linux-kernel - Re: [PATCH 06/11] Export fragmentation index via /proc/extfrag

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Message-Id: <20100317114321.4C9A.A69D9226@jp.fujitsu.com>
Date:	Wed, 17 Mar 2010 11:49:49 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	kosaki.motohiro@...fujitsu.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 06/11] Export fragmentation index via /proc/extfrag_index

> +/*
> + * A fragmentation index only makes sense if an allocation of a requested
> + * size would fail. If that is true, the fragmentation index indicates
> + * whether external fragmentation or a lack of memory was the problem.
> + * The value can be used to determine if page reclaim or compaction
> + * should be used
> + */
> +int fragmentation_index(unsigned int order, struct contig_page_info *info)
> +{
> +	unsigned long requested = 1UL << order;
> +
> +	if (!info->free_blocks_total)
> +		return 0;
> +
> +	/* Fragmentation index only makes sense when a request would fail */
> +	if (info->free_blocks_suitable)
> +		return -1000;
> +
> +	/*
> +	 * Index is between 0 and 1 so return within 3 decimal places
> +	 *
> +	 * 0 => allocation would fail due to lack of memory
> +	 * 1 => allocation would fail due to fragmentation
> +	 */
> +	return 1000 - ( (1000+(info->free_pages * 1000 / requested)) / info->free_blocks_total);
> +}

Dumb question.

your paper (http://portal.acm.org/citation.cfm?id=1375634.1375641) says

fragmentation_index = 1 - (TotalFree/SizeRequested)/BlocksFree

but your code have extra '1000+'. Why?



Probably, I haven't understand the intention of this calculation.



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