30 ms·

An age old question... Is the code wrong for not being optimal, or is the compiler/JIT wrong for not optimizing it? I've seen optimizing compilers like clang, g

An age old question... Is the code wrong for not being optimal, or is the compiler/JIT wrong for not optimizing it? I've seen optimizing compilers like clang, gcc, rust optimize constructs similar to this to the same machine code. Ideally you want your high-level stuff that makes code easier to understand to be completely transparent and not have a performance impact when the code actually runs.

winrid 3y ago
For my simple case, it would. Generally, the code around the allocation is complicated so maybe it's not getting optimized for some reason. Or maybe the object is passed through some complicated path... or maybe it's added to a list and then cleared at the end of the frame...
mike_hearn 3y ago
Neither is wrong, and there's very likely a lot of low hanging JVM performance fruit here still to claim. Right now they're in a bit of a transitional period. Since a few weeks ago the most obvious thing to try is using the latest Oracle GraalVM with ZGC. ZGC is a pauseless GC like Shenandoah and the Graal compiler is a lot better at escape analysis and removing allocations than the stock C2 compiler especially now Oracle made the enterprise compiler free to use. The main problem is going to be that ZGC Generational is not quite launched yet. Without generational GC it may not be able to keep up with those very high allocation rates, even with a better compiler reducing them down again. Generational ZGC should be in Java 21 but then you'd have to wait for GraalVM to catch up. So, probably it's worth trying that combination in the next six months or so. Whilst I don't know about ZGC vs Shenandoah, the performance improvements from using Graal EE over C2 can be large even for Java (the wins are much bigger still for other higher level/more dynamic languages). But the Minecraft community isn't really known for adopting the latest JVM tech. They're pretty conservative. Edit: someone downthread linked to https://github.com/brucethemoose/Minecraft-Performance-Flags-Benchmarks https://github.com/brucethemoose/Minecraft-Performance-Flags... which talks about all of that so I guess they're getting more experimental!

winrid 3y ago

For my simple case, it would. Generally, the code around the allocation is complicated so maybe it's not getting optimized for some reason. Or maybe the object is passed through some complicated path... or maybe it's added to a list and then cleared at the end of the frame...

mike_hearn 3y ago

Neither is wrong, and there's very likely a lot of low hanging JVM performance fruit here still to claim. Right now they're in a bit of a transitional period. Since a few weeks ago the most obvious thing to try is using the latest Oracle GraalVM with ZGC. ZGC is a pauseless GC like Shenandoah and the Graal compiler is a lot better at escape analysis and removing allocations than the stock C2 compiler especially now Oracle made the enterprise compiler free to use. The main problem is going to be that ZGC Generational is not quite launched yet. Without generational GC it may not be able to keep up with those very high allocation rates, even with a better compiler reducing them down again. Generational ZGC should be in Java 21 but then you'd have to wait for GraalVM to catch up. So, probably it's worth trying that combination in the next six months or so. Whilst I don't know about ZGC vs Shenandoah, the performance improvements from using Graal EE over C2 can be large even for Java (the wins are much bigger still for other higher level/more dynamic languages). But the Minecraft community isn't really known for adopting the latest JVM tech. They're pretty conservative. Edit: someone downthread linked to https://github.com/brucethemoose/Minecraft-Performance-Flags-Benchmarks https://github.com/brucethemoose/Minecraft-Performance-Flags... which talks about all of that so I guess they're getting more experimental!