Re: [PATCH] sched: avoid large irq-latencies in smp-balancing

[Eric St-Laurent <ericstl34@sympatico.ca>]

On Wed, 2007-11-07 at 17:10 -0500, Steven Rostedt wrote:
> > 
> > It would be nice if sched_nr_migrate didn't exist, really.  It's hard to
> > imagine anyone wanting to tweak it, apart from developers.
> 
> I'm not so sure about that. It is a tunable for RT. That is we can tweak
> this value to be smaller if we don't like the latencies it gives us.
> 
> This is one of those things that sacrifices performance for latency.
> The higher the number, the better it can spread tasks around, but it
> also causes large latencies.
> 
> I've just included this patch into 2.6.23.1-rt11 and it brought down an
> unbounded latency to just 42us. (previously we got into the
> milliseconds!).
> 
> Perhaps when this feature matures, we can come to a good defined value
> that would be good for all. But until then, I recommend keeping this a
> tunable.


Why not use the latency-expectation infrastructure?

Iterate under lock until (or before...) the system global latency is
respected.


- Eric