linux-kernel - Re: [PATCH] sched/fair: Optimize sum computation with a lookup table

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Message-ID: <20160408162245.GB11241@pablo>
Date:	Fri, 8 Apr 2016 09:22:45 -0700
From:	Juri Lelli <juri.lelli@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Joe Perches <joe@...ches.com>, Yuyang Du <yuyang.du@...el.com>,
	mingo@...nel.org, linux-kernel@...r.kernel.org, bsegall@...gle.com,
	pjt@...gle.com, morten.rasmussen@....com,
	vincent.guittot@...aro.org, dietmar.eggemann@....com
Subject: Re: [PATCH] sched/fair: Optimize sum computation with a lookup table

Hi,

On 08/04/16 12:54, Peter Zijlstra wrote:
> On Fri, Apr 08, 2016 at 03:31:41AM -0700, Joe Perches wrote:
> > On Fri, 2016-04-08 at 10:07 +0800, Yuyang Du wrote:
> > > __compute_runnable_contrib() uses a loop to compute sum, whereas a
> > > table lookup can do it faster in a constant time.
> > 
> > Perhaps this becomes rather fragile code overly dependent on the
> > current #define values of LOAD_AVG_MAX_N and LOAD_AVG_PERIOD.
> 
> Too late for that, these tables already heavily depend on these values.
> 
> Commit 5b51f2f80b3b ("sched: Make __update_entity_runnable_avg() fast"),
> which introduced these tables, includes a program to re-compute the
> values if we ever need to change them.
> 
> It might maybe make sense for Yuyang to update that program to also
> compute this new table and include the whole new program in the
> Changelog of this patch.
> 

+1 on this. That program turns out to be really useful if one needs to
recompute tables.

Best,

- Juri