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SubjectRe: [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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