FIFO is a scheduling mechanism applied to processes with priorities higher than 0 (0 is assigned to normal processes). FIFO processes run sans any timeslice allocation; in other words, they invariably run until they block for some event or explicitly yield to another process. A FIFO process also gets preempted when the scheduler encounters a higher-priority runnable FIFO, RR, or deadline task. When scheduler encounters more than one fifo task with the same priority, it runs the processes in round robin, starting with the first process at the head of the list. On preemption, the process is added back to the tail of the list. If a higher-priority process preempts the FIFO process, it waits at the head of the list, and when all other high-priority tasks are preempted, it is again picked up to run. When a new fifo process becomes runnable, it is added to the tail of the list.