Use Opis to Diagnose Lag

Opis is a Forge mod that can help pinpoint the sources of lag in a modded Minecraft server. You can run Opis from within your game to get details on the slowest chunks, entities, tile entities, and other objects and data on your server and client. Opis is only available for Forge based modpacks 1.6.4 and later. You can check your Minecraft version after you start your game on the lower left corner of the Minecraft screen. Some Feed the Beast mod packs already include Opis.

Remove/Disable Existing MapWriter

Opis includes MapWriter. If your server and client already have a MapWriter mod, remove or disable it. You can disable MapWriter by moving the mapwriter...zip out of the mods folder.

Install Opis and MobiusCore Onto the Server

  1. Download Opis and MobiusCore for your version of Minecraft.
  2. Place the .zip into your server game’s mods folder using FTP.

Install Opis and MobiusCore Onto Your Client

  1. Place Opis and MobiusCore into the mods folder of your client. Instruction may vary for different launchers, for example if you’re using the Technic Launcher, select the modpack you want to add these mods to, click on Modpack Options next to the gear icon on the far right, and click Open.

After Installation, Start the Game

  1. Run /opis_start to collect performance data.
  2. After a few moments, run /opis_stop to stop collection to create a sample.
  3. Type /opis_chunk to view the slowest chunks. Click on them to go to the affected locations.
  4. Opis highlights the blocks that are causing the slowdowns in red.

Useful Commands

  • /opis_nent Will return a list of amount of each entity type
  • /opis_killall <entitytype>

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