Help against CMS old gen fragmentation (was: G1GC success examples (compared to CMS) ?)

[Andreas Loew]

Hi Andreas,

Am 15.11.2012 10:46, schrieb Andreas Müller:
> 2. CMS
> Cons:
> - I ran into very bad heap fragmentation after 1 day of full load with 
> only 1 GB of heap and untuned gen sizes (promotion rate was too high). 
> I solved that but do not know at what timescale such problems could 
> reappear in production (risk remains!)
as you noticed, there are multiple things that you can do in case you 
run into CMS old gen fragmentation issues:

* firstly, monitor the old gen promotion rate of your application by 
using -XX:PrintFLSStatistics=<n> with n=1 or n=2 to collect some data
* if you notice object allocation requests into old that are 
particularly large in size, try to find out which ones and possibly look 
at whether the application can be redesigned to use smaller objects
* increase both the young gen and the survivor space size in order to be 
able to garbage collect more of these short and medium lived objects 
during the tenuring process while still in the young gen (i.e. reduce 
the over all promotion rate and object size from young into old)
* have the CMS GC kick in earlier (i.e. lower initial occupancy ratio 
and set initial occupancy ratio only)
* or increase the old gen heap - if there is more space available, the 
remaining fragments of free space will automatically become larger.

> - I had to increase heap size from 1GB to 1.5 GB to solve the 
> fragmentation issue
Maybe applying some more of the above options could even allow you to 
get down a less drastic increase in heap size (50% is much)...

Hope this helps & best regards,
Andreas

-- 
Andreas Loew | Senior Java Architect
Oracle Advanced Customer Services
ORACLE Germany