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Java garbage collector comparison

Context: Check out the README file first. This is an adapted excerpt from our research paper "The Case for Writing Network Drivers in High-Level Languages" [BibTeX].

OpenJDK 12 offers 7 different garbage collectors that all impact performance and latency in our driver in different ways. ixy.java tries to avoid allocations wherever possible, but it's virtually impossible to write allocation-free idiomatic Java code, so we still allocate around 20 bytes on average per forwarded packet.

Performance with different garbage collectors, CPU at 3.3 GHz

Epsilon, a garbage collector that never frees memory, is the fastest but it also crashes after a few minutes as it runs out of memory. The new Shenandoah collector doesn't do too well, unfortunately. Some GCs are as fast as the no-op implementation, this is because never freeing memory isn't ideal either: leaking memory leads to poor data locality as the heap fills up.

But throughput isn't everything, let's look at the latency.

Performance with different garbage collectors, CPU at 3.3 GHz

That's... not great. Go and C# managed to keep tail latency below 100µs, see language comparison. Even Epsilon is quite slow, so there's something else going on, probably the JIT compiler. These tests already show the steady state by excluding the first 5 seconds because Java drops packets and shows excessive latencies during startup.

The best garbage collectors for drivers written in Java seem to be ZGC and Parallel with the best trade-offs between performance and latency.

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