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Relative time spent in GC and the number of surviving generations
- Relative time - measured as a percentage of the total execution time that
the target JVM spends running garbage collection with all application threads
suspended.
- Surviving generations - the number of different
ages for all objects allocated on the JVM heap since the moment Profiler was
attached to it.
- Age of object - measured as the number of garbage
collections survived
A steady growth of
this number usually indicates that some objects are leaking. On the other
hand, if this metrics stabilizes and does not grow noticeably thereafter,
it's guaranteed that your application does not have any growing memory leaks.
This may be valuable information if you observe memory usage going up. In
such situations, people usually start to suspect a memory leak. However,
if you observe that in this case the number of surviving generations remains
stable, it is very likely that the root of the problem is not a memory leak,
but some other problem.
One example is a growing number of concurrent threads,
each of which allocates a large amount of memory for a very short time.
In that case, you may observe gradual increase in the memory usage, which
may ultimately cause an out of memory error. However, there is no leak,
since each thread releases its allocated objects quickly, and memory recycling
rate is in fact quite high.
Note that for technical reasons this monitor does not work when your target
JVM uses the Concurrent Mark and Sweep garbage collector. This is the garbage
collector that is activated using the -XX:+UseConcMarkSweepGC
or -Xconcgc JVM command line option.
See also
