On Tue, Feb 22, 2011 at 03:42:00PM +0100, Andrea Arcangeli wrote:

<SNIP>

I'm also intrigued by reducing this from 2 to 1:

    /* Make sure at least 2 hugepages are free for MIGRATE_RESERVE */
    recommended_min = pageblock_nr_pages * nr_zones * 2;

Do we really need 2 pages instead of just 1 here to provide the
guarantee?

For workloads that cause a lot of fragmentation - yes. Simplistically with 1,
the trace event mm_page_alloc_extfrag will trigger more frequently and
it's more likely to be severe. The problem is that if it's not "* 2",
there is a very low probability that there will pages free in a suitable
pageblock and "mixing" occurs. It can take a very long time for
allocation success rates to go down but it happens eventually.

-- 
Mel Gorman
SUSE Labs

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