Concurrent Chunk Management Engine (Fabric)

A Fabric mod designed to improve the chunk performance of Minecraft.

Looking for NeoForge versions?

Benchmarks for 1.21.10 w/ Terralith (Nov 21, 2025)

2t-80t	1t-16t

So what is C2ME?

C^2M-Engine, or C2ME for short, is a Fabric mod designed to improve the performance of chunk generation, I/O, and loading. This is done by taking advantage of multiple CPU cores in parallel. For the best performance it is recommended to use C2ME with Lithium and ScalableLux.

What does C2ME stand for?

Concurrent chunk management engine, it's about making the game better threaded and more scalable in regard to world gen and chunk io performance.

Vanilla parity

C2ME does not sacrifice vanilla functionality or behavior, or alter the vanilla world generation in the name of raw speed by default.
However, due to the non-determinism of vanilla world generation, worlds will vary
significantly run-to-run even with the same seed. This is not a bug on our side.

While we carefully check that we do not modify any vanilla behavior, bugs are unavoidable after all.
So, if you do encounter an issue where C2ME deviates from the intended vanilla behavior, don't hesitate to open an issue.

Mod and Datapack compatibility

World generation datapacks that can run on vanilla Minecraft are fully supported.
Custom world generators implemented in mods usually runs well, but may cause compatibility issues due to certain
design assumption used by mod authors being broken for further speedups of world generation.
As a world generation mod author, if you find your mod broken, don't hesitate to look for help in our discord server (linked below).
We are willing to help mod authors to embrace scalable world generation.

Undefined behavior sanitization

C2ME includes CheckedThreadLocalRandom for world random (included in UWRAD) plus a few others.
These detections exist to prevent mods from screwing up Minecraft internals and causing undebuggable problems.
The detection should almost never produce false positives, and should be taken seriously and reported
to corresponding mod authors instead.

Usage notice

Backup your worlds and practice good game modding skills.

Support status for Minecraft versions

Only the latest Minecraft release and the latest Minecraft snapshot are fully supported.
Older Minecraft releases are in long-term support and will receive critical bug fixes.
Older Minecraft snapshots are not supported.

Support

Our issue tracker: link
Our discord server: link

More information

Check out the README.md here: https://github.com/RelativityMC/C2ME-fabric

Previous benchmarks

Benchmarks for 1.21.3 (Nov 24, 2024)

2t-80t	1t-10t

Footnotes: https://gist.github.com/ishland/9601a033afad4be5b17ffaed859577fd

Footnotes

Footnotes for benchmarks

Benchmark setup

All benchmark done with dual-socket Intel(R) Xeon(R) Gold 5218R limited to 2GHz max, on Debian 13.

openjdk version "25.0.1" 2025-10-21 LTS
OpenJDK Runtime Environment Zulu25.30+17-CA (build 25.0.1+8-LTS)
OpenJDK 64-Bit Server VM Zulu25.30+17-CA (build 25.0.1+8-LTS, mixed mode)

Datapack used: Terralith_1.21.5_v2.5.13.zip
https://modrinth.com/datapack/terralith/version/RFNry3l0

C2ME benchmark

Fabric mods used for C2ME benchmark:

c2me-fabric-mc1.21.10-0.3.5.1.0.jar
Chunky-Fabric-1.4.51.jar
fabric-api-0.138.3+1.21.10.jar
lithium-fabric-0.20.1+mc1.21.10.jar
ScalableLux-0.1.6+fabric.c25518a-all.jar
spark-1.10.152-fabric.jar
vmp-fabric-mc1.21.10-0.2.0+beta.7.217-all.jar

Changed settings (c2me.toml):

• globalExecutorParallelism set to thread count

Paper benchmark

Paper used: paper-1.21.10-113.jar
Paper plugin used: Chunky-Bukkit-1.4.40.jar
Changed settings (paper-global.yml):

• worker-pool.io-threads = 4
• worker-pool.worker-threads set to thread count

Vanilla benchmark

Fabric mods used for vanilla benchmark:

Chunky-Fabric-1.4.51.jar
fabric-api-0.138.3+1.21.10.jar
spark-1.10.152-fabric.jar

Extra JVM flags: -Dmax.bg.threads= set to thread count

All benchmarks are run on tmpfs, on overworld, with world folder emptied before each JVM execution.
Seed: 114862964781274189
force-load-existing-chunks is turned on in Chunky config.

2-80 threads benchmark

JVM flags: -Xmx32G -Xms32G -Xss4m -Xshare:off -XX:+DisableExplicitGC -XX:+UseNUMA -XX:+UseZGC -XX:-ZUncommit -XX:+UseLargePages -XX:+UseTransparentHugePages -XX:+UnlockExperimentalVMOptions -XX:-OmitStackTraceInFastThrow -XX:+AlwaysPreTouch -XX:+UnlockDiagnosticVMOptions -XX:+DebugNonSafepoints -Dchunky.maxWorkingCount=256
For vanilla tests only: -Xmx64G -Xms64G is used to avoid crashes during benchmark
Warmup phase: 2048 block radius with 0 0 as center (data discarded)
Measurement phase: 4096 block radius with 1000000 1000000 as center (263169 chunks in total)

Thread count	C2ME cps	Paper cps	Vanilla cps
2	22.62	17.37	10.63
4	45.02	34.17	12.67
6	69.42	50.87	13.64
8	89.94	63.14	13.26
10	115.53	72.66	14.25
12	135.93	78.63	14.58
14	159.01	82.09	14.57
16	182.38	84.76	14.22
18	208.45	88.94
20	230.04	89.91
22	252.08	87.66
24	274.85	88.82
26	297.70	88.43
28	317.26	86.31
30	334.40	87.69
32	358.87	85.06
34	377.21	87.52
36	392.01	87.40
38	407.59	85.14
40	415.31	84.62
42	423.33	86.06
44	425.38	85.56
46	424.47	86.28
48	438.13	85.03
50	440.57	85.31
52	441.81	86.43
54	444.29	83.81
56	446.81	84.13
58	444.29	85.03
60	449.86	83.81
62	453.48	85.50
64	451.40	84.89
66	454.26	83.60
68	454.26	84.21
70	457.16	83.89
72	456.36	83.47
74	460.09	84.70
76	455.05	83.31
78	459.82	83.39
80	460.35	81.43	14.58

1-16 threads benchmark

JVM flags: -Xmx16G -Xms16G -Xss4m -Xshare:off -XX:+DisableExplicitGC -XX:+UseNUMA -XX:+UseZGC -XX:-ZUncommit -XX:+UseLargePages -XX:+UseTransparentHugePages -XX:+UnlockExperimentalVMOptions -XX:-OmitStackTraceInFastThrow -XX:+AlwaysPreTouch -XX:+UnlockDiagnosticVMOptions -XX:+DebugNonSafepoints -Dchunky.maxWorkingCount=256
Warmup phase: 1024 block radius with 0 0 as center (data discarded)
Measurement phase: 2048 block radius with 1000000 1000000 as center (66049 chunks in total)

Thread count	C2ME cps	Paper cps	Vanilla cps
1	11.62	8.05	7.37
2	22.65	16.19	10.01
3	33.46	24.45	11.46
4	44.95	32.52	12.05
5	56.94	40.65	12.41
6	68.55	48.32	12.61
7	79.82	54.95	12.83
8	91.57	60.15	12.83
9	104.76	63.88	13.08
10	114.42	68.66	13.53
11	126.83	70.72	13.01
12	138.73	72.34	13.39
13	148.35	75.57	13.58
14	162.25	76.01	13.34
15	172.34	76.27	13.56
16	183.78	77.43	13.72