An app running with higher priority gets more access to cpu time as compared to an app running with low priority. This is primarily used to keep an app (like the Arista transcoder) from eating all of the cpu time and slowing the system to a crawl, OR, for allowing one app to suck up as much CPU as it can regardless of the consequences.
It is not possible by default to raise the priority of running apps. This includes after you have lowered the priority. However if you wish to do so you simply need to add the following to /etc/security/limits.conf :
USER - nice -20
Replacing USER with your username and then rebooting the machine.
I think you should ask in Unix & Linux but in any case, the ones that say that NICE values for CPU in Linux vary from 0 to 99 are just for Real Time OS. The nice values are between -20 to 20, being -20 the higher priority, in normal (the one that most of users uses) *NIX kernels for CPU. Which is for most cases.
Also, the file copying wouldn't need actually that much CPU (depending the filesystems) but the I/O priority. From kernel 2.6.26 the ionice values are determined by the CPU nice values using the following formulae:
io_priority = (cpu_nice + 20) / 5
Also, all process (unless modified source code) starts with "Best effort" priority class which uses the formulae above. But, in any case, you probably don't need to set this as default.
A way to set the I/O priority to start a process is doing:
ionice -c2 -n6 SOME_COMMAND
Process that would like to set/get their own I/O scheduling class and priority should use the ioprio_set and ioprio_get functions.
Also a nice warning from Linux Poison:
Read the description for realtime carefully. Realtime IO nice processes will starve the system completely until they are done using the disk. You can easily make your system non responsive for long periods of time.
Also, IO niceness only has effect if you are using the CFQ io scheduler . If you have an elevator= (as, deadline,noop) line in your menu.lst, IO niceness will have no effect.
Sources:
Best Answer
Press r. That's it! :-)
Reference: The 9th item in 12 TOP Command Examples in Linux