memory not release after close

There are about 70~80 MB of memory haven't released to system, after the launcher closed even I haven't clicked "play" (I know it in Task Manager, also it tells the source is javaw.exe). Number of tasks increased every time after I start the launcher.
I searched for it which told some jvm may happen this problem but I am using official jvm and there are no problem when I using the java application I created, which contain lwjgl with natives, reflection. They have some background memory, but release immediately after closed.