Nice Priority – Why Can’t Renice Increase a Process’ Nice Value?

nicepriority

From man renice:

Users other than the super-user may only alter the priority of processes
they own, and can only monotonically increase their “nice value'' (for
security reasons) within the range 0 to PRIO_MAX (20) […]

So, I can renice my own processes upwards (give them lower priority) but never downwards:

$ renice 10 22316
22316 (process ID) old priority 0, new priority 10
$ renice 9 22316
renice: failed to set priority for 22316 (process ID): Permission denied

Why is this? I can understand why normal users cannot set nice values lower than 0, but why since I can decrease the priority to 10 can't I increase it again to 9? What "security reason" is there for this? I have the right to launch a process with a nice value of 9, so why can't I renice it to 9?

EDIT: I should learn to scroll down. Turns out this is listed as a bug in man renice:

BUGS
     Non super-users can not increase scheduling priorities of their own
     processes, even if they were the ones that decreased the priorities 
     in the first place.

That's even more confusing. If they consider this behavior to be a bug, why not change it? The renice command appeared in 4.0BSD which I think is from 1980. This should be very easy to fix so on the one hand they seem to have chosen to leave it and on the other they list it as a bug.

Best Answer

Since linux 2.6.12, that depends on the value of the RLIMIT_NICE limit (ulimit -e). Which can take values from 0 to 40. That limit is more the limit on the priority of the process (the greater that number, the higher the priority a user can set for a process).

You'll notice the default value is 20 on ubuntu 10.04 and 0 in Debian jessie for instance.

A value of n for that limit means that a process without the CAP_NICE capability can only increase a process priority to up to n, which means decrease niceness down to a niceness of 20 - n. So for a value of 0, that means no non-privileged user can lower the niceness below 20, so no non-privileged user can lower the niceness.

With a value of 20, non-privileged users can decrease the niceness back to 0.

It's up to the administrator to choose whether they allow users to lower their process priority, and to what level by setting the hard limit for that.

As to why an administrator may not want users to lower their process priority, see Flup's answer.

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