Sunday, August 16, 2009

JVM memory usage stats -- reconciliation

Suppose I start a jvm with initial heap of 1G and max 4G. Halfway through, 2G is in use.

Q: What would unix ps SIZE column show?
A: committed. Note RSS column is less useful. It's the physical portion of the virtual memory allocated.

Q: What would windows task manager -> PageFile Usage graph show?
A: committed.

Q: What would Runtime.totalMemory() show?
A: committed

Q: What would jconsole show?
A: committed, used and max. {--- These meanings are explained in the java.lang.Runtime api. JDK 5 onwards offers more instrumentation under MemoryMXBean

I believe "committed" is the actual amount of virtual memory dedicated to the JVM and not available to competing processes. Max is higher, but is partly available to other processes.

Difference between committed vs used? See post on wholesale/retail. JVM has grabbed the committed amount from OS syscalls, but only allocated the used amount.

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