PrintHeapAtGC and PrintTenuringDistribution

Jon Masamitsu Jon.Masamitsu at Sun.COM
Fri Dec 12 10:11:56 PST 2008


Victor Cheung wrote On 12/12/08 09:44,:

> Hihi all,
>
> I was able to (finally) get admin to add some options to the JBoss
> start script on staging and I’m trying to understand the output. I
> have a couple of questions about the following snippet:
>
> Application time: 0.7699290 seconds
>
> {Heap before GC invocations=1 (full 0):
>
> PSYoungGen total 305856K, used 73435K [0x00002aac78cb0000,
> 0x00002aac8e200000, 0x00002aad4e200000)
>
> eden space 262208K, 28% used
> [0x00002aac78cb0000,0x00002aac7d466ca8,0x00002aac88cc0000)
>
> from space 43648K, 0% used
> [0x00002aac8b760000,0x00002aac8b760000,0x00002aac8e200000)
>
> to space 43648K, 0% used
> [0x00002aac88cc0000,0x00002aac88cc0000,0x00002aac8b760000)
>

The above two lines tells you what the survivor space sizes are.

> PSOldGen total 699072K, used 0K [0x00002aaace200000,
> 0x00002aaaf8cb0000, 0x00002aac78cb0000)
>
> object space 699072K, 0% used
> [0x00002aaace200000,0x00002aaace200000,0x00002aaaf8cb0000)
>
> PSPermGen total 524288K, used 17851K [0x00002aaaae200000,
> 0x00002aaace200000, 0x00002aaace200000)
>
> object space 524288K, 3% used
> [0x00002aaaae200000,0x00002aaaaf36efe0,0x00002aaace200000)
>
> 2008-12-12T11:34:45.080-0500: 1.795: [GC
>
> Desired survivor size 44695552 bytes, new threshold 7 (max 15)
>

After the collection the GC thinks that the above is the best size for
the survivor spaces.
The actual survivor space may not be at that size because of constraints on
how the survivor spaces can be changed. In this cause it looks like the
survivor
spaces are already there. The new threshold of 7 is the tenuring
threshold that
GC thinks is best.



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