Off-heap Hash Map in Java, part 1

A month ago I had to deal with an interesting case: huge hash maps in Java! When input started to grow we switched to larger AWS instance. But that didn’t help because input kept growing. And I observed huge heap consumption and very long GC pauses. Essentially application was partly doing its job and partly doing GC. But when it started to hit max heap I had to do something. And I started investigating off-heap solutions.

My “googling” quickly led me to two Java solutions: Java Large Off Heap Cache and Binary Off Heap Hash Map. Both solutions are treating keys and values as blob of bytes. I chose BinaryOffheapHashMap because it is a small code which I can understand. Even that code was experimental it solved my task: creating a hash map outside of GC world. You can read more about that project here. OHC looks more “professional” and is something I will try next time.

So my experiments allowed me to look on Java from a different angle: “greedy” memory consumption and really nasty GC cycles. I will publish my test results in my next post.

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