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Posted Jul 23, 2010 22:33 UTC (Fri) by giraffedata (subscriber, #1954)
In reply to: Example by blitzkrieg3
Parent article: Adding periods to SCHED_DEADLINE

I think where people really get lost in these explanations is that they appear to describe a system where the scheduler gets to to decide exactly who to give CPU time and when, or where a process requests a precise CPU schedule for itself for the rest of time.

But in reality, the scheduler must wait for a process to request each chunk of time, and even the process can't know exactly when it will do that.

I presume the parameters actually say, "I am going to be requesting CPU time in the future. There will be at least 10 ms between requests. Each one will be for less than 4 ms, but whatever it is, I'll need it all within 5 ms of when I ask for it.

Based on that, the scheduler is supposed to say, "As long as you stick to those constraints, I guarantee I will fill all your requests" or "I can't guarantee that, so don't bother trying to run."

I really don't get the one statement about it being OK to miss the deadline by a bounded amount. If the process is OK with getting its 4ms within 10 ms, why did it state a deadline of 5 ms?

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