Xms!=Xmx

Setting the initial Java heap minimum equal to the Java heap maximum is not recommended. Doing so can delay the start of garbage collection until the heap is full. Therefore, the first time the Garbage Collector runs is a very expensive operation.
The heap is also likely to be very fragmented by the time heap compaction is required. This is also a very expensive operation.

http://www-01.ibm.com/support/docview.wss?uid=swg21160795&acss=was121610