Made an account just for this... Anyway, in TrainCarts I make heavy use of LongHashSets to store chunk coordinate information of trains/minecarts. A few days ago, dockter messaged me about a memory issue. A stunning 3 gb was 'consumed' by the offline train storage system. I then spent a few days debugging this issue. Now, after profiling the memory usage per code block with Dockter's help, we have identified the issue, and that is the LongHashSet. A single LongHashSet instance, with an initial size of about 30 elements, consumed 4 mb of RAM memory, persistently! On CraftBukkit the ENTIRE storage consumes less than 1 mb. How can a single element (out of 400 of them) consume well over that amount, 8 mb?! (I use two Long Hashsets) Now, I looked at Spigots changelog, and well...here we go. I have no idea why a single LongHashSet instance uses a 2-dimensional Object array to check whether elements are contained. md_5 will have to explain that himself I suppose. And yes, a HashSet<Long> consumed far less memory than this implementation.