G1: higher perm gen footprint or a possible perm gen leak?

[Wolfgang Pedot]

Hi,

I am using G1 on 7u45 for an application-server which has a "healthy" 
permGen churn because it generates a lot of short-lived dynamic classes 
(JavaScript). Currently permGen is sized at a little over 1GB and 
depending on usage there can be up to 2 full GCs per day (usually only 
1). I have not noticed an increased permGen usage with G1 (increased 
size just before switching to G1) but I have noticed something odd about 
the permGen-usage after a collect. The class-count will always fall back 
to the same level which is currently 65k but the permGen usage after 
collect can either be ~0.8GB or ~0.55GB. There are always 3 collects 
resulting in 0.8GB followed by one scoring 0.55GB so there seems to be 
some kind of "rythm" going on. The full GCs are always triggered by 
permGen getting full and the loaded class count goes significantly 
higher after a 0.55GB collect (165k vs 125k) so I guess some classes 
just get unloaded later...

I can not tell if this behaviour is due to G1 or some other factor in 
this application but I do know that I have no leak because the 
after-collect values are fairly stable over weeks.

So I have not experienced this but am sharing anyway ;)

happy new year
Wolfgang

Am 03.01.2014 10:12, schrieb Srinivas Ramakrishna:
> I haven't narrowed it down sufficiently yet, but has anyone noticed if
> G1 causes a higher perm gen footprint or, worse, a perm gen leak perhaps?
> I do realize that G1 does not today (as of 7u40 at least) collect the
> perm gen concurrently, rather deferring its collection to a stop-world full
> gc. However, it has just come to my attention that despite full
> stop-world gc's (on account of the perm gen getting full), G1 still uses
> more perm gen
> space (in some instacnes substantially more) than ParallelOldGC even
> after the full stop-world gc's, in some of our experiments. (PS: Also
> noticed
> that the default gc logging for G1 does not print the perm gen usage at
> full gc, unlike other collectors; looks like an oversight in logging
> perhaps one
> that has been fixed recently; i was on 7u40 i think.)
>
> While I need to collect more data using non-ParallelOld, non-G1
> collectors (escpeially CMS) to see how things look and to get closer to
> the root
> cause, I wondered if anyone else had come across a similar issue and to
> check if this is a known issue.
>
> I'll post more details after gathering more data, but in case anyone has
> experienced this, please do share.
>
> thank you in advance, and Happy New Year!
> -- ramki
>
>
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