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

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Message-ID: <20100317113326.GD12388@csn.ul.ie>
Date:	Wed, 17 Mar 2010 11:33:26 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	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

On Wed, Mar 17, 2010 at 11:49:49AM +0900, KOSAKI Motohiro wrote:
> > +/*
> > + * 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?

To get an approximation to three decimal places.

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

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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