UHC Chunk Preloader — preload chunks and stop UHC lag

UHC Chunk Preloader: Pre-Generate Chunks for Lag-Free UHC

Every Ultra Hardcore server administrator knows the sinking feeling. The match is tense, players are sprinting across the landscape, and suddenly the server stutters. Chunks fail to load, mobs freeze mid-stride, and the TPS plummets. The culprit is almost always the same: the server is trying to generate new terrain on the fly, and the CPU is choking under the load.

This is a classic problem for UHC, where maps are often enormous, with radii reaching thousands of blocks. Generating that much world data in real time is a recipe for disaster. The solution, however, is elegantly simple: generate everything before the game starts. That is exactly what the UHC Chunk Preloader does, and it is a game-changer for anyone serious about hosting smooth, competitive matches.

What Makes This Preloader Different?

Instead of forcing your server to build the world while players are actively exploring it, this tool shifts the heavy lifting to a pre-match phase. You, as the administrator, define the world, the radius, and the central coordinates. The plugin then systematically walks through every chunk in that designated area, loading it into memory and saving it to disk. When the match finally begins, the world is already fully formed. The processor is free to handle the real-time demands of combat, redstone, and player movement, keeping your TPS high and your gameplay fluid.

For UHC, where every second counts and stability is paramount, this pre-generation approach is invaluable. It transforms a potentially laggy, frustrating experience into a seamless one, allowing players to focus entirely on survival and PvP.

Performance Benchmarks: What to Expect

To give you a realistic idea of the time investment, the developer ran a series of tests on a modest setup: a Core 2 Quad Q6600 processor, 8 GB of RAM, and a Velociraptor hard drive. The results clearly show how pre-generation time scales with the radius and the required Java heap size (XMX).

Overworld Generation Times

• Radius 500 blocks (4,096 chunks): 3 minutes with XMX 1024M
• Radius 1,000 blocks (16,129 chunks): 9 minutes with XMX 1024M
• Radius 1,500 blocks (35,721 chunks): 23 minutes with XMX 1536M
• Radius 2,000 blocks (63,504 chunks): 56 minutes with XMX 2560M
• Radius 2,500 blocks (98,596 chunks): 1 hour 24 minutes with XMX 4096M

For the Nether (world_nether), the times are similar but generally a bit shorter due to the unique generation patterns of the fiery dimension. It is important to note that these figures are from an older processor. On modern hardware, the speed increase is dramatic. For example, a Core i7-2600K delivers roughly three times the MIPS of the Q6600, meaning the same pre-generation task would complete in a fraction of the time. In tests with a processor twice as fast as the Q6600, the developer observed a 66-75% reduction in generation time.

How to Install and Configure

Getting the plugin up and running is straightforward, but it requires a specific sequence of steps to avoid memory issues or crashes. Following this guide will ensure a safe and successful setup. If you are looking for a streamlined way to manage your server plugins, you might want to consider a modern launcher that allows you to download UHC Chunk Preloader: Pre-Generate Chunks for Lag-Free UHC for Minecraft directly from a menu, avoiding manual file hunting and config editing.

Step-by-Step Setup Guide

1. Start your CraftBukkit server with a minimum XMX value as indicated in the table above. For a 2,000-block radius, for instance, you would use -Xmx2560M.
2. Ensure no players are online. Even a single connected player can cause a crash during the generation process.
3. In the server console, type the command: /ChunkPreloader world radius 0 0. Replace "world" with your world's name (usually "world" or "world_nether"), "radius" with your desired size, and "0 0" with the center coordinates.
4. Wait for the process to finish. The plugin will announce when it is complete.
5. Stop the server completely. A simple reload will not clear the memory; you need a full restart of the JAR file.
6. Repeat steps 1-4 for the Nether world if you are using it.
7. Start the server for the final time and invite your players.

You can also add the border true/false argument to the command. Setting it to true will automatically build a square bedrock wall at the edge of your specified radius after the chunks are loaded. This is perfect for UHC games with a limited map size. There is an optional FORCE parameter that allows you to run the command from the game itself, but the developer strongly recommends using the console to avoid potential issues.

Known Issues and Caveats

While the plugin is immensely helpful, it does have a few quirks you should be aware of before you start.

Memory Leaks

During chunk generation, three objects are not properly released even after the world is fully unloaded. The Java garbage collector cannot remove them, leading to a gradual filling of the heap. Two of these are tied to the creation of new chunks, while the third is related to the FileIOThread, which holds onto data after it is no longer needed. This is why allocating sufficient XMX and performing a full server restart after pre-generation is critical. Without the restart, the memory remains occupied, and the server could crash with an OutOfMemoryError during gameplay.

FileIOThread Crashes

Occasionally, the input/output thread will terminate abruptly, which can look alarming in the logs. However, in practice, this is not a negative event. In fact, this crash clears one of the memory leaks. Bukkit automatically restarts the thread, and the occupied heap becomes available again. The developer suspects this issue will resolve itself once the underlying memory leak is fixed. If you observe any side effects, it is worth reporting them to the author.

Final Verdict: Is It Worth It?

If you regularly host UHC matches, the answer is a resounding yes. This plugin completely eliminates the lag associated with world generation, offering your players a smooth, stutter-free experience. Yes, you will need to invest time in the initial setup and allocate a generous amount of RAM, but the payoff in gameplay quality is substantial.

The ability to pre-build a bedrock wall and clearly define the arena is a fantastic bonus. And if you have a modern processor, even a 2,500-block radius can be processed in under an hour. The key is to follow the installation instructions carefully, never skip the full server restart, and monitor your allocated heap size. With this approach, the UHC Chunk Preloader: Pre-Generate Chunks for Lag-Free UHC becomes a reliable foundation for dozens of exciting, hardcore matches. It is a must-have tool for any admin looking to provide a professional, lag-free UHC experience.