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

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Message-Id: <1194481486.6606.6.camel@perkele>
Date:	Wed, 07 Nov 2007 19:24:46 -0500
From:	Eric St-Laurent <ericstl34@...patico.ca>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>, mingo@...e.hu,
	pwil3058@...pond.net.au, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: avoid large irq-latencies in smp-balancing


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

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