this information in combination with PRRTE’s
sequential mapper to obtain their specific layout:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o"><</span><span class="n">launcher</span><span class="o">></span> <span class="o">--</span><span class="n">hostfile</span> <span class="n">dummyhosts</span> <span class="o">--</span><span class="n">hostfile</span> <span class="n">mylayout</span> <span class="o">--</span><span class="n">prtemca</span> <span class="n">rmaps</span> <span class="n">seq</span> <span class="o">./</span><span class="n">my_app</span>
</pre></div>
</div>
<p>which will result in:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">rank0</span> <span class="n">being</span> <span class="n">mapped</span> <span class="n">to</span> <span class="n">dummy3</span>
<span class="n">rank1</span> <span class="n">to</span> <span class="n">dummy1</span> <span class="k">as</span> <span class="n">the</span> <span class="n">first</span> <span class="n">empty</span> <span class="n">node</span>
<span class="n">rank2</span> <span class="n">to</span> <span class="n">dummy4</span>
<span class="n">rank3</span> <span class="n">to</span> <span class="n">dummy3</span>
<span class="n">rank4</span> <span class="n">to</span> <span class="n">dummy2</span> <span class="ow">and</span> <span class="n">rank5</span> <span class="n">to</span> <span class="n">dummy5</span> <span class="k">as</span> <span class="n">the</span> <span class="n">last</span> <span class="n">remaining</span> <span class="n">unused</span> <span class="n">nodes</span>
</pre></div>
</div>
<p>Note that the sequential mapper ignores the number of slots arguments
as it only maps one rank at a time to each node in the list.</p>
<p>If the default round-robin mapper had been used, then the mapping
would have resulted in:</p>
<ul class="simple">
<li><p>ranks 0 and 1 being mapped to dummy3 since two slots were specified</p></li>
<li><p>ranks 2-5 on dummy1 as the first empty node, which has four slots</p></li>
<li><p>rank6 on dummy4 since the hostfile specifies only a single slot from
that node is to be used</p></li>
<li><p>ranks 7 and 8 on dummy3 since only two slots remain available</p></li>
<li><p>ranks 9-12 on dummy2 since it is the next available empty node and
has four slots</p></li>
<li><p>ranks 13-16 on dummy5 since it is the last remaining unused node and
has four slots</p></li>
</ul>
<p>Thus, the use of relative indexing can allow for complex mappings to
be ported across allocations, including those obtained from automated
resource managers, without the need for manual manipulation of scripts
and/or command lines.</p>
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