Kintegrityd and why it has -20 nice value

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.

10 minutes is very much long-term as far as Linux's scheduler is concerned. Time slices are something like 10ms.

When you're looking at CPU usage percentages, keep in mind that top adds up the per-thread usage of multi-threaded processes. So a 10-thread process that has each thread getting 10% active time will show up as using 100% of a CPU.

Linux's scheduler won't starve a nice 19 task (because deadlock bugs are hard to avoid if a process can be descheduled forever), so even nice 19 won't stop a task from getting some CPU time. If it has a lot of threads, it may still use significant CPU resources.

If some of the processes are blocking on I/O, especially virtual memory paging, their CPU usage % will go way down. Run something like dstat to see CPU usage breakdowns, disk, network, paging, and context switches. It's like vmstat but colourized and nicer.

Make sure your processes really are niced the way you think they are, by looking at the NI column in top. (It's unlikely that different threads in the same process will have different nice levels, but I think possible.)

If you've been using renice, remember that it's not recursive. renice-ing a parent process won't affect existing children, only future children.