info > CHRT

📘 CHRT(1) – User Commands CHRT(1)

📖 NAME

chrt – manipulate the real-time attributes of a process

🚀 Quick Reference

Use Case	Command	Description
Run a command with FIFO scheduling	chrt -f 99 command [args]	Run command with SCHED_FIFO at priority 99
Run a command with RR scheduling	chrt -r 50 command [args]	Run command with SCHED_RR at priority 50
Retrieve scheduling attributes of a PID	chrt -p PID	Show current policy and priority of an existing task
Set scheduling attributes of a PID	chrt -r -p 30 PID	Set existing PID to SCHED_RR at priority 30
Display valid priority range	chrt -m	Show minimum and maximum valid priorities for each policy
Set SCHED_DEADLINE with period	chrt -d --sched-period 1000000 command	Run command with deadline scheduling (period 1ms)

📋 SYNOPSIS

chrt [options] priority command argument ...

chrt [options] -p [priority] PID

📄 DESCRIPTION

chrt sets or retrieves the real-time scheduling attributes of an existing PID, or runs command with the given attributes.

⚙️ POLICIES

🔵 -o, --other

Set scheduling policy to SCHED_OTHER (time-sharing scheduling). This is the default Linux scheduling policy.

🔴 -f, --fifo

Set scheduling policy to SCHED_FIFO (first in-first out).

🟠 -r, --rr

Set scheduling policy to SCHED_RR (round-robin scheduling). When no policy is defined, SCHED_RR is used as the default.

🟢 -b, --batch

Set scheduling policy to SCHED_BATCH (scheduling batch processes). Linux-specific, supported since 2.6.16. The priority argument must be set to zero.

🟡 -i, --idle

Set scheduling policy to SCHED_IDLE (scheduling very low priority jobs). Linux-specific, supported since 2.6.23. The priority argument must be set to zero.

🔴 -d, --deadline

Set scheduling policy to SCHED_DEADLINE (sporadic task model deadline scheduling). Linux-specific, supported since 3.14. The priority argument must be set to zero. See also --sched-runtime, --sched-deadline and --sched-period. The relation between the options required by the kernel is runtime ≤ deadline ≤ period. chrt copies period to deadline if --sched-deadline is not specified and deadline to runtime if --sched-runtime is not specified. It means that at least --sched-period has to be specified. See sched(7) for more details.

🔧 SCHEDULING OPTIONS

⏱️ -T, --sched-runtime nanoseconds

Specifies runtime parameter for SCHED_DEADLINE policy (Linux-specific).

⏱️ -P, --sched-period nanoseconds

Specifies period parameter for SCHED_DEADLINE policy (Linux-specific).

⏱️ -D, --sched-deadline nanoseconds

Specifies deadline parameter for SCHED_DEADLINE policy (Linux-specific).

🔄 -R, --reset-on-fork

Use SCHED_RESET_ON_FORK or SCHED_FLAG_RESET_ON_FORK flag. Linux-specific, supported since 2.6.31.

Each thread has a reset-on-fork scheduling flag. When this flag is set, children created by fork(2) do not inherit privileged scheduling policies. After the reset-on-fork flag has been enabled, it can be reset only if the thread has the CAP_SYS_NICE capability. This flag is disabled in child processes created by fork(2).

More precisely, if the reset-on-fork flag is set, the following rules apply for subsequently created children:

• If the calling thread has a scheduling policy of SCHED_FIFO or SCHED_RR, the policy is reset to SCHED_OTHER in child processes.
• If the calling process has a negative nice value, the nice value is reset to zero in child processes.

⚙️ OPTIONS

🔢 -a, --all-tasks

Set or retrieve the scheduling attributes of all the tasks (threads) for a given PID.

📊 -m, --max

Show minimum and maximum valid priorities, then exit.

🔗 -p, --pid

Operate on an existing PID and do not launch a new task.

📝 -v, --verbose

Show status information.

ℹ️ -V, --version

Display version information and exit.

❓ -h, --help

Display help text and exit.

📝 USAGE

The default behavior is to run a new command:

chrt priority command [arguments]

You can also retrieve the real-time attributes of an existing task:

chrt -p PID

Or set them:

chrt -r -p priority PID

🔒 PERMISSIONS

A user must possess CAP_SYS_NICE to change the scheduling attributes of a process. Any user can retrieve the scheduling information.

📝 NOTES

Only SCHED_FIFO, SCHED_OTHER and SCHED_RR are part of POSIX 1003.1b Process Scheduling. The other scheduling attributes may be ignored on some systems.

Linux' default scheduling policy is SCHED_OTHER.

👤 AUTHORS

Robert Love <rml AT tech9.net>, Karel Zak <kzak AT redhat.com>

📚 SEE ALSO

• nice(1)
• renice(1)
• taskset(1)
• sched(7)

See sched_setscheduler(2) for a description of the Linux scheduling scheme.

🐛 REPORTING BUGS

For bug reports, use the issue tracker at https://github.com/karelzak/util-linux/issues.

📦 AVAILABILITY

The chrt command is part of the util-linux package which can be downloaded from Linux Kernel Archive <https://www.kernel.org/pub/linux/utils/util-linux/>.

util-linux 2.37.2   2021-06-02   CHRT(1)