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How do I configure my linux system to allow JACK to use realtime scheduling?

Realtime (RT) scheduling is a feature of all Linux operating systems that enables an application to meet timing deadlines more reliably. Almost all Linux distributions consider RT scheduling to be a potential source of system abuse, and so access to it is limited to varying degrees. As mentioned in the FAQ, it is very important to understand that you do NOT need an "RT kernel" to use RT scheduling (this is a very common misconception.)

JACK requires real time (RT) scheduling privileges for reliable, dropout-free operation. Modern versions of JACK request RT scheduling by default, though it can be disabled with the -r option. Older versions require the -R option to request RT scheduling. If your system is not setup correctly, then JACK will print a message like when it tries to run with RT scheduling (it will appear in the messages window of QJackctl if you use that to start JACK):

cannot use real-time scheduling (FIFO at priority 10) ... (1: Operation not permitted)

Some distributions configure everything so that any user can use RT scheduling with absolutely no work at all. This is great, when it happens. However, many if not most Linux distributions do not allow applications run by ordinary users to request RT scheduling by default. Some distributions' JACK packages will configure things more or less correctly for you, but in almost every case, some work is required.

Distributions that are known to get this completely right include:

  • AVLinux
  • 64Studio
These distributions have JACK packages that will create and configure a group with the required priviledges for RT scheduling, but will not add you to that group.
  • Debian Squeeze
  • Fedora 13 and later
  • Ubuntu 10.4 and later
Unfortunately, on these systems you will need to determine which group name is used, and then skip to step 2 (reading carefully).

Ultimately, the only way to find out if your system is configured properly is to run JACK with RT scheduling and see if it works.

This may not work!

If you follow all these steps carefully and they do not work, you should review the information over here and consider whether this may apply to your system.

Systems using PAM

PAM (Pluggable Authentication Modules) are used by almost all modern Linux distributions nowadays to handle various tasks related to granting a user permission to do something (e.g. to login). If your system has PAM, it will also have a file called /etc/security/limits.conf and/or a directory called /etc/security/limits.d.

You need to carry out 3 steps to be able to run JACK with RT scheduling. In what follows, several references are made to the "realtime" group. If this group name already exists, pick a different name and use it instead of "realtime" when following the rest of these instructions. The actual name of the group is completely irrelevant.

1. Editing the configuration file

If your system has no directory called /etc/security/limits.d then you will need to edit /etc/security/limits.conf. If /etc/security/limits.d does exist on your machine, then you will need to create and edit a file called /etc/security/limits.d/99-realtime.conf. The file must contain (at least) the following two lines:

@realtime   -  rtprio     99
@realtime   -  memlock    unlimited
Contrary to a lot of misinformation on the web, there is no reason to include a line here that provides enhanced "niceness" control, which is completely irrelevant for realtime scheduling and low latency audio applications.

2. Creating a "realtime" group

As the super-user ("root") run the following commands from a terminal window:

groupadd realtime
usermod -a -G realtime yourUserID

You should substitute your actual user id or "login" for "yourUserID".

If you prefer, you can carry out these two steps using the graphical tools that are available under the "System Adminstration" section of your desktop's main menu, but this text-based method is faster and much easier to explain.

If you are using a distribution that has already created the group and configured the "limits" file, you will need to determine the name of the group (it is likely called "audio" or "jackuser") and then you can just add yourself to the group with this command (run as the superuser inside a terminal window):

usermod -a G theGroupName yourUserId
substituting the real names for theGroupName and yourUserId

3. Logout and back in

None of the changes you have made above will have any effect until you logout and back in. You do not need to reboot your system or reinstall any software.