The tools nice, renice and ionice are used to adjust the scheduling of a process giving it a higher or lower priority than normal. Run without arguments, nice just shows the niceness level of the current process which by default should be 0:
The levels of niceness go from -20 (the least nice or nastiest) to +19 (the nicest). The more negative the figure the more resources the process will use and the fastest it will complete. The opposite happens with “positive niceness”: less resources used per time interval and more time required to complete.
Let’s see some examples of use:
root:~> nice batchjob.sh → increases the niceness by +10 (lower priority)
root:~> nice -n 19 batch.sh → increases the niceness to +19 (lower priority)
root:~> nice -n 20 batch.sh → decreases the niceness to 20 (highest priority)
Non-privileged users can only set niceness to positive values (lower priority than normal). Only root can set negative niceness values and thus higher-than-normal priority.
Once a process is running we cannot “nice” it but we can renice it:
root:~> renice -n +1 -p 4564 5566 6576 4535 → renices +1 all 4 processes
root:~> renice +1 -p 4564 5566 6576 4535 → same as above
root:~> renice +1 4564 5566 6576 4535 → same as above
root:~> renice +1 -u marc john matt → renices +1 all processes owned by those 3 users
root:~> renice +1 -g 45 770 → renices +1 both process leaders and all children
The ionice command is used to query and set the I/O class scheduling and priority. There are three I/O scheduling classes:
– best-effort (class 0 or 2) is the class by default. Processes in this class with the same priority get served in a round-robin fashion the same amount of resources. There are 8 priorities within this class: 0 (highest priority) up to 7 (lowest).
– real-time (class 1) processes get first access to disk. Only root can set real-time I/O class to processes and it also has 8 priority levels.
– idle (class 3) processes only get access to disk when no other process in best effort or realtime class needs it. There are no priorities in this class.
Let’s query the class:prio of a PID
root:~> ionice -p 2569
none: prio 0 → it has class none or best-effort and priority 0
We’ll keep the PID above in the same class but will increase priority to 0 or highest:
root:~> ionice -c 2 -n 0 -p 2569
Now we will change to real-time a few processes:
root:~> ionice -c 1 -n 4 -p 453 456 470
We need to run a batch job but time completion is not important so we’ll set it to idle class:
root:~> nohup ionice -c 3 batchjob9.sh &
We now want user daemon processes to be switched to real-time:
root:~> ionice -c 1 -n 4 -u daemon
Finally we need to change the class to real-time for all processes in process group 2565:
root:~> ionice -c 1 -n 4 -g 2565