Hi guys, Users frequently ask questions regarding memory tuning and heap usage; one of the more common being: "why does top report resident memory usage larger than -Xmx?".
I'd like to get some clarification on the usage patterns in Cassandra so I write something up in the Wiki and generally answer questions better. I understand that the -Xm(s|x) flags only affect the JVM managed heap, and that memory is also allocated for the stack (per-thread), program code and native heap. Of these, I can only see the native heap being a major contributor to memory usage. Would I be right in saying that this is due to Cassandra's liberal use of NIO direct ByteBuffers? I couldn't find a whole lot of information on JVM memory usage, but what I did find[1] suggests that these are allocated outside of the native heap (with only a pointer object in the JVM heap). If that's correct, what specifically are direct ByteBuffers used for and when were they introduced? I seem to recall a lot of porting to ByteBuffers in 0.7, yet our production 0.6 cluster exhibits a high non-JVM heap usage that indicates otherwise. If I'm way off the mark, please could you guys point me in the right direction. [1]: http://www.ibm.com/developerworks/java/library/j-nativememory-linux/ Cheers, Nick Telford
