This post was originally published on go2linux.org. The domain is no longer mine, but I am the original author. I am republishing it here on garron.me with corrections and improvements. It merges several short posts about nice and renice into one.

You are working normally, a cron job or a long compilation starts, and everything else slows down. You can avoid that by telling the kernel which processes matter less. nice starts a command with a different priority, and renice changes the priority of a process that is already running.

Niceness

Every process has a niceness value from -20 to 19:

  • -20 is the highest priority (the least "nice" to other processes).
  • 0 is the default.
  • 19 is the lowest priority.

Niceness only matters when processes compete for the CPU. On an idle machine a process with niceness 19 runs at full speed. When the CPU is busy, a higher niceness means a smaller share of it. It is a weight, not a limit: a niced process is never stopped completely, and it never has a cap on the CPU it may use.

Start a command with nice

Without options nice adds 10 to the niceness:

nice make all

Choose the value with -n:

nice -n 19 tar czf backup.tar.gz /home

The number is added to the niceness of the shell that starts the command, which is normally 0. Run nice with no arguments to print the current value:

nice

0

A negative value raises the priority, and only root may do that:

sudo nice -n -5 ./realtime-encoder

Change a running process with renice

Find the process ID with pgrep, ps or top, then:

renice -n 10 -p 1234

1234 (process ID) old priority 0, new priority 10

Unlike nice, the number given to renice is the new niceness, not an increment.

Several processes at once:

renice -n 10 -p 1234 5678

Every process of a program, using pgrep to get the IDs:

renice -n 15 -p $(pgrep ffmpeg)

Every process owned by a user:

sudo renice -n 5 -u backup

Every process in a process group, with -g:

renice -n 10 -g 4321

Old tutorials, including the original version of this post, use the form renice +1 987. It still works, but -n and -p are clearer.

What a normal user may do

  • Raise the niceness (lower the priority) of their own processes.
  • Nothing else.

A normal user cannot give a negative niceness, cannot touch the processes of other users, and cannot lower the niceness again after raising it, not even back to the original value:

renice -n 10 -p 1234    # works
renice -n 0 -p 1234     # Permission denied

Use sudo for that. An administrator can allow a user or group to go down to a given niceness with the nice item in /etc/security/limits.conf.

See the niceness of a process

With ps:

ps -o pid,ni,comm -p 1234

    PID  NI COMMAND
   1234  10 tar

In top and htop it is the NI column. Both can change it interactively:

  • In top, press r, type the PID and then the new value.
  • In htop, highlight the process with the arrow keys and press F8 to raise the niceness (lower priority) or F7 to reduce it (higher priority, root only).

Disk priority: ionice

nice only affects CPU scheduling. A backup at niceness 19 can still make the machine sluggish by saturating the disk. ionice sets the I/O priority:

ionice -c 3 tar czf backup.tar.gz /home

Class 3 is "idle": the process gets disk time only when nobody else wants it. Combine both for background jobs:

nice -n 19 ionice -c 3 tar czf backup.tar.gz /home

Change a running process with -p:

ionice -c 3 -p 1234

ionice is only honored by I/O schedulers that implement priorities, such as BFQ. With none or mq-deadline, common on NVMe drives, it has little or no effect. Check the scheduler in use with:

cat /sys/block/sda/queue/scheduler

Practical examples

A nightly backup from cron that does not disturb the rest of the system:

0 3 * * * nice -n 19 ionice -c 3 /usr/local/bin/backup.sh

Compile using all cores but stay out of the way:

nice -n 15 make -j"$(nproc)"

Calm down a process that is already hogging the CPU:

renice -n 19 -p $(pgrep -f "convert-videos.sh")

For a systemd service, set it in the unit file instead of wrapping the command:

[Service]
Nice=10
IOSchedulingClass=idle

See also

man nice, man renice and man ionice — full reference.