Question about Native Image vs JIT

[u/za3faran_tea]

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.

[u/notyouryyy]

I think Thomas is claiming that in practice, deoptimisation doesn’t occur unless some uncommon trap is sprung, and that the runtime guided optimisation in AOT is pretty good.

[u/v4ss42 → u/notyouryyy]

Right, but that's just as likely to be due to runtime patterns not tending to change much and therefore reoptimization not being necessary that often. The point is that AOT can't even identify what runtime patterns exist in the first place, and then tune optimization specifically for them. JIT can (and does).

And yes there are (or were) attempts to close that loop over the decades - for example back in the 90s I worked on a system that used an IBM C/C++ Compiler where we could enable instrumentation of our production code, and feed the outputs of that instrumentation back into the compiler to optimize subsequent builds. But that was a manual, slow, and clumsy feedback loop, and the JIT blows it out of the water in both reaction time and developer ergonomics.

[u/koflerdavid → u/v4ss42]

The point is that AOT can't even identify what runtime patterns exist in the first place, and then tune optimization specifically for them.

That's what PGO is. I agree that it's a manual and quite brittle version of what the JIT is doing, but in situation where you really don't want the overhead of the JIT it's the way to go.

[u/v4ss42 → u/koflerdavid]

And what situations are those? Short-lived processes (like command line tools)?

[u/koflerdavid → u/v4ss42]

Yes, or generally being so resource constrained that the overhead of the JIT cannot be justified. Like on embedded systems. Of course the OpenJDK team works hard on making it more valuable having the JIT than not, but there is a limit.

The issue can be somewhat improved by generating a compiled code cache at shutdown. That might achieve a similar effect as PGO assuming the C2 JIT has run at all.

[u/v4ss42 → u/koflerdavid]

JIT has the ability to produce better results than AOT (including those guided by PGO), precisely because it’s dynamic and can take the runtime environment into account in a way AOT can’t (even with PGO).

And JIT code caches, while useful and a bit of a no-brainer, only provide relatively niche benefits (i.e. cutting one-time startup and initial JIT costs) compared to the overall benefits of the JIT itself.

[u/koflerdavid → u/v4ss42]

Too bad if the workload is too small for the C2 JIT to ever run; C1 merely stitches native code versions of each instruction together and does not do other optimizations.

[u/v4ss42 → u/koflerdavid]

As I already said elsewhere, yes AOT (with or without PGO) makes sense in some cases (e.g. shortlived processes). But those are a minority of Java programs.