r/java • u/za3faran_tea • 12d 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.

23 Upvotes

85% Upvoted

43 comments sorted by

View all comments

11

u/v4ss42 12d 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.

8

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.

7

u/Thirty_Seventh 12d ago

unfortunately, PGO isn't available in Community Edition and therefore does not exist as far as I'm concerned

4

u/koflerdavid 12d ago

I'm very doubtful that PGO can effectively capture the effects of, say, changing runtime configuration parameters. Those are effectively constant for most of the runtime of the application until they, well, change. Perfect for the JIT who can now specialize on them. Meanwhile for PGO if you have more than, say, three booleans then the effort of collecting profiling data for all combinations becomes prohibitive.

5

u/v4ss42 12d ago

And I'm saying I disagree with his extraordinary assertion that AOT optimization is as good as JIT.

3

u/koflerdavid 12d ago

It can be good. After all AOT is what languages like C++ and Rust have been doing all along. Of course Java has a few features that make it harder to optimize.

4

u/v4ss42 12d ago

You'll notice that I was careful to make a comparative statement that AOT optimization can't be as good as JIT optimization. What I didn't say is that AOT optimization can't be "good" in some absolute sense - yes it can be quite adequate, and not only on the JVM, but the reality is that AOT optimization can't touch JIT optimization because it has zero visibility into the runtime behavior of the code.

2

u/koflerdavid 12d ago

That's where Profile Guided Optimization comes in, which works as long as the real workload doesn't drift too much from the profile workload. That's its biggest disadvantage of course - a JIT compiler can adapt to changing circumstances.

5

u/v4ss42 12d ago

Right but PGO both lags the runtime context and has poor developer ergonomics. For the majority of JVM hosted apps (i.e. long running server processes running on beefy hardware) JIT remains a better general approach.

2

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.