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Linux: CFS And Nice

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Linux

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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