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OpenACC Tutorial - Profiling: Difference between revisions
More information for the analysis
(Interpretation of Intensity) |
(More information for the analysis) |
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== Understanding the code == <!--T:22--> | == Understanding the code == <!--T:22--> | ||
Let's look closely at the | Let's look closely at the main loop in the | ||
[https://github.com/calculquebec/cq-formation-openacc/blob/main/cpp/matrix_functions.h#L29 <code>matvec()</code> function implemented in <code>matrix_functions.h</code>]: | |||
</translate> | </translate> | ||
<syntaxhighlight lang="cpp" line highlight="1,5,10,12"> | <syntaxhighlight lang="cpp" line highlight="1,5,10,12"> | ||
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Given the code above, we search for data dependencies: | Given the code above, we search for data dependencies: | ||
* Does one loop iteration affect other loop iterations? | * Does one loop iteration affect other loop iterations? | ||
* Do loop iterations read from and write to | ** For example, when generating the '''[https://en.wikipedia.org/wiki/Fibonacci_number Fibonacci sequence]''', each new value depends on the previous two values. Therefore, efficient parallelism is very difficult to implement, if not impossible. | ||
* | * Is the accumulation of values in <code>sum</code> a data dependency? | ||
** No, it’s a '''[https://en.wikipedia.org/wiki/Reduction_operator reduction]'''! And modern compilers are good at optimizing such reductions. | |||
* Do loop iterations read from and write to the same array, such that written values are used or overwritten in other iterations? | |||
** Fortunately, that does not happen in the above code. | |||
<!--T:25--> | <!--T:25--> | ||