How to read the output of +PrintTenuringDistribution

[Ji Cheng]

Hello,

I have gc log enabled with -XX:+PrintTenuringDistribution. But I'm quite
confused with the tenuring distribution below.

=============
2013-10-19T19:46:30.244+0800: 169797.045: [GC2013-10-19T19:46:30.244+0800:
169797.045: [ParNew
Desired survivor size 87359488 bytes, new threshold 4 (max 4)
- age   1:   10532656 bytes,   10532656 total
- age   2:   14082976 bytes,   24615632 total
- age   3:   15155296 bytes,   39770928 total
- age   4:   13938272 bytes,   53709200 total
: 758515K->76697K(853376K), 0.0748620 secs] 4693076K->4021899K(6120832K),
0.0756370 secs] [Times: user=0.42 sys=0.00, real=0.07 secs]
2013-10-19T19:47:10.909+0800: 169837.710: [GC2013-10-19T19:47:10.909+0800:
169837.711: [ParNew
Desired survivor size 87359488 bytes, new threshold 4 (max 4)
- age   1:    9167144 bytes,    9167144 total
- age   2:    9178824 bytes,   18345968 total
- age   3:   16101552 bytes,   34447520 total
- age   4:   21369776 bytes,   55817296 total
: 759449K->63442K(853376K), 0.0776450 secs] 4704651K->4020310K(6120832K),
0.0783500 secs] [Times: user=0.43 sys=0.00, real=0.07 secs]
=============

>From What I read, there are 10532656 bytes in age 1 (survived from 1 GC) in
the first gc. In the second gc, 9178824 bytes in age 2 (survived from 2
GCs). This is fine since some objects died between the first and second GC.

But in the second GC, 16101552 bytes are in age 3 while only 14082976 bytes
in age 2 in the first GC. I don't why this number is increasing. Shouldn't
all bytes in age n come from age n-1 in the previous GC? Or I
misinterpreted those numbers?

btw, the jvm version is 1.7.0_40.

Thanks.

Ji Cheng
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