Chapter 20. Setting scheduler priorities

Red Hat Enterprise Linux for Real Time kernel allows fine-grained control of scheduler priorities. It also allows application-level programs to be scheduled at a higher priority than kernel threads.

Warning

Setting scheduler priorities can carry consequences and may cause the system to become unresponsive or behave unpredictably if crucial kernel processes are prevented from running as needed. Ultimately, the correct settings are workload-dependent.

20.1. Viewing thread scheduling priorities

Thread priorities are set using a series of levels, ranging from 0 (lowest priority) to 99 (highest priority). The systemd service manager can be used to change the default priorities of threads after the kernel boots.

Procedure

  • To view scheduling priorities of running threads, use the tuna utility: 

    # tuna --show_threads
                      thread       ctxt_switches
    pid SCHED_ rtpri affinity voluntary nonvoluntary             cmd
  2      OTHER     0    0xfff       451            3        kthreadd
  3       FIFO     1        0     46395            2     ksoftirqd/0
  5      OTHER     0        0        11            1    kworker/0:0H
  7       FIFO    99        0         9            1   posixcputmr/0
  ...[output truncated]...

20.2. Changing the priority of services during booting

Using systemd, you can set up real-time priority for services launched during the boot process.

Unit configuration directives are used to change the priority of a service during boot process. The boot process priority change is done by using the following directives in the service section of /etc/systemd/system/service.service.d/priority.conf:

CPUSchedulingPolicy=

Sets the CPU scheduling policy for executed processes. Takes one of the scheduling classes available on Linux:

  • other
  • batch
  • idle
  • fifo
  • rr

CPUSchedulingPriority=

Sets the CPU scheduling priority for an executed processes. The available priority range depends on the selected CPU scheduling policy. For real-time scheduling policies, an integer between 1 (lowest priority) and 99 (highest priority) can be used.

Prerequisites

  • You have administrator privileges.
  • A service that runs on boot.

Procedure

For an existing service:

  1. Create a supplementary service configuration directory file for the service. 

    # cat <<-EOF > /etc/systemd/system/mcelog.service.d/priority.conf

  2. Add the scheduling policy and priority to the file in the [Service] section.

    For example: 

    [Service]
CPUSchedulingPolicy=fifo
CPUSchedulingPriority=20
EOF

  3. Reload the systemd scripts configuration. 

    # systemctl daemon-reload

  4. Restart the service. 

    # systemctl restart mcelog

Verification

  • Display the service’s priority. 

    $ tuna -t mcelog -P

    The output shows the configured priority of the service.

    For example: 

                        thread       ctxt_switches
  pid SCHED_ rtpri affinity voluntary nonvoluntary             cmd
826     FIFO    20  0,1,2,3        13            0          mcelog

Additional resources

20.3. Configuring the CPU usage of a service

Using systemd, you can specify the CPUs on which services can run.

Prerequisites

  • You have administrator privileges.

Procedure

  1. Create a supplementary service configuration directory file for the service. 

    # md sscd

  2. Add the CPUs to use for the service to the file using the CPUAffinity attribute in the [Service] section.

    For example: 

    [Service]
CPUAffinity=0,1
EOF

  3. Reload the systemd scripts configuration. 

    # systemctl daemon-reload

  4. Restart the service. 

    # systemctl restart service

Verification

  • Display the CPUs to which the specified service is limited. 

    $ tuna -t mcelog -P

    where service is the specified service.

    The following output shows that the mcelog service is limited to CPUs 0 and 1. 

                        thread       ctxt_switches
  pid SCHED_ rtpri affinity voluntary nonvoluntary             cmd
12954   FIFO    20      0,1         2            1          mcelog

20.4. Priority map

Scheduler priorities are defined in groups, with some groups dedicated to particular kernel functions.

Table 20.1. Thread priority table

PriorityThreadsDescription

1

Low priority kernel threads

This priority is usually reserved for the tasks that need to be just above SCHED_OTHER.

2 - 49

Available for use

The range used for typical application priorities.

50

Default hard-IRQ value

This priority is the default value for hardware-based interrupts.

51 - 98

High priority threads

Use this range for threads that execute periodically and must have quick response times. Do not use this range for CPU-bound threads, because it will prevent responses to lower level interrupts.

99

Watchdogs and migration

System threads that must run at the highest priority.

20.5. Additional resources