alt Hacker NewsSet JVM heap to half available memory. Mincraft is very much the kind of appication that benifits from filesystem cache to help with loading chunks.
Also if for some reason you are setting the heap size high avoid going past 32 gb unless you tune object alignment. Allocating between 32 and 47 gb will result in an effectively smaller heap. Allocating more than 32 gb can result in performance penalties.
Wow. When I played Minecraft, 1GB was enough and 4GB was a lot. I know that since 1.8 Microsoft rewrote large parts of the engine to make them less efficient and more ivory-tower, but I didn't know it was that bad.