alt.hn

5/22/2025 at 8:24:30 AM

JEP 519: Compact Object Headers

by Skinney

5/22/2025 at 9:08:24 AM

The previous JEP 450 [1] has a lot more implementation details for those who are interested.

> They have been tested at Oracle by running the full JDK test suite. They have also been tested at Amazon by hundreds of services in production, most of them using backports of the feature to JDK 21 and JDK 17.

One of the underappreciated perks of working on platform teams in large (and very large in the case of Amazon) companies is that you've got a playground to see and quantify the impact of your performance work that few others have.

[1]: https://openjdk.org/jeps/450

by SerCe

5/22/2025 at 9:58:48 AM

I find it a bit bizarre that this JEP doesn't enable Compact Object Headers by default. Most users will not know to specifically enable it, so if they're that confident in its stability and performance, why not enable it for everyone?

The JVM used to have a reputation for requiring byzantine flags to properly optimise its performance (mostly GC configuration). We've mostly left that behind these days, but it feels like JEP 519 takes a step backwards here.

by nicktelford

5/22/2025 at 10:11:34 AM

The JVM world is slow and extremely careful. If there's a chance something that once works breaks, it'll take many years before the change is applied by default.

Many JVM users can make huge performance gains by switching the GC to a better once and by toggling all kinds of options.

by jeroenhd

5/22/2025 at 10:35:20 AM

That would be the LTS track. A change has no effect on the LTS version which is supported for a long time

by zaphirplane

5/22/2025 at 10:06:11 AM

These things tend to be done gradually out of an abundance of caution. Assuming the experience remains positive, it will be made the default in some future release.

by pron

5/22/2025 at 8:47:55 AM

This particular JEP is just: "Change compact object headers from an experimental feature to a product feature."

Very good performance results though. Particularly like the json parsing benchmark showing a 10% performance improvement.

by hmottestad

5/22/2025 at 10:04:58 AM

I wonder if this also improves the efficiency of native image graal built executables?

by kosolam

5/22/2025 at 10:45:33 AM

'this', making the future non-experimental won't do anything. It was already available. With this change it still won't be the default setting.

So, you still need to run some perf. testing on your own to decide.

by xxs

5/22/2025 at 12:24:53 PM

This is a change to the OpenJDK JVM (HotSpot) only. I'm not sure, but I think that Graal Native Image (based on the Substrate VM) may already have small headers.

by pron

5/22/2025 at 11:49:43 AM

I’m asking if this JEP will also improve the performance of native image graal built executables, or only when an app is ran using the jdk?

by kosolam