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

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Message-ID: <4BF44156.2070807@am.sony.com>
Date:	Wed, 19 May 2010 12:51:50 -0700
From:	Frank Rowand <frank.rowand@...sony.com>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/2] cpuidle: Add a repeating pattern detector to the
 menu governor

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)"?

> +	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;
> +}


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