Linux wont release memory at the end of a render or for anything else. It's not a bug it's the way memory management works under Linux.

Windows releases memory as soon as it's finished with it (in theory) Linux will hold everything in memory in case it's needed again it will release it as and when it's needed by another application. If you want to flush your memory try this

sync; echo 3 > /proc/sys/vm/drop_caches

Really though your memory under Linux should be full. Unlike under windows.

Regards
Dave