What is tomcat memory heap committed?

Question

We are monitoring tomcat servers and i found whenever Committed Heap memory reached Max heap memory my tomcat got crashed or OOM error, But at same time Heap Used memory is under normal used. Could some one explain what is committed used and why its crashing while Heap used is normal.

See following graph so you get some idea.

Answer 1

Committed space is space that is not virtual. Namely space that is statically assigned to a given partition of the JVM. When you use the optional sizing switches -Xms and -Xmx not everything is initially committed to particular partition of memory. As the generations have a need to expand they are permitted to grow into the "virtual" space.

Your chart looks like how things should be working. As your used space grows toward your committed, the committed space expands towards the maximum. When committed reaches the maximum size thats it, it has no where to go. If it can't save itself with a last ditch GC then down your JVM goes.