"The Linux scheduler, in both the mainline and realtime versions, provides a couple of scheduling classes for realtime tasks. These classes implement the classic POSIX priority-based semantics, wherein the highest-priority runnable task is guaranteed to have access to the CPU. While this scheduler works as advertised, priority-based scheduling has a number of problems and has not been the focus of realtime research for some time. Cool schedulers in this century are based on deadlines instead. Linux does not yet have a deadline scheduler, though there is one in the works. A recent discussion on implementing the full deadline model has shown, once again, just how complex it can be to get deadline scheduling right in the real world.
"Deadline scheduling does away with priorities, replacing them with a three-parameter tuple: a worst-case execution time (or budget), a deadline, and a period. In essence, a process tells the scheduler that it will require up to a certain amount of CPU time (the budget) by the given deadline, and that the deadline optionally repeats with the given period. So, for example, a video-processing application might request 1ms of CPU time to process the next incoming frame, expected in 10ms, with a 33ms period thereafter for subsequent frames. Deadline scheduling is appealing because it allows the specification of a process's requirements in a natural way which is not affected by any other processes running in the system. There is also great value, though, in using the deadline parameters to guarantee that a process will be able to meet its deadline, and to reject any process which might cause a failure to keep those guarantees."
