Using Multiple Consecutive Minor Frames

Using Multiple Consecutive Minor Frames

There are cases when a process sometimes or always requires more than one minor frame to complete its work. Possibly the work is lengthy, or possibly the process could be delayed by a system call or a lock or semaphore wait.

You must decide the absolute maximum time the process could consume between starting up and calling frs_yield(). If this is unpredictable, or if it is predictably longer than the major frame, the process cannot be scheduled by the Frame Scheduler. It should probably run in another CPU under the IRIX scheduler.

However, when the worst case time is bounded and is less than the major frame, you can enqueue the process to enough consecutive minor frames to allow it to finish. A combination of disciplines is used in these frames to ensure that the process starts when it should, finishes when it must, and does not cause an error if it finishes early.

The discipline settings for each frame should be:

First frame: Realtime + Overrunnable + Continuable--the process must start in this frame (not Underrunable) but is not required to yield (Overrunnable). If it yields, it is not restarted in the following minor frame (Continuable).
Intermediate: Realtime+Underrunable+Overrunnable+Continuable--the process need not start (it might already have yielded, or might be blocked) but is not required to yield. If it does yield (or if it had yielded in a preceding minor frame), it is not restarted in the following minor frame (Continuable).
Final frame: Realtime+Underrunable--the process need not start (it might already have yielded) but if it starts, it must yield in this frame (not Overrunnable). The process can start a new run in the next minor frame to which it is queued (not Continuable).

A process can be enqueued for one or more of these multi-frame sequences in one major frame. For example, suppose that the minor frame rate is 60 Hz, and a major frame contains 60 minor frames (1 Hz). You have a process that should run at a rate of 5 Hz and can use up to 3/60 second at each dispatch. You would enqueue the process to 5 sequences of 3 consecutive frames each. It would start in frames 0, 12, 24, 36, and 48. Frames 1, 13, 25, 37 and 49 would be intermediate frames, and 2, 14, 26, 38 and 50 would be final frames.