Wednesday, August 16, 2006

JVM mem leak detector basics + force GC

Some JVM profilers like JProbe can "force" GC daemon to wake up and run. We know that A regular java method can't "force" GC daemon thread to wake up and run. But a JVM profiler is not written in java.

jconsole can invoke GC on the target JVM. It's on the Memory tab, and also in Mbeans -> java.lang->Memory->operations

I think JProbe uses JVM Tool Interface (aka JVM Profiler Interface) to do that? Probably. See
http://java.sun.com/j2se/1.5.0/docs/guide/jvmti/jvmti.html#ForceGarbageCollection. The profiler runs in the same process as the JVM,
sharing the address space and have access to all the jvm variables, GC, threads..

Many memory leak detectors work by counting instances for each class and detect growth over time. The assumption -- instance count for any class should rise and fall, not rise and rise. IMHO, This is the #1 fundamental assumption in memory leak detectors.

In general, if a system is supposed to generate increasing "live" objects then it will crash sooner or later. However, if under normal load the increase is slow enough relative to the lifespan between VM restarts, then it's tolerable but still, such a design remains worrisome and questionable for a robust server-side engine.

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