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<h4 class="subsectionHead"><span class="titlemark">7.15 </span> <a
id="x83-1200007.15"></a>psb_nrm2 — Global 2-norm reduction</h4>
<pre class="verbatim" id="verbatim-87">
call psb_nrm2(ctxt, dat [, root, mode, request])
</pre>
<!--l. 964--><p class="nopar" >
<!--l. 966--><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">ctxt</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.
 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"><</span>= <span
class="cmmi-10">root <</span>= <span
class="cmmi-10">np </span><span
class="cmsy-10">- </span>1, default -1. <br
class="newline" />
</dd><dt class="description">
<span
class="cmbx-10">mode</span> </dt><dd
class="description">Whether the call is started in non-blocking mode and completed later,
or is executed synchronously.<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. The action to be taken is determined by
its bit fields, which can be set with bitwise <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">OR</span></span></span>. Basic action values are
<span class="obeylines-h"><span class="verb"><span
class="cmtt-10">psb_collective_start_</span></span></span>, <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">psb_collective_end_</span></span></span>. Default: both fields are
selected (i.e. require synchronous completion).<br
class="newline" />
</dd><dt class="description">
<span
class="cmbx-10">request</span> </dt><dd
class="description">A request variable to check for operation completion.<br
class="newline" />Scope: <span
class="cmbx-10">local</span>.<br
class="newline" />Type: <span
class="cmbx-10">optional</span>.<br
class="newline" />Intent: <span
class="cmbx-10">inout</span>.<br
class="newline" />If <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">mode</span></span></span> does not specify synchronous completion, then this variable must
be present.</dd></dl>
<!--l. 1008--><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><dt class="description">
<span
class="cmbx-10">request</span> </dt><dd
class="description">A request variable to check for operation completion.<br
class="newline" />Scope: <span
class="cmbx-10">local</span>.<br
class="newline" />Type: <span
class="cmbx-10">optional</span>.<br
class="newline" />Intent: <span
class="cmbx-10">inout</span>.<br
class="newline" />If <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">mode</span></span></span> does not specify synchronous completion, then this variable must
be present.</dd></dl>
<!--l. 1026--><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=" ∘ ∑------
res = dat2i,
i
" class="math-display" ></center>
<!--l. 1032--><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>
<li
class="enumerate" id="x83-120008x4">The <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">mode</span></span></span> argument can be built with the bitwise <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">IOR()</span></span></span> operator; in the
following example, the argument is forcing immediate completion, hence the
<span class="obeylines-h"><span class="verb"><span
class="cmtt-10">request</span></span></span> argument needs not be specified:
<div class="center"
>
<!--l. 1047--><p class="noindent" >
<div class="minipage"><pre class="verbatim" id="verbatim-88">
  call psb_nrm2(ctxt,dat,mode=ior(psb_collective_start_,psb_collective_end_))
</pre>
<!--l. 1051--><p class="nopar" ></div></div>
</li>
<li
class="enumerate" id="x83-120010x5">When splitting the operation in two calls, the <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">dat</span></span></span> argument <span
class="cmti-10">must not </span>be
accessed between calls:
<div class="center"
>
<!--l. 1067--><p class="noindent" >
<div class="minipage"><pre class="verbatim" id="verbatim-89">
  call psb_nrm2(ctxt,dat,mode=psb_collective_start_,request=nrm2_request)
  ....... ! Do not access dat
  call psb_nrm2(ctxt,dat,mode=psb_collective_end_,request=nrm2_request)
</pre>
<!--l. 1073--><p class="nopar" ></div></div>
</li></ol>
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