Monday, May 4, 2009

garbage collection in java

Recently one of our developers wanted to know why the application seems to use a lot more memory than the steady state (amount of memory JVM consumes after the startup of application server with application deployed) when there were no users on the system for a while.

For the purpose of discussion, say the JVM has max of 1GB heap and steady state memory consumption is 300MB. The heap usage was around 700MB on an idle system.

Well, this is an expected behavior from the JVM. (SUN JVM)
Garbage collection is a condition based trigger. This would mean garbage collection is reactive and not proactive.
Most probable reason for this is GC is resource intensive operation and we only want this when needed.
Only a request will trigger a Full GC , If the request needs 100MB of memory (as an example) and JVM is not able to find 100MB of contiguous memory, this will trigger a Full GC, else, it will not initiate a GC cycle. (even if there are objects that have no references and are ready to be GCed).
Just because the system/jvm is idle, GC will not be "proactive" to clean up the unused objects.

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