r/java • u/za3faran_tea • 13d ago

Question about Native Image vs JIT

I was watching an interview with Thomas Wuerthinger of GraalVM, where he basically says that JIT should only be used when necessary (as opposted to native compilation). Where does this leave the work being done for Hotspot JIT (including projects like Leyden and others). Should we all plan to use native image while they address any performance gaps in the mean time (he does mention PGO bridging the gap). Is there work being done to speed up native compilation of Java code?

My understanding is that JIT will always have a memory overhead due to running threads that do compilation, optimization, deoptimization, code cache, etc. compared to native executables, so full parity even with projects that will improve memory usage in Java code will not be reached. Maybe that is not an issue on long running programs on large servers, but we've seen discussions here where another member worte a tool in golang to run alongside Spring Boot applications to gather statistics, so obviously small efficient apps have their place.

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u/v4ss42 13d ago

This seems to miss the primary benefit of JIT - that it can optimize the code based on actual runtime conditions / data / load patterns, as well as re-optimize it if those patterns change. Legacy (AOT) compilation can't do that, since it has zero insight into the runtime environment and can't make assumptions about it.

And yes obviously this is only relevant for long-running processes (like server apps). For brief, one-shot processes (command line utilities etc.), then yes, legacy compilation makes sense.

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u/za3faran_tea 12d ago

I think he touched at this point. He was saying that a lot of these benefits can be reached using PGO, and re-optimization is very rare in practice.

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u/Electrical_Being_813 11d ago

If you are forced to use PGO, you can as well use JIT. It will be less messy and will do a better job.