linux-kernel - Re: [PATCH 2/2] cpuidle: Add a repeating pattern detector to the menu governor

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Message-ID: <20100521222323.4799b1ca@infradead.org>
Date:	Fri, 21 May 2010 22:23:23 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	frank.rowand@...sony.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] cpuidle: Add a repeating pattern detector to the
 menu governor

On Fri, 21 May 2010 12:43:09 -0700
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Wed, 19 May 2010 12:51:50 -0700
> Frank Rowand <frank.rowand@...sony.com> wrote:
> 
> > Aplogies if this is a duplicate, my outgoing email seems to have not
> > been working.
> > 
> > On 05/09/10 16:04, Arjan van de Ven wrote:
> > 
> > > +/*
> > > + * Try detecting repeating patterns by keeping track of the last
> > > 8
> > > + * intervals, and checking if the standard deviation of that set
> > > + * of points is below a threshold. If it is... then use the
> > > + * average of these 8 points as the estimated value.
> > > + */
> > > +static void detect_repeating_patterns(struct menu_device *data)
> > > +{
> > > +	int i;
> > > +	uint64_t avg = 0;
> > > +	uint64_t stddev = 0; /* contains the square of the std
> > > deviation */ +
> > > +	/* first calculate average and standard deviation of the
> > > past */
> > > +	for (i = 0; i < INTERVALS; i++)
> > > +		avg += data->intervals[i];
> > > +
> > > +	/* if the avg is beyond the known next tick, it's
> > > worthless */
> > > +	if (avg > data->expected_us)
> > > +		return;
> > > +
> > 
> > Should the following division by INTERVALS be moved up 6 lines to
> > before "if (avg > data->expected_us)"?
> 
> Quite possibly.

yeah that's a bug; I'll make a patch to fix it

> 
> > > +	avg = avg / INTERVALS;
> > > +	for (i = 0; i < INTERVALS; i++)
> > > +		stddev += (data->intervals[i] - avg) *
> > > +			  (data->intervals[i] - avg);
> > > +
> > > +	stddev = stddev / INTERVALS;
> > > +
> > > +	/*
> > > +	 * now.. if stddev is small.. then assume we have a
> > > +	 * repeating pattern and predict we keep doing this.
> > > +	 */
> > > +
> > > +	if (avg && stddev < STDDEV_THRESH)
> > > +		data->predicted_us = avg;
> > > +}
> 
> wakey wakey, Arjan.

sorry, seems this email got lost in 400 "Suspend block API" emails ;(


> 
> 
> Also, expected_us is 32-bit and predicted_us is 64-bit.  Was that
> rational?

predicted holds intermediate results of some calculations.. and those
overflow 32 bit...


-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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