G1 collector consuming up to ~35GB memory?

[Neil Ferguson]

Hi all.

When using the G1 collector with large heap sizes I'm seeing the JVM
process grow much larger than my heap size, sometimes by as much as 35GB.
Here are some examples:

Heap size 60GB, java process size: 85GB
Heap size 120GB, java process size: 158GB
Heap size 240GB, java process size: 274GB
Heap size 480GB, process killed by Linux

I'm quoting the resident set as reported by the Linux ps command. The
virtual memory size of the process is typically a few GB larger. This is on
a machine with 512GB RAM, running Linux  2.6.32-220.el6.x86_64 and
Java 1.7.0_05-b05.

Typically the process size grows gradually over the first 30 minutes or so
of my test, then flattens out and doesn't grow any more. I'm running a
fairly trivial benchmark, which repeatedly adds items to a bounded (LRU)
cache. There's typically around 10GB free on the heap in each of my tests.

Does anyone know if this is the expected behaviour for G1? It seems rather
a lot of memory to be using, over the heap size. I've run the same test
with the parallel collector with a heap size of 60GB, and the process size
stabilizes quickly to around 62GB.

Thanks.

Neil.
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