Chapter 12. RHEL for Real Time scheduler

The chrt utility checks and adjusts scheduler policies and priorities. It can start new processes with the specified properties, or change the current properties of a running process.

The chrt utility takes the either --pid or the -p option to specify the process ID (PID).

The chrt utility takes the following policy options:

The following example shows the attributes for a specified process.

# chrt -p 468
pid 468’s current scheduling policy: SCHED_FIFO
pid 468’s current scheduling priority: 85

Real-time preemption is the mechanism to temporarily interrupt an executing task, with the intention of resuming it later. It occurs when a higher priority process interrupts CPU usage. Preemption can negatively impact performance, and constant preemption can lead to thrashing, where processes are constantly preempted and no process ever runs completely. Changing task priority can help reduce involuntary preemption.

You can check for voluntary and involuntary preemption occurring on a single process by viewing the contents of the /proc/PID/status file, where PID is the process identifier.

The following example shows the preemption status of a process with PID 1000.

# grep voluntary /proc/1000/status
voluntary_ctxt_switches: 194529
nonvoluntary_ctxt_switches: 195338

The real-time processes use a different set of library calls to control policy and priority. The functions require the inclusion of the sched.h header file. The symbols SCHED_OTHER, SCHED_RR and SCHED_FIFO must also be defined in the sched.h header file.

The table lists the functions that set the policy and priority for the real-time scheduler.