GC memory overhead

[Serguei Mourachov]

Hello

We have a system what is essentially big concurrent hashmap containing 
tens of millions of entries for relatively small long-lived objects.
We are running it with following GC related parameters: -Xms7168M 
-Xmx7168M -Xmn256M -XX:+UseParallelOldGC .
The code is optimized for GC, with minor collections happening 
approximately every 5 min and full collections once a week.
Looking at the Linux process RSS values we can see that, apparently, JVM 
is using significant amount of non-heap memory, and major increase in 
this value happening around time of very first full GC.
In our case that "additional" used RAM space is almost 1Gb.
So my questions are: is that expected to have such significant non-heap 
memory consumption? How can we estimate the GC memory overhead  based on 
number of objects and references in the system?



Thank you

Serguei Mourachov