linux-kernel - Re: [RFC tg_shares_up improvements - v1 00/12] [RFC tg_shares_up - v1 00/12] Reducing cost of tg->shares distribution

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Date:	Sat, 16 Oct 2010 21:46:54 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	pjt@...gle.com
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Chris Friesen <cfriesen@...tel.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Pierre Bourdon <pbourdon@...ellency.fr>,
	Bharata B Rao <bharata@...ux.vnet.ibm.com>
Subject: Re: [RFC tg_shares_up improvements - v1 00/12] [RFC tg_shares_up -
 v1 00/12] Reducing cost of tg->shares distribution

On Fri, 2010-10-15 at 21:43 -0700, pjt@...gle.com wrote:
> Hi all,
> 
> Peter previously posted a patchset that attempted to improve the problem of
> task_group share distribution.  This is something that has been a long-time
> pain point for group scheduling.  The existing algorithm considers
> distributions on a per-cpu-per-domain basis and carries a fairly high update
> overhead, especially on larger machines.
> 
> I was previously looking at improving this using Fenwick trees to allow a
> single sum without the exorbitant cost but then Peter's idea above was better :).
> 
> The kernel is that by monitoring the average contribution to load on a
> per-cpu-per-taskgroup basis we can distribute the weight for which we are
> expected to consume.
> 
> This set extends the original posting with a focus on increased fairness and
> reduced convergence (to true average) time.  In particular the case of large
> over-commit in the case of a distributed wake-up is a concern which is now
> fairly well addressed.
> 
> Obviously everything's experimental but it should be stable/fair.

I like what you've done with it, my only worry is 10/12 where you allow
for extra updates to the global state -- I think they should be fairly
limited in number, and I can see the need for the update if we get too
far out of whack, but it is something to look at while testing this
stuff.

> TODO:
> - Validate any RT interaction

I don't think there's anything to worry about there, the only
interaction which there is between this and the rt scheduling classes is
the initial sharing of the load-avg window, but you 'cure' that in 7/12.

(I think that sysctl wants a _us postfix someplace and we thus want some
NSEC_PER_USEC multiplication in there).

> - Continue collecting/analyzing performance and fairness data

Yes please ;-), I'll try and run this on some machines as well.

> - Should the shares period just be the sched_latency?

Interesting idea.. lets keep it a separate sysctl for now for easy
tuning, if things settle down and we're still good in that range we can
consider merging them.


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