Development Assistance Unlaod world method is not stable

I setted up a system to continusly loading worlds and unload the world after 10-15 min. later of loaded. But I think, the world is not cleaning from ram after unload. This reason, ram loads a lot after server's start and crashing by "out of memory" error. What you think about this?

Yourkit memory snapshot:
https://yadi.sk/d/hkSemzoDchKXA

Spigot version:


Crash report:

http://pastebin.com/n0QUzhXF