Ludovic Marie
74 lines
4.0 KiB
Markdown
74 lines
4.0 KiB
Markdown
# Compact object headers experiment
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Java 25 HotSpot offers `-XX:+UseCompactObjectHeaders`. This changes object-header
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representation without replacing game logic. Oracle documents the header format
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and enable flag in its [Java 25 GC guide](https://docs.oracle.com/en/java/javase/25/gctuning/other-considerations.html).
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Object alignment and fields determine whether a particular allocation gets smaller.
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Native renderer buffers and GPU allocations are outside this optimization.
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## Try on your Mac
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Import the balanced pack normally. In Prism, edit the instance's Java settings
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check the Java Arguments override box, and add `-XX:+UseCompactObjectHeaders` to its JVM arguments, retaining any existing
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arguments. Use Java 25 HotSpot matching your CPU architecture. Restart the game.
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Remove the argument to return to the launcher's default, or use
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`-XX:-UseCompactObjectHeaders` to explicitly test the disabled mode. Do not supply
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both flags. Packs do not automatically enable this option.
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For an isolated comparison, duplicate the same balanced instance and use explicit
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off/on flags. Keep collector, heap, world copy, camera route and all video settings
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matched. Alternate at least three warmed 120-second recordings. The 0.8 recorder
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captures the actual `compact_object_headers` setting and `garbage_collectors` names;
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check these in the offline viewer. Older reports show "not captured", rather than
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an assumed default. Launcher and JVM defaults can differ.
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Local demo and server tools accept `--compact-headers`. This enables the option
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only in that process. The server still requires local EULA acceptance before it runs.
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## Isolated allocation measurement
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```sh
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python3 tools/build_mod.py --benchmark-headers
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```
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The original benchmark uses the verified game/Distant Horizons publisher types
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on its classpath, without redistributing them. It forks separate JVMs with explicit
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off/on flags three times. Each type gets three discarded warm-up batches followed
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by three measured batches of 100,000 objects. `ThreadMXBean` counts allocated bytes;
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reference-array storage is allocated before measurement. Objects escape through a
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volatile array, and their values are checked after measurement. The benchmark
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asserts that the running VM reports the requested flag. Output includes Java/OS/CPU
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architecture and goes to gitignored `dist/object-header-benchmark.txt`.
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Local Linux x64 and hosted Linux x64, Windows x64 and Apple Silicon macOS 26
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measurements yielded the same medians:
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| Type | Headers off, bytes/object | Headers on, bytes/object |
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| --- | ---: | ---: |
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| Minecraft `AABB` | 64 | 56 |
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| Minecraft `Vec3` | 40 | 32 |
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| Distant Horizons `DhChunkPos` | 24 | 24 |
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| Boxed `Long`, outside cache | 24 | 16 |
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| Plain `Object` | 16 | 8 |
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[Raw platform results](../benchmarks/object-headers-2026-10-03) come from
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[CI run 37153955462](https://github.com/ludovic111/minecraft-optimization-lab/actions/runs/37153955462).
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Each platform has nine measured batches per type/mode across three JVM forks.
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The hosted ARM Mac has three logical CPUs and is not the user's M2 Pro.
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These are allocations for specific object layouts, not a measured reduction in
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whole-game retained heap, process RSS, FPS or server TPS. Changing headers can
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also change runtime tradeoffs. Real workloads and the user's M2 Pro remain the
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decision point. CI runs the probe on Windows, Linux and Apple Silicon macOS and
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runs the metric tests with compact headers enabled as well as the default.
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## Fresh video settings correction in 0.8
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Minecraft 26.3 applies its selected graphics preset at startup. A fresh minimal
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0.7 options file had the correct data-version marker but omitted an explicit
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custom preset, allowing the default Fancy preset to override full-detail distance
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to 16 chunks. Version 0.8 explicitly selects the custom preset so the pack's
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12-chunk budget (8 in low-memory) is retained. This fixes configuration, without
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reducing simulation distance or changing gameplay. Runtime reports capture the
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effective settings; always compare those, not just pack labels.
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