Minor GCs grow longer after Full GC

[Andreas Müller]

Hi all,

for some time I have been looking at an application (Tomcat 7, Java 7 on Linux, ParallelGC collector, 4GB of heap, 3 GB of perm heap, 20 virtual CPUs on single KVM guest) which shows strange GC behavior:

-        immediately after every Full GC run minor GCs are fast (below 100ms)

-        with growing heap occupation minor GC pauses grow to around 800ms immediately before every Full GC
Find a plot of heap occupation (blue line, left scale) and GC pause duration (grey columns, right scale) attached.
We use the following GC settings:
-Xms4g -Xmx4g -XX:NewSize=800m -XX:MaxNewSize=800m -XX:PermSize=3g -XX:MaxPermSize=3g -XX:TargetSurvivorRatio=50 -XX:SurvivorRatio=8 -XX:ParallelGCThreads=20

I am not sure whether minor GC pauses grow with old gen or perm gen occupation but assume that one of these is the cause.
Perm gen occupation also grows over time because there are (re)generated classes and this would eventually trigger a Full GC if old gen were not filled first.
That's the reason why we configured a rather large perm space.

I can imagine that old gen occupation might cause such a problem if there are many references from old gen to new gen objects.
In this case, management of "remembered sets" during new gen collections  could require growing effort when the number of old gen objects grows. Is this correct?
I have tested G1 for this application and results were worse. As I understand it, management of "remembered sets" is more of a limiting factor with G1 than with ParallelGC, and
therefore this seems to corroborate my assumption. Is this correct?

Are there any other ideas about the cause of growing minor GC pauses?
Are there any ideas how to investigate further?
How to achieve shorter minor GCs without more frequent major GCs?

Best regards / Mit freundlichen Grüßen

Andreas Müller

mgm technology partners GmbH
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E-Mail Andreas.Mueller at mgm-tp.com<mailto:Andreas.Mueller at mgm-tp.com>
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