Force who parameters to be interpreted as process group IDs.
Instead of changing the specified processes to the given priority, interpret the following argument as an increment to be applied to the current priority of each process.
Force the who parameters to be interpreted as user names or user IDs.
Reset the who interpretation to be (the default) process IDs.
"renice +1 987 -u daemon root -p 32"
would change the priority of process IDs 987 and 32, and all processes owned by users daemon and root.
Users other than the super-user may only alter the priority of processes they own, and can only monotonically increase their nice value within the range 0 to PRIO_MAX (20). (This prevents overriding administrative fiats.) The super-user may alter the priority of any process and set the priority to any value in the range PRIO_MIN (-20) to PRIO_MAX. Useful priorities are: 20 (the affected processes will run only when nothing else in the system wants to), 0 (the base scheduling priority), anything negative (to make things go very fast).