POSIX operating systems have a concept of process priorities, usually
from 0 to 39 (or 40) with 20 being a normal priority and (somewhat
confusingly) larger numeric values denoting lower priority. To add to
the confusion, there is a niceness value, the amount by which
the priority numerically exceeds 20 (which can be negative).
Processes with high niceness will receive less CPU time than those
with normal priority. On some OSes, processes with niceness
+19 are only run when the system would otherwise be idle. On many OSes utilities such as top report the priority and
not the niceness. Niceness is used by the utility
/usr/bin/renice: /usr/bin/nice (and
/usr/bin/renice -n) specifies an increment in niceness.
Only privileged users (usually super-users) can lower the niceness.
Windows has a slightly different concept of priority
classes. We have mapped the idle priority to niceness 19,
below normal to 15, normal to 0, above
normal to -5 and realtime to -10. Unlike
Unix-alikes, a non-privileged user can increase the priority class on
Windows (but using realtime is inadvisable).