I have a Cassandra cluster and I set java Xmx to 2G in cassandra.in.sh in every node, but the memory usage of cassandra daemon is 3 or 4 times of that (so 6-8 Gb).

Have you got any idea to decrease the memory usage of cassandra?

There are 4 nodes in the cluster and the system has approx 1000 reads and writes per sec.
Every node has debian.