Hello WanMil, during the last days I tried a lot of different techniques to further improve memory usage, but nothing really improved the throughput of splitter. The default settings are now so well tuned that the only situation where --optimize-mem=true really helped was when spttiting rather small files (pbf with 100MB or less). But these small files are never a problem regarding memory, and I guess that most users that want to split large files (europe or so) have machines with 8GB or more. So, the attached patch removes all code handling the --optimize-mem parameter to remove useless complex code, keeping only those changes that really improve something. I've also added code to the unit tests and small correction to the now unused SparseInt2ShortMap implementations. Ciao, Gerd