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diff --git a/Documentation/scheduler/sched-eevdf.rst b/Documentation/scheduler/sched-eevdf.rst new file mode 100644 index 000000000000..83efe7c0a30d --- /dev/null +++ b/Documentation/scheduler/sched-eevdf.rst @@ -0,0 +1,43 @@ +=============== +EEVDF Scheduler +=============== + +The "Earliest Eligible Virtual Deadline First" (EEVDF) was first introduced +in a scientific publication in 1995 [1]. The Linux kernel began +transitioning to EEVDF in version 6.6 (as a new option in 2024), moving +away from the earlier Completely Fair Scheduler (CFS) in favor of a version +of EEVDF proposed by Peter Zijlstra in 2023 [2-4]. More information +regarding CFS can be found in +Documentation/scheduler/sched-design-CFS.rst. + +Similarly to CFS, EEVDF aims to distribute CPU time equally among all +runnable tasks with the same priority. To do so, it assigns a virtual run +time to each task, creating a "lag" value that can be used to determine +whether a task has received its fair share of CPU time. In this way, a task +with a positive lag is owed CPU time, while a negative lag means the task +has exceeded its portion. EEVDF picks tasks with lag greater or equal to +zero and calculates a virtual deadline (VD) for each, selecting the task +with the earliest VD to execute next. It's important to note that this +allows latency-sensitive tasks with shorter time slices to be prioritized, +which helps with their responsiveness. + +There are ongoing discussions on how to manage lag, especially for sleeping +tasks; but at the time of writing EEVDF uses a "decaying" mechanism based +on virtual run time (VRT). This prevents tasks from exploiting the system +by sleeping briefly to reset their negative lag: when a task sleeps, it +remains on the run queue but marked for "deferred dequeue," allowing its +lag to decay over VRT. Hence, long-sleeping tasks eventually have their lag +reset. Finally, tasks can preempt others if their VD is earlier, and tasks +can request specific time slices using the new sched_setattr() system call, +which further facilitates the job of latency-sensitive applications. + +REFERENCES +========== + +[1] https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=805acf7726282721504c8f00575d91ebfd750564 + +[2] https://lore.kernel.org/lkml/a79014e6-ea83-b316-1e12-2ae056bda6fa@linux.vnet.ibm.com/ + +[3] https://lwn.net/Articles/969062/ + +[4] https://lwn.net/Articles/925371/ |