View source for OpenMP in Small Bites/Scoping

[[Category:Tutorials]]<nowiki />
{{Syllabus OpenMP in Small Bites}}<nowiki />
__TOC__

This video provides an overview on managing one of the most challenging parts of OpenMP: Data Scoping. It discusses the differences between <code>private</code>, <code>firstprivate</code>, <code>lastprivate</code> and <code>shared</code> variables and shows how to implement a scalable reduction. 

=== Video === <!--T:5-->

<youtube width="600" height="340" right>3iU7rd7jLL0</youtube>


=== Quiz === <!--T:5-->  

{{hidden begin 
|title = 1. What is the default data scoping of variable in a parallel region when this variable is declared before a parallel region? 
}}
<quiz display=simple>
{
|type="()"}
+ Click and submit to see the answer
|| shared
</quiz>
{{hidden end}}

{{hidden begin 
|title = 2. What is the data scoping of the variables <code>a</code>, <code>b</code>, <code>c</code> and <code>d</code> in the following code snippet in the parallel region? What is printed when executing the code? <br>
<code>int a = 0;<br /> int b = 23;<br /> int c = -3;<br /> # pragma omp parallel num_threads(2) private(a) reduction(+:c)<br /> { </code> <div style="margin-left: 1em;"><code>int d = omp_get_thread_num();<br /> a = 42 + d;<br /> # pragma omp critical<br /> b = 1;<br /> c += a + b;</code></div> <code>} <br />c = c / 2;<br /> printf("a=%d, b=%d, c=%d\n", a, b, c) </code><br />
}}
<quiz display=simple>
{
|type="()"}
+ Click and submit to see the answer
||{{Note|'''a: shared, b: private, c: firstprivate (due to the reduction clause), d: private '''<br />'''output: <code>a:0, b:1, c:42</code>'''}}  
</quiz>
{{hidden end}}

{{hidden begin  
|title = 3. The following code snippet is wrong due to a missing data sharing attribute. How can you fix it? <br>
<code> int i, int s = 0; <br /> # pragma omp parallel for <br /> for (i = 1; i < 100; i++){</code> <div style="margin-left: 1em;"><code> s = s + a[i];</code></div> <code>}  </code> <br /> 
}}
<quiz display=simple>
{
|type="()"}
+ Click and submit to see the answer
|| {{Note|'''Add <code>reduction(+:s)</code> clause to the parallel for construct'''}} 
</quiz>
{{hidden end}}