Repeated ParNews when Young Gen is Empty?

[Bernd Eckenfels]

Am 01.07.2013, 22:44 Uhr, schrieb Andrew Colombi <acolombi at palantir.com>:
> My question is, Why would it do three ParNews, only 300ms apart from  
> each other, when the young gen is essentially empty?  Here are three  
> hypotheses that I have:
> * Is the application trying to allocate something giant, e.g. a 1  
> billion element double[]? Is there a way I can test for this, i.e. some  
> JVM level logging that would indicate very large objects being allocated.

That was a suspicion of me as well. (And I dont know a good tool for Sun  
VM (with IBM you can trace it)).

> * Is there an explicit System.gc() in 3rd party code? (Our code is  
> clean.) We're going to disable explicit GC in our next maintenance  
> period.  But this theory doesn't explain concurrent mode failure.

I think System.gc() will also not trigger 3 ParNew in a row.

> * Maybe a third explanation is fragmentation? Is ParNew compacted on  
> every collection?  I've read that CMS tenured gen can suffer from  
> fragmentation.

ParNew is a copy collector, this is automatically compacting. But the  
promoted objects might of course fragment due to the PLABs in old. Your  
log is from 13h uptime, do you see it before/after as well?

Because there was no follow up, I will just mention some more candidates  
to look out for, the changes around CMSWaitDuration (RFE 7189971) I think  
they have been merged to 1.7.0.

And maybe enabling more diagnostics can help:

-XX:PrintFLSStatistics=2

Greetings
Bernd