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Linux Forge 36.0 high sys CPU


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On my up-to-date debian buster server installation:

# cat /etc/debian_version
# java --version
openjdk 2020-11-04
OpenJDK Runtime Environment (build
OpenJDK 64-Bit Server VM (build, mixed mode, sharing)
# apt-cache policy openjdk-11-jre-headless
  Tabla de versión:
 *** 500
        500 http://debian.redimadrid.es/debian buster/main amd64 Packages
        100 /var/lib/dpkg/status
     11.0.9+11-1~deb10u1 500
        500 http://security.debian.org/debian-security buster/updates/main amd64 Packages

# uname -a
Linux eneri 4.19.0-14-amd64 #1 SMP Debian 4.19.171-2 (2021-01-30) x86_64 GNU/Linux

model name      : AMD Athlon(tm) 64 X2 Dual Core Processor 4400+
cpu MHz         : 2200.000

Minecraft 1.16.5, Forge 36.0.13. I've also tried previous forge versions (all of them 36.0.x) with the same result


When running vanilla server, two java threads are using around 10-20% CPU time when no players are connected. Almost all of this CPU time is user CPU time


Same scenario with forge 36.0, one java thread is using around 10-20% CPU, but another one is always using 100% of one complete core with around 75% sys (kern) CPU time


I've straced the problematic java thread, and it's running this syscall

clock_gettime(CLOCK_MONOTONIC, {tv_sec=204463, tv_nsec=427087879}) = 0

around 11900 times a second.


Is this behavior normal with forge installed or is it a bug ?

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39 minutes ago, diesieben07 said:

Do you see the same behavior with a vanilla Minecraft server?

No, vanilla server uses far less CPU time than forge.


I've just started vanilla server and straced java thread. It seems it's running the same syscall but only around 7500 times per second.

This seems to be the difference that makes forge use much more kern CPU time that vanilla.


Edited by Davide
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Forge uses more threads and uses more of your CPU's power, when running your tests were you viewing CPU usage as a single value or was it broken down by core or thread?

This is my Forum Signature, I am currently attempting to transform it into a small guide for fixing easier issues using spoiler blocks to keep things tidy.


As the most common issue I feel I should put this outside the main bulk:

The only official source for Forge is https://files.minecraftforge.net, and the only site I trust for getting mods is CurseForge.

If you use any site other than these, please take a look at the StopModReposts project and install their browser extension, I would also advise running a virus scan.


For players asking for assistance with Forge please expand the spoiler below and read the appropriate section(s) in its/their entirety.


Logs (Most issues require logs to diagnose):


Please post logs using one of the following sites (Thank you Lumber Wizard for the list):

https://gist.github.com/100MB Requires member (Free)

https://pastebin.com/: 512KB as guest, 10MB as Pro ($$$)

https://hastebin.com/: 400KB

Do NOT use sites like Mediafire, Dropbox, OneDrive, Google Drive, or a site that has a countdown before offering downloads.


What to provide:

...for Crashes and Runtime issues:

Minecraft 1.14.4 and newer:

Post debug.log

Older versions:

Please update...


...for Installer Issues:

Post your installer log, found in the same place you ran the installer

This log will be called either installer.log or named the same as the installer but with .log on the end

Note for Windows users:

Windows hides file extensions by default so the installer may appear without the .jar extension then when the .log is added the log will appear with the .jar extension


Where to get it:

Mojang Launcher: When using the Mojang launcher debug.log is found in .minecraft\logs.


Curse/Overwolf: If you are using the Curse Launcher, their configurations break Forge's log settings, fortunately there is an easier workaround than I originally thought, this works even with Curse's installation of the Minecraft launcher as long as it is not launched THROUGH Twitch:

  1. Make sure you have the correct version of Forge installed (some packs are heavily dependent on one specific build of Forge)
  2. Make a launcher profile targeting this version of Forge.
  3. Set the launcher profile's GameDir property to the pack's instance folder (not the instances folder, the folder that has the pack's name on it).
  4. Now launch the pack through that profile and follow the "Mojang Launcher" instructions above.






or alternately, 


Fallback ("No logs are generated"):

If you don't see logs generated in the usual place, provide the launcher_log.txt from .minecraft


Server Not Starting:


If your server does not start or a command window appears and immediately goes away, run the jar manually and provide the output.


Reporting Illegal/Inappropriate Adfocus Ads:


Get a screenshot of the URL bar or copy/paste the whole URL into a thread on the General Discussion board with a description of the Ad.

Lex will need the Ad ID contained in that URL to report it to Adfocus' support team.


Posting your mod as a GitHub Repo:


When you have an issue with your mod the most helpful thing you can do when asking for help is to provide your code to those helping you. The most convenient way to do this is via GitHub or another source control hub.

When setting up a GitHub Repo it might seem easy to just upload everything, however this method has the potential for mistakes that could lead to trouble later on, it is recommended to use a Git client or to get comfortable with the Git command line. The following instructions will use the Git Command Line and as such they assume you already have it installed and that you have created a repository.


  1. Open a command prompt (CMD, Powershell, Terminal, etc).
  2. Navigate to the folder you extracted Forge’s MDK to (the one that had all the licenses in).
  3. Run the following commands:
    1. git init
    2. git remote add origin [Your Repository's URL]
      • In the case of GitHub it should look like: https://GitHub.com/[Your Username]/[Repo Name].git
    3. git fetch
    4. git checkout --track origin/master
    5. git stage *
    6. git commit -m "[Your commit message]"
    7. git push
  4. Navigate to GitHub and you should now see most of the files.
    • note that it is intentional that some are not synced with GitHub and this is done with the (hidden) .gitignore file that Forge’s MDK has provided (hence the strictness on which folder git init is run from)
  5. Now you can share your GitHub link with those who you are asking for help.

[Workaround line, please ignore]


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  • 1 month later...
On 2/12/2021 at 4:57 PM, DaemonUmbra said:

Forge uses more threads and uses more of your CPU's power, when running your tests were you viewing CPU usage as a single value or was it broken down by core or thread?

Sorry for my late answer. Still happening in latest forge.


What I'm seeing is a single thread always using a full core, regardless of whether the server is loaded or not.


The screeshot below was taken with the server idle, no users connected. Thread 30991 is using a complete core by itself. 75% of the CPU usage of this thread is system CPU.




When there are users connected and the server is loaded, the thread is still consuming a full core by itself. The rest of the load is spread over several threads, but all tied to the idle core.


What seems strange is that with the server idle, with no users connected, a full core is being used. This behavior leaves only one free core to serve users when they connect.


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