4.5. Setting Real-time Scheduler Priorities

Using systemd to set scheduler priorities is described in Procedure 3.1, “Using systemd to Set Priorities”. In the example given in that procedure, some kernel threads could 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 to 89.

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 50 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 filesystem journaling.

Use extreme caution when scheduling any application thread above priority 49. If any application threads are scheduled above priority 89, ensure that the threads only run a very short code path. Failure to do so would undermine the low latency capabilities of the Red Hat Enterprise Linux for Real Time kernel.