Steps to reproduce:
1. Start fuseki in memory mode with updates enabled.
2. Repeatedly PUT an N-Triple or turtle file to the same graph.
3. Attach VisualVM.
4. Memory use slowly grows and is not all collected.
The parser uses the sink SinkTriplesToGraph, which registers itself with the global RIOT event manager. The sink is never closed by fuseki, and so is never unregistered, which means the sink remains reachable. The sink also has a reference to the graph, so each added graph remains forever.
Closing the sink seems to fix the issue. Not clear if this is especially a memory mode issue.