I'm running a test to write 30 million columns (700bytes each) to Cassandra: the process ran smoothly for about 20mil then the heap usage suddenly jumped from 2GB to 3GB which is the up limit of JVM, --from this point Cassandra will freeze for long time (terrible latency, no response to nodetool that I have to stop the import client ) before it comes back to normal . It's a single node cluster with JVM maximum heap size of 3GB. So what could cause this spike? What kind of tool can I use to find out what are the objects that are filling the additional 1GB heap? I did a heap dump but could get jhat to work to browse the dumped file.