[GRASS-dev] r.clump4p - OpenMP version of r.clump

[Markus Metz]

There is an OpenMP version of r.clump available at

http://sil.uc.edu/downloads.html#software

called r.clump4p. The reported performance gain is 450 times over the
original r.clump.

The performance gain over the original r.clump vanished:
r.clump4p with one thread is now about 12x slower than r.clump, and
r.clump4p with 4 threads is now about 5x slower than r.clump.
Moreover, the results of r.clump4p are wrong, it clumps together areas
with different cell values (no multithreading effect).

I tested in a region with 650 million cells, both modules produced
about 17.6 million clumps. All differences in the resultant clumps
were due to errors in r.clump4p.

Tests were performed in GRASS 7 where I have optimized r.clump and
also added support for diagonal clump tracing.

Markus M