Linux: CFS and Nice

“The recently merged Completely Fair Scheduler changes how the
Linux kernel handles scheduling priorities set with the nice
command. Ingo Molnar explained that each level of nice adds or
substracts 10% of CPU utilization, ‘the ‘10% effect’ is relative
and cumulative: from _any_ nice level, if you go up 1 level, it’s
-10% CPU usage, if you go down 1 level it’s +10% CPU usage.’ Ingo
noted that with the earlier scheduler the nice level was tied to
the HZ, offering three examples in which HZ is set to 100, 250, and
300, ‘a nice +19 task (the most commonly used nice level in
practice) gets 9.1%, 3.9%, 3.1% of CPU time on the old scheduler,
depending on the value of HZ. This is quite inconsistent and
illogical. This HZ dependency of nice levels existed for many
years, and the new scheduler solves that inconsistency–every nice
level will get the same amount of time, regardless of HZ…'”

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