CMS GC can not recycle objects

I have a trouble in my cluster that CMS full gc can not reduce the size of old gen. Days ago I post this problem to the maillist, people think it will be solved by tuning the gc setting, however it doesn't work for me.

Then I saw a similar bug in CASSANDRA-8447, but benedict think it is not related. With the jstack on https://gist.github.com/yangzhe1991/755ea2a10520be1fe59a, benedict find a bug and resolved it in CASSANDRA-8459. So I build a latest version on 2.1 branch and run the SNAPSHOT version on the nodes with gc trouble.

However, there is still the gc issue. So I think opening a new tick and post more information is a good idea. Thanks for helping me.