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142 lines
5.8 KiB
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142 lines
5.8 KiB
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<h4 class="subsectionHead"><span class="titlemark">7.15 </span> <a
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id="x83-1200007.15"></a>psb_nrm2 — Global 2-norm reduction</h4>
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<pre class="verbatim" id="verbatim-75">
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call psb_nrm2(ctxt, dat, root)
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</pre>
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<!--l. 595--><p class="nopar" >
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<!--l. 597--><p class="indent" > This subroutine implements a 2-norm value reduction operation based on the
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underlying communication library.
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<dl class="description"><dt class="description">
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<span
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class="cmbx-10">Type:</span> </dt><dd
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class="description">Synchronous.
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</dd><dt class="description">
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<span
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class="cmbx-10">On Entry</span> </dt><dd
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class="description">
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</dd><dt class="description">
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<span
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class="cmbx-10">ctxt</span> </dt><dd
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class="description">the communication context identifying the virtual parallel machine.<br
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class="newline" />Scope: <span
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class="cmbx-10">global</span>.<br
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class="newline" />Type: <span
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class="cmbx-10">required</span>.<br
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class="newline" />Intent: <span
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class="cmbx-10">in</span>.<br
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class="newline" />Specified as: an integer variable.
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</dd><dt class="description">
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<span
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class="cmbx-10">dat</span> </dt><dd
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class="description">The local contribution to the global minimum.<br
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class="newline" />Scope: <span
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class="cmbx-10">local</span>.<br
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class="newline" />Type: <span
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class="cmbx-10">required</span>.<br
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class="newline" />Intent: <span
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class="cmbx-10">inout</span>.<br
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class="newline" />Specified as: a real variable, which may be a scalar, or a rank 1 array.
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 Kind, rank and size must agree on all processes.
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</dd><dt class="description">
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<span
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class="cmbx-10">root</span> </dt><dd
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class="description">Process to hold the final value, or <span
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class="cmsy-10">-</span>1 to make it available on all processes.<br
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class="newline" />Scope: <span
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class="cmbx-10">global</span>.<br
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class="newline" />Type: <span
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class="cmbx-10">optional</span>.<br
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class="newline" />Intent: <span
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class="cmbx-10">in</span>.<br
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class="newline" />Specified as: an integer value <span
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class="cmsy-10">-</span>1 <span
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class="cmmi-10"><</span>= <span
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class="cmmi-10">root <</span>= <span
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class="cmmi-10">np </span><span
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class="cmsy-10">- </span>1, default -1. <br
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class="newline" /></dd></dl>
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<!--l. 624--><p class="indent" >
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<dl class="description"><dt class="description">
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<span
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class="cmbx-10">On Return</span> </dt><dd
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class="description">
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</dd><dt class="description">
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<span
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class="cmbx-10">dat</span> </dt><dd
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class="description">On destination process(es), the result of the 2-norm reduction.<br
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class="newline" />Scope: <span
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class="cmbx-10">global</span>.<br
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class="newline" />Type: <span
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class="cmbx-10">required</span>.<br
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class="newline" />Intent: <span
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class="cmbx-10">inout</span>.<br
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class="newline" />Specified as: a real variable, which may be a scalar, or a rank 1 array. <br
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class="newline" />Kind, rank and size must agree on all processes.</dd></dl>
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<!--l. 636--><p class="noindent" ><span
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class="cmbx-12">Notes</span>
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<ol class="enumerate1" >
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<li
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class="enumerate" id="x83-120002x1">This reduction is appropriate to compute the results of multiple (local)
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NRM2 operations at the same time.
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</li>
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<li
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class="enumerate" id="x83-120004x2">Denoting by <span
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class="cmmi-10">dat</span><sub><span
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class="cmmi-7">i</span></sub> the value of the variable <span
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class="cmmi-10">dat </span>on process <span
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class="cmmi-10">i</span>, the output <span
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class="cmmi-10">res</span>
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is equivalent to the computation of
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<center class="math-display" >
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<img
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src="userhtml29x.png" alt=" ∘ -------
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∑ 2
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res = dati,
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i
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" class="math-display" ></center>
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<!--l. 642--><p class="nopar" > with care taken to avoid unnecessary overflow.
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</li>
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<li
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class="enumerate" id="x83-120006x3">The <span class="obeylines-h"><span class="verb"><span
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class="cmtt-10">dat</span></span></span> argument is both input and output, and its value may be changed
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even on processes different from the final result destination.</li></ol>
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