Re: [PATCH] Minor scheduler fix to get rid of skipping in xmms

[Nick Piggin]

Andrew Morton wrote:

> Nick Piggin <piggin@xxxxxxxxxxxxxxx> wrote:
>
> > > My concern is the (large) performance regression with specjbb and
> > >volanomark, due to increased idle time.
> > >
> > >We cannot just jam all this code into Linus's tree while crossing our
> > >fingers and hoping that something will turn up to fix this problem. 
> > >Because we don't know what causes it, nor whether we even _can_ fix it.
> > >
> > >So this is the problem which everyone who is working on the CPU scheduler
> > >should be concentrating on, please.
> > >
> >
> > IIRC my (equivalent to Andrew's CAN_MIGRATE) patch fixed this. There was 
> > still a small (~8%?) performance regression, but idle times were on par 
> > with -linus. I don't have easy access to a largeish NUMA box, so I
> > can't do much more.
>
> That is not clear at this time.  We do know that the reaim regression was
> introduced by sched-2.6.0-test2-mm2-A3, but we don't know why.  Certainly
> that patch did not introduce the problem which Andrew's patch fixed.  And
> we have theorised that Andrew's patch brought back the reaim throughput. 
> And we have extrapolated those observations to possible improvements in
> volanomark throughput.

Earlier we _saw_ my patch do what it was supposed to:
http://members.optusnet.com.au/ckolivas/kernel/2.5/volano/
Idle time is back to mainline levels although throughput is still down
a bit. (I'd say thats due to overbalancing which could be tuned back,
I'm going to attack the SMP and NUMA stuff in the scheduler soon).

> It's all foggy and I'd like to see a clean rerun of specjbb and volanomark
> by Mark Peloquin and co, confirming that -mm6 is performing OK.
>
>
> Also, I'm concerned that sched-2.6.0-test2-mm2-A3 caused slowdowns and
> Andrew's patch caused speedups and they just cancelled out.  Let's get
> Andrew's patch into Linus's tree and see if it speeds things up.  If it
> does, we probably still have a problem.

The slowdowns are due to CPUs becoming idle too long, and the patches
fix that. If CPUs aren't idle, the patch will have no effect.

I have no idea how volanomark really works, so I have no idea why the
patch causes queue imbalances. Thats the problem with those jumbo
patches :P


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