Explain the Completely Fair Scheduler (CFS) in Linux.

CFS aims for ideal multitasking where each process gets equal CPU time. Uses red-black tree keyed by virtual runtime (vruntime). Process with smallest vruntime runs next. When running, vruntime increases; lower priority means faster increase. Sleeping processes don't accumulate vruntime, so they get priority when waking. No fixed timeslices - scheduling granularity based on number of runnable tasks. Achieves O(log n) scheduling decisions. Replaced O(1) scheduler in Linux 2.6.23.