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I've got a running java webapp, that I'm monitoring with visualVM.
Here's the graph of the heap:
The was tested with two sets of requests, one at 3:20 and the other at 4:40 aprox (they are represented in the graph as the only two peaks).
My question is: does this means I have a memory leak? I'm worried about the middle part where, although the GC runs, the heap stays in 250MB all the time.
Thanks a lot for your insights.
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Running out of memory is the simplest way to identify a memory leak, and it's also the most common approach to uncovering one. That's also the most inconvenient way to find a leak. You'll probably notice your system slowing down before you run out of RAM and crash your application.
DEFINITION A memory leak is the gradual deterioration of system performance that occurs over time as the result of the fragmentation of a computer's RAM due to poorly designed or programmed applications that fail to free up memory segments when they are no longer needed.
There are two types of memory leaks: apparent and subtle. An apparent memory leak is a chunk of heap memory that's never referred from active memory, a subtle leak is memory that is still referred to but shouldn't be, i.e. a hash or dynamic array holds the references.
The first request at 3:20 caused some memory to be held, but notice that the GCs after the second request reclaimed most of it. Also I think that major GC was performed only after the second request at 4:40.
It looks like there is no leak. My theory is that the request at 3:20 caused the young generation to fill up, and the resulting minor GC promoted some objects to older generation. The next major GC, caused by the request at 4:40 cleaned most of those up.
You can verify this by using a profiler to mark the heap before issuing the same request as the one at 3:20, forcing a full GC, and then checking what objects are lingering. I am not sure if VisualVM lets you (1) mark the heap and (2) force a full GC, but OptimizeIt used to do it.
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