Chapter 12. RHEL for Real Time scheduler

The chrt utility checks and adjusts scheduler policies and priorities. It can start new processes with the desired 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

The real-time preemption is the mechanism to temporarily interrupt an executing task, with the intention of resuming it at a later time. It occurs when a higher priority process interrupts the CPU usage. Preemption can have a particularly negative impact on performance, and constant preemption can lead to a state known as thrashing. This problem occurs when processes are constantly preempted and no process ever gets to run completely. Changing the priority of a task 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.