Re: [RFC] scheduler: improve SMP fairness in CFS

[Bill Huey (hui) <billh@gnuppy.monkey.org>]

On Tue, Jul 24, 2007 at 05:22:47PM -0400, Chris Snook wrote:
> Bill Huey (hui) wrote:
> Well, you need enough CPU time to meet your deadlines.  You need 
> pre-allocated memory, or to be able to guarantee that you can allocate 
> memory fast enough to meet your deadlines.  This principle extends to any 
> other shared resource, such as disk or network.  I'm being vague because 
> it's open-ended.  If a medical device fails to meet realtime guarantees 
> because the battery fails, the patient's family isn't going to care how 
> correct the software is.  Realtime engineering is hard.
...
> Actually, it's worse than merely an open problem.  A clairvoyant fair 
> scheduler with perfect future knowledge can underperform a heuristic fair 
> scheduler, because the heuristic scheduler can guess the future incorrectly 
> resulting in unfair but higher-throughput behavior.  This is a perfect 
> example of why we only try to be as fair as is beneficial.

I'm glad we agree on the above points. :)

It might be that there needs to be another more stiff policy than what goes
into SCHED_OTHER in that we also need a SCHED_ISO or something has more
strict rebalancing semantics for -rt applications, sort be a super SCHED_RR.
That's definitely needed and I don't see how the current CFS implementation
can deal with this properly even with numerical running averages, etc...
at this time.

SCHED_FIFO is another issue, but this actually more complicated than just
per cpu run queues in that a global priority analysis. I don't see how
CFS can deal with SCHED_FIFO efficiently without moving to a single run
queue. This is kind of a complicated problem with a significant set of
trade off to take into account (cpu binding, etc..)

>> Tong's previous trio patch is an attempt at resolving this using a generic
>> grouping mechanism and some constructive discussion should come of it.
>
> Sure, but it seems to me to be largely orthogonal to this patch.

It's based on the same kinds of ideas that he's been experimenting with in
Trio. I can't name a single other engineer that's posted to lkml recently
that has quite the depth of experience in this area than him. It would be
nice to facilitted/incorporate some his ideas or get him to and work on
something to this end that's suitable for inclusion in some tree some where.

bill

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