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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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