[Rcpp-devel] parallel distance matrix calculation

[JJ Allaire]

> On a 4 core machine (8 with hyperthreading) I'm observing a 10x speedup.
> The parallel related speedup is 4x. There is an additional 2.5x speedup
> which appears to be related to the lower level access to the Matrix memory
> done by RMatrix<T> (and perhaps some elimination of copying).
>

It turns out that the additional slowdown in the serial version was due to
repeatedly calling Vector::length as a loop termination condition. I
re-wrote the serial version using iterators and now the speedup from
parallel is about 5x (more in line with expectations).
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