Using rtctl to set scheduler priorities is described at Using rtctl to Set Priorities. In the example given in that chapter, some kernel threads have been given a very high priority. This is to have the default priorities integrate well with the requirements of the Real Time Specification for Java (RTSJ). RTSJ requires a range of priorities from 10-89, so many kernel thread priorities are positioned at 90 and above. This avoids unpredictable behavior if a long-running Java application blocks essential system services from running.

For deployments where RTSJ is not in use, there is a wide range of scheduling priorities below 90 which are at the disposal of applications. It is usually dangerous for user level applications to run at priority 90 and above - despite the fact that the capability exists. Preventing essential system services from running can result in unpredictable behavior, including blocked network traffic, blocked virtual memory paging and data corruption due to blocked filesytem journaling.

Extreme caution should be used if scheduling any application thread above priority 89. If any application threads are scheduled above priority 89 you should ensure that the threads only run a very short code path. Failure to do so would undermine the low latency capabilities of the MRG Realtime kernel.

Setting Real-time Priority for Non-privileged Users

Generally, only root users are able to change priority and scheduling information. If you require non-privileged users to be able to adjust these settings, the best method is to add the user to the Realtime group.

Further Reading

For more information, or for further reading, the following links are related to the information given in this section.