The scheduler is operating system software that decides which processes run when. A typical scheduler handles process priorities, allocation of resources, load balancing across processors.

At every context switch, all the processes in the queue are scanned and the best one is picked. It was discontinued because of scalability issues.

Constant time required to pick the next process to execute. Hence, it easily scales to a large number of processes. It was discontinued because of complex heuristics used to distinguish between interactive and non-interactive processes.

At every context switch, the process with the least processor runtime is picked in O(log N) time. It uses red-black trees to achieve this.

xv6 is a modern reimplementation of Sixth Edition Unix in ANSI C for multiprocessor x86 and RISC-V systems.

XV6 uses a First In First Out scheduling policy. Hence, there is no support for prioritization and dynamic time slicing.

The O(1) scheduler supports priority-based scheduling and implements dynamic behavior-based bonuses using certain heuristics. It achieves this through a pair of identical multi-level queue data structures, the active queue, and passive queue.

Dynamic priorities range from 0 to 39. All new processes start with a priority of 20. A process stops running when its time slice is over(or through a voluntary system call). It is now placed into a passive queue after recomputing its dynamic priority based on its latest run.

The first process in the first non-empty queue within the active multi-queue is picked to run. Its time slice is calculated based on its dynamic priority and several heuristics.

The active and passive multi queues are swapped when the active multi-queue becomes empty.