RLIMITRTPRIO

RLIMIT_RTPRIO is a Linux resource limit that governs the maximum real-time scheduling priority a process and its threads may use. It applies to real-time scheduling policies such as SCHED_FIFO and SCHED_RR, where priorities range from 1 to 99. A value of 0 effectively disables real-time priorities for the process.

The limit is enforced per process via the rlimit structure, with rlim_cur representing the soft (current) limit and rlim_max representing the hard (maximum) limit. A process can raise its real-time priority only up to the hard limit, and typically only if it has the necessary privileges (for example, CAP_SYS_NICE or root). Attempts to set a priority beyond the soft limit usually fail with an error such as EPERM or EINVAL, depending on the API used.

Viewing and modifying RLIMIT_RTPRIO can be done in several ways. In code, programs use getrlimit and setrlimit to query and change the limits. User-space tools include the shell command ulimit -r to display the soft limit, and reading /proc/<pid>/limits to view both soft and hard limits. The prlimit utility can be used to adjust limits for a running process or for a new command, including the real-time priority limit.

Security and performance considerations are important because real-time scheduling can monopolize CPU time and cause other processes to be starved of CPU resources. Limiting high real-time priorities to trusted components helps prevent system-wide disruption. Related topics include real-time scheduling (SCHED_FIFO and SCHED_RR), and the system interfaces getrlimit, setrlimit, prlimit, and the CAP_SYS_NICE capability.

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