Java old gen keeps growing - need help interpreting Java GC output

Question

I work on a performance-critical server-side Java application.

Once the system has started, I expect no long-lived objects to be created - only ones with short-ish lives (up to 10 seconds). I therefore want to tune the JVM so that the Old Generation is constant once the system has started up.

I think I've succeeded but I don't understand why (see below).

Here are our settings:

-Xmx3000m -Xms3000m -verbose:gc -XX:+PrintGCTimeStamps -XX:+PrintGCDetails -XX:+UseConcMarkSweepGC -XX:SurvivorRatio=5 -XX:TargetSurvivorRatio=90 -XX:MaxTenuringThreshold=31 -XX:+PrintTenuringDistribution -XX:NewSize=1250m -XX:MaxNewSize=1250m

We're using Oracle's Java 1.6.

I'm really confused by the output of the "PrintTenuringDistribution" flag. I see output like this:

893.866: [GC 893.866: [ParNew
Desired survivor size 168512712 bytes, new threshold 16 (max 31)
- age   1:   13251072 bytes,   13251072 total
- age   2:    1135456 bytes,   14386528 total
- age   3:      59104 bytes,   14445632 total
- age   4:     467384 bytes,   14913016 total
- age   5:      68688 bytes,   14981704 total
- age   6:      43336 bytes,   15025040 total
- age   7:      41344 bytes,   15066384 total
- age   8:       6872 bytes,   15073256 total
- age   9:      87568 bytes,   15160824 total
- age  10:      23840 bytes,   15184664 total
- age  11:      99712 bytes,   15284376 total
- age  12:       5224 bytes,   15289600 total
- age  13:       1000 bytes,   15290600 total
- age  14:      59056 bytes,   15349656 total
- age  15:   65225584 bytes,   80575240 total
: 1015468K->95478K(1097152K), 0.0268070 secs] 1387738K->467748K(2889152K), 0.0269910 secs] [Times: user=0.31 sys=0.00, real=0.03 secs]
899.179: [GC 899.180: [ParNew
Desired survivor size 168512712 bytes, new threshold 16 (max 31)
- age   1:   12438336 bytes,   12438336 total
- age   2:    1531984 bytes,   13970320 total
- age   3:      87920 bytes,   14058240 total
- age   4:      58824 bytes,   14117064 total
- age   5:     463368 bytes,   14580432 total
- age   6:      68688 bytes,   14649120 total
- age   7:      43336 bytes,   14692456 total
- age   8:      40136 bytes,   14732592 total
- age   9:       6872 bytes,   14739464 total
- age  10:      87568 bytes,   14827032 total
- age  11:      23840 bytes,   14850872 total
- age  12:      99712 bytes,   14950584 total
- age  13:       2896 bytes,   14953480 total
- age  14:       1000 bytes,   14954480 total
- age  15:   65282456 bytes,   80236936 total
: 1009782K->98799K(1097152K), 0.0383370 secs] 1382052K->471069K(2889152K), 0.0385490 secs] [Times: user=0.36 sys=0.00, real=0.03 secs]

Given that I have a lot of objects with age=15, and the threshold is 16, I would expect each object in a GC to either (a) get promoted into the Old Generation, or (b) garbage collected.

But according to JConsole my old generation is not increasing. So what's happening to all these objects?

Answer 1

The ages you have are for objects in the survivor space. As you have a large eden space most objects which make into the survivor space will be relatively old.