I've encountered a dataset which indicates there is probably a performance bug lurking in the FPGrowth implementation. This set may be a bit of an unusual target for FPG - there's a relatively modest number itemsets, and many items with a Zipfy distribution. I am attaching a patch (addSynth.patch) to add a similar dataset as core/src/test/resources/FPGsynth.dat.
FPGsynth.dat can take minutes or a few hours to process, depending on how it is grouped out to machines. If run in sequential mode, or with "-g 50" it will take considerable time. Most reducers/"anchor items" are processed quickly, but a small number take a handful of minutes, and one or two take a long time. If you experiment with this data, I suggest using '-s 50 -regex "[ ]+"'.
Digging into this, I've found that the tree pruning code sometimes creates surprising trees. One oddity I've observed is 0-count nodes, sometimes with non-zero children. The other is that sometimes subtrees seem to get repeated. I'm attaching a sample input file (smallexample.dat, use the whitespace regex with this one, too) and a patch which adds some logging in pruneFPTree and growthBottomUp which will print out some interesting trees when run with the smallexample.dat input.