horizon-airdef/docs/optimization-plan.md

Optimization research plan

Starting point

The first playable Java foundation is described in Java performance packs. Client optimization has priority. Vanilla gameplay, world formats and multiplayer behavior must be preserved; existing modpack compatibility is not required. Any language, renderer or architecture is in scope when measured gains justify it. Windows/Linux and Apple Silicon/Intel macOS are targets, with the M2 Pro/16 GB Mac prioritized.

Inspect both pinned game JARs locally, retain bytecode as the reference, and measure actual Minecraft workloads before selecting a subsystem to replace. The Rust face-count experiment establishes a small correctness/benchmark workflow; it does not identify a Minecraft bottleneck.

Whole-game translation is a long-term research objective. Decompiled Java is an inspection artifact, not the original source tree: build configuration, comments, tooling history, and some source-level constructs cannot be recovered faithfully. A mechanically translated Rust game would also have to preserve behavior, data formats, networking and rendering. An isolated hot-path replacement is a useful first experiment.

The Rust rewrite now has an original horizon-core foundation and a bytecode differential runner for selected position, random-stream and packed-storage operations. Rust parity status distinguishes that tested slice from the unimplemented gameplay and renderer. This establishes compatibility testing infrastructure; it does not select an FPS bottleneck or supply a playable Rust client.

Local code map

These entry points were verified present in the decompiled 26.3 client. Paths below are relative to .local/minecraft/26.3/sources/client/net/minecraft/; shared/server logic can also be inspected in the dedicated-server tree.

Area	Local entry points	Questions to measure
Client loop	client/Minecraft.java	CPU frame-time distribution, allocation/GC, render-thread stalls
Server loop	server/MinecraftServer.java, server/level/ServerLevel.java	Tick time, entity/block tick work, waiting on chunk jobs
Section compilation	client/renderer/chunk/SectionCompiler.java, SectionRenderDispatcher.java in that directory	Work per rebuild, queue latency, allocations, upload overhead
World generation	world/level/levelgen/NoiseChunk.java, NoiseBasedChunkGenerator.java in that directory	Time per newly generated chunk, cache reuse, generation scheduling
Lighting	world/level/lighting/LevelLightEngine.java	Update propagation work and queue contention
Block storage	world/level/chunk/PalettedContainer.java	Palette access costs, locality, mutation behavior
Region I/O	world/level/chunk/storage/RegionFile.java	Serialization/compression cost versus disk waits

The local inventory contains every class, bytecode size, method count, and source-file relationship. Large classes are navigation hints; bytecode size does not establish execution cost. Generated registries can be large while irrelevant to steady-state frame time.

Capture a real baseline

Use a licensed launcher installation and a copy of a test world. Keep raw captures and world data in .local/. Record game version/hash, mod versions, Java version/flags, CPU/GPU, OS, resolution, render/simulation distance, world seed, route, and warm-up duration.

Use at least these separate scenarios:

1. A fixed stationary view after chunks settle.
2. A reproducible movement route through already generated chunks.
3. The same route through newly generated terrain.
4. A controlled entity/redstone workload on the dedicated server.
5. World loading/saving as a separate latency test.

For a running local Java process, JDK tooling can capture a bounded recording:

jcmd -l
# Replace 12345 with the Minecraft JVM PID, and use an absolute capture path.
jcmd 12345 JFR.start name=mc-baseline settings=profile duration=120s filename=/absolute/path/to/repo/.local/baseline.jfr
jfr summary .local/baseline.jfr
jfr print --events jdk.ExecutionSample,jdk.GarbageCollection .local/baseline.jfr > .local/baseline-events.txt

Collect CPU profiles, allocation/GC information, frame-time percentiles, server tick-time percentiles, and relevant GPU timings. JFR samples alone do not measure FPS or GPU cost. JVM compilation/warm-up, profiling overhead, world generation, and background activity must be controlled when comparing runs. The core comparison now contains stationary Linux software-rendering observations; Mac GPU, movement/generation and populated dedicated-server measurements remain open.

First native integration experiment

Choose a kernel only after a profile shows enough inclusive cost to justify it. A candidate should have a clear input/output boundary and deterministic reference tests. Candidate areas include occupancy preprocessing, palette scans, or batched generation calculations; these are hypotheses to investigate.

Measure an improved Java implementation first. Then compare an original Rust implementation with equivalent work and account for Java/native transition cost, data marshaling, allocation, and scheduling. Use coarse batches rather than a call per block. Preserve Minecraft behavior for boundaries, partial/transparent blocks, lighting, saved data, and deterministic world generation wherever the selected kernel touches them.

Ship any integration as an original mod/native library loaded into the user's local game. Do not bundle a modified Minecraft client/server. Add a fallback to the reference implementation and differential tests using locally generated fixtures.

Gates before expanding scope

Gate	Evidence required
Bottleneck selected	Reproducible real-game profile and workload definition
Kernel equivalent	Boundary cases and randomized differential tests, including real input shapes
Native call worthwhile	End-to-end measurements including copying and transitions
Game improves	Repeated real-game frame/tick distributions, allocations, memory, and CPU/GPU cost
More Rust justified	Improvement persists across workloads without changing gameplay

If a subsystem occupies fraction p of total measured time, even eliminating its cost gives a theoretical maximum overall speedup of 1 / (1 - p). For example, a subsystem consuming 10% of frame time cannot alone yield a 2× whole-game speedup. Apply this before undertaking a large rewrite.