Messages in this thread | | | Date | Thu, 16 Jul 2009 16:47:48 -0400 | From | Raj Rajkumar <> | Subject | Re: [Fwd: Re: RFC for a new Scheduling policy/class in the Linux-kernel] |
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Jim:
Good discussion. THanks for taking the time to educate me on past exchanges.
> We didn't talk about the low processor utlization (Dhall effect) > mentioned in your last email. However, that applies to hard real-time > workloads, not soft real-time workloads. This discussion has been > touching on both.
For hard real-time workloads, partitioning (static binding to specific processors) works well, with developer control over where tasks run and their contenders. For soft real-time workloads, global scheduling (dynamic binding to available processors) should do well. The situation is analogous to what we see in banks and airports. There is a common global queue serviced by multiple counters for "soft" real-time customers, and for those (business or first-class/special) customers needing higher QoS, separate queues are provided. In the OS context, we need to ensure that the two queues/servers do not interfere. Ceilings would help even when the hard and soft real-time tasks use the same processors.
However, the question of dealing with mutexes shared by processes allocated to different processors remains. As Ted has pointed out, avoiding them would be best! In practice, moving them to a synchronization processor (as was pointed out by Peter? and also discussed in one of my earlier papers on synchronization on multi-processors) ought to be considered. I think the first-order improvements are from
(1) ensuring that task waiting times are bounded as a function of critical sections only (i.e. avoiding the "unbounded" priority inversion problem) - this is accomplished by having critical sections execute at ceiling priorities (or higher) in the multiprocessor case, (2) avoiding the wait from very long critical sections used only by lower-priority tasks - using priority ceilings instead of higher priority values for long critical sections mitigates this problem.
--- Raj
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