The effectiveness of the language-based model of parallelism lies entirely in its ability to expose the dependency structure of the computation by not introducing any dependencies that are not forced on us by the nature of the computation itself. And the key to this is functional programming, which manifests itself here in the transformational approach to computation: sorting is conceived of as a mathematical function that transforms a given sequence into another sequence. It does not destroy the given sequence any more than adding two numbers destroys those numbers! Since Quicksort is a mathematical function, we need not worry that execution of qs xsl
interferes with (depends on)qs xsg; we can readily run them in parallel without fear of untoward consequences. The payoff is that there are many fewer dependencies among the subcomputations, and hence many more opportunities for parallelism that can be exploited, in accord with Brent’s Principle, when scheduling the work onto a parallel fabric.The upshot of all this is that functional programming is of paramount importance for parallelism.
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