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

Markus Metz markus.metz.giswork at gmail.com
Sun Feb 23 14:10:36 PST 2014

There is an OpenMP version of r.clump available at


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

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