CFS: >From my reading of the source, the 'nice'/static priority of a SCHED_NORMAL task is used by the CFS scheduler by mapping it to a weight (https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/tree/kernel/sched/sched.h?id=refs/tags/v3.10.94#n918). This weight comes in to play in 2 main areas: 1) When adjusting the currently running task's 'vruntime' : https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/tree/kernel/sched/fair.c?id=84bb5b645ec5a54744180a1edc5dc72adc862457#n680 When the 'vruntime' is being updated for a task I see that if the task's 'nice' value is less than 0 the execution time delta added to its 'vruntime' is less than its actual execution time. Similarly, for task's with 'nice' values greater than 0 the execution time delta added to its 'vruntime' is larger than its actual execution time. Essentially, vruntime = actual_execution_delta * NICE_0_LOAD / task_weight 2) When computing the ideal amount of time a task should execute within a scheduling period: https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/tree/kernel/sched/fair.c?id=84bb5b645ec5a54744180a1edc5dc72adc862457#n650 It seems a task's ideal execution time is determined by its weight relative to the total weight of all tasks on the same runqueue: ideal_execution_time = scheduling_period * task_weight / total_weight_of_all_tasks_on_runqueue With (1) and (2) above I don't exactly see how the comments above the weight array definition are consistent with this: /* * Nice levels are multiplicative, with a gentle 10% change for every * nice level changed. I.e. when a CPU-bound task goes from nice 0 to * nice 1, it will get ~10% less CPU time than another CPU-bound task * that remained on nice 0. * * 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. (to achieve that we use a multiplier of 1.25. * If a task goes up by ~10% and another task goes down by ~10% then * the relative distance between them is ~25%.) */ static const int prio_to_weight[40] = { If I take an example of 2 tasks both with weight=1024 (NICE_0) they should each get 50% of the CPU time. If 1 of the tasks is reniced to NICE_1 then the NICE_0 task will get 1024/(1024+820)=56% of the CPU time and the NICE_1 task will get 44% of the CPU time. I'm obviously not understanding something since I can't come at those 10% figures in the comments. tl;dr: How does going from 1 nice level to the next get you to the 10% figure mentioned in the comments of the 'prio_to_weight[]' above? Thanks. _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
