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<head><title>psb_nrm2 &#8212; Global 2-norm reduction</title> 
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   <h4 class="subsectionHead"><span class="titlemark">7.15   </span> <a 
 id="x83-1200007.15"></a>psb_nrm2 &#8212; Global 2-norm reduction</h4>
                                                                  

                                                                  
   <div class="verbatim" id="verbatim-98">
call&#x00A0;psb_nrm2(icontxt,&#x00A0;dat,&#x00A0;root)
</div>
<!--l. 595--><p class="nopar" >
<!--l. 597--><p class="indent" >   This subroutine implements a 2-norm value reduction operation based on the
underlying communication library.
     <dl class="description"><dt class="description">
<span 
class="cmbx-10">Type:</span> </dt><dd 
class="description">Synchronous.
     </dd><dt class="description">
<span 
class="cmbx-10">On Entry</span>  </dt><dd 
class="description">
     </dd><dt class="description">
<span 
class="cmbx-10">icontxt</span> </dt><dd 
class="description">the communication context identifying the virtual parallel machine.<br 
class="newline" />Scope: <span 
class="cmbx-10">global</span>.<br 
class="newline" />Type: <span 
class="cmbx-10">required</span>.<br 
class="newline" />Intent: <span 
class="cmbx-10">in</span>.<br 
class="newline" />Specified as: an integer variable.
     </dd><dt class="description">
<span 
class="cmbx-10">dat</span> </dt><dd 
class="description">The local contribution to the global minimum.<br 
class="newline" />Scope: <span 
class="cmbx-10">local</span>.<br 
class="newline" />Type: <span 
class="cmbx-10">required</span>.<br 
class="newline" />Intent: <span 
class="cmbx-10">inout</span>.<br 
class="newline" />Specified as: a real variable, which may be a scalar, or a rank 1 array.
     &#x00A0;Kind, rank and size must agree on all processes.
     </dd><dt class="description">
<span 
class="cmbx-10">root</span> </dt><dd 
class="description">Process to hold the final value, or <span 
class="cmsy-10">-</span>1 to make it available on all processes.<br 
class="newline" />Scope: <span 
class="cmbx-10">global</span>.<br 
class="newline" />Type: <span 
class="cmbx-10">optional</span>.<br 
class="newline" />Intent: <span 
class="cmbx-10">in</span>.<br 
class="newline" />Specified as: an integer value <span 
class="cmsy-10">-</span>1 <span 
class="cmmi-10">&#x003C;</span>= <span 
class="cmmi-10">root &#x003C;</span>= <span 
class="cmmi-10">np </span><span 
class="cmsy-10">- </span>1, default -1. <br 
class="newline" /></dd></dl>
<!--l. 624--><p class="indent" >
     <dl class="description"><dt class="description">
<span 
class="cmbx-10">On Return</span> </dt><dd 
class="description">
     </dd><dt class="description">
<span 
class="cmbx-10">dat</span> </dt><dd 
class="description">On destination process(es), the result of the 2-norm reduction.<br 
class="newline" />Scope: <span 
class="cmbx-10">global</span>.<br 
class="newline" />Type: <span 
class="cmbx-10">required</span>.<br 
class="newline" />Intent: <span 
class="cmbx-10">inout</span>.<br 
class="newline" />Specified as: a real variable, which may be a scalar, or a rank 1 array. <br 
class="newline" />Kind, rank and size must agree on all processes.</dd></dl>
<!--l. 636--><p class="noindent" ><span 
class="cmbx-12">Notes</span>
                                                                  

                                                                  
     <ol  class="enumerate1" >
     <li 
  class="enumerate" id="x83-120002x1">This reduction is appropriate to compute the results of multiple (local)
     NRM2 operations at the same time.
     </li>
     <li 
  class="enumerate" id="x83-120004x2">Denoting by <span 
class="cmmi-10">dat</span><sub><span 
class="cmmi-7">i</span></sub> the value of the variable <span 
class="cmmi-10">dat </span>on process <span 
class="cmmi-10">i</span>, the output <span 
class="cmmi-10">res</span>
     is equivalent to the computation of
     <center class="math-display" >
     <img 
src="userhtml29x.png" alt="     &#x2218; -------
       &#x2211;     2
res =     dati,
         i
     " class="math-display" ></center>
     <!--l. 642--><p class="nopar" > with care taken to avoid unnecessary overflow.
     </li>
     <li 
  class="enumerate" id="x83-120006x3">The <span class="obeylines-h"><span class="verb"><span 
class="cmtt-10">dat</span></span></span> argument is both input and output, and its value may be changed
     even on processes different from the final result destination.</li></ol>
                                                                  

                                                                  
                                                                  

                                                                  
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