VTDecoderXPCService using 100% of your CPU? Use cputhrottle

By | December 5, 2016

Since upgrading to macOS Sierra (10.12.1) I’ve been having problems with the VTDecoderXPCService process spinning out of control. Closing the Messages app will fix this, but inevitably starting Messages at a later time will cause the problem to resurface. I didn’t really want to fire up IDA Pro and dig into why VTDecoderXPCService was doing this, so I instead thought I’d use cputhrottle to limit how much CPU it is allowed to consume.

You can download and compile cputhrottle yourself, or install it with brew:

Unfortunately VTDecoderXPCService is owned by launchd, which means cputhrottle isn’t allowed to throttle it. This is due to an error when calling task_for_pid (error 5, KERN_FAILURE). So you’ll have to disable the System Integrity Protection (SIP), sadly.

Simply reboot your machine and hold Command-R to enter recovery mode. Then start terminal and run this command:

Now cputhrottle can limit VTDecoderXPCService!

I created a ruby script called limit_decoder.rb which will throttle the service for you. You could simply run this in Terminal and call it a day, or you could configure it to start with your machine. Steps to achieve this:

  1. Download limit_decoder.rb and save it somewhere.
  2. Make it executable:
  3. Create a plist within /Library/LaunchDaemons/, mine is called com.geesu.limitdecoder.plist
  4. Insert the following into the above plist (change the path!):
  5. Now launch the command with the following:

    and you can unload it with:

    If you run into any problems you can view the /var/log/system.log for any errors.


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