linux-kernel - Re: High wake up latencies with FAIR_USER

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Message-ID: <20080129162658.GN1044@linux.vnet.ibm.com>
Date:	Tue, 29 Jan 2008 21:56:58 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	"Guillaume Chazarain" <guichaz@...oo.fr>
Cc:	"Ingo Molnar" <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	a.p.zijlstra@...llo.nl, dhaval@...ux.vnet.ibm.com
Subject: Re: High wake up latencies with FAIR_USER_SCHED

On Tue, Jan 29, 2008 at 04:53:56PM +0100, Guillaume Chazarain wrote:
> I just thought about something to restore low latencies with
> FAIR_GROUP_SCHED, but it's possibly utter nonsense, so bear with me
> ;-) The idea would be to reverse the trees upside down. The scheduler
> would only see tasks (on the leaves) so could apply its interactivity
> magic, but the hierarchical groups would be used to compute dynamic
> loads for each task according to their position in the tree:

I think this is equivalent to flattening the hierarchy? We discussed this a bit 
sometime back [1], but one of its weaknesses is providing strong
partitioning between groups when it comes to ensuring fairness. Ex: imagine a 
group which does a fork-bomb. With the flattened tree, it affects other groups 
more than it would with a 1-level deep hierarchy.

Having said that, I would be interested to hear other solutions that maintain 
this good partitioning b/n groups and still provide good interactivity!

1. http://lkml.org/lkml/2007/5/30/300

> - now:
>   - we schedule each level of the tree starting from the root
> 
> - with my proposition:
>   - we schedule tasks like with !FAIR_GROUP_SCHED, but
> calc_delta_fair() would traverse the tree starting from the leaves to
> compute the dynamic load.

-- 
Regards,
vatsa
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