How to know the root reason to cause RegionServer OOM?

[David chen]

A RegionServer was killed because OutOfMemory(OOM), although  the process 
killed can be seen in the Linux message log, but i still have two following 
problems:
1. How to inspect the root reason to cause OOM?
2  When RegionServer encounters OOM, why can't it free some memories occupied? 
if so, whether or not killer will not need.
Any ideas can be appreciated!