A single visibility test costs a few nanoseconds, so the thread start up is
only repaid by a large matrix. Measured on sixteen cores the parallel form
gains from a few hundred satellites and loses below that, so the note says
so and points a small shell at the sequential form.
Building a topology asks the same geometry question for every pair of
satellites, which is a quadratic number of independent tests, and that loop
is the dominant cost of the construction. The parallel form returns the
same row major matrix in the same order as the sequential one.
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