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    SubjectRe: [PATCH 4/4] capture pages freed during direct reclaim for allocation by the reclaimer
    On Thu, Oct 02, 2008 at 04:24:14PM +0900, KAMEZAWA Hiroyuki wrote:
    > On Wed, 1 Oct 2008 13:31:01 +0100
    > Andy Whitcroft <> wrote:
    > > When a process enters direct reclaim it will expend effort identifying
    > > and releasing pages in the hope of obtaining a page. However as these
    > > pages are released asynchronously there is every possibility that the
    > > pages will have been consumed by other allocators before the reclaimer
    > > gets a look in. This is particularly problematic where the reclaimer is
    > > attempting to allocate a higher order page. It is highly likely that
    > > a parallel allocation will consume lower order constituent pages as we
    > > release them preventing them coelescing into the higher order page the
    > > reclaimer desires.
    > >
    > > This patch set attempts to address this for allocations above
    > > ALLOC_COSTLY_ORDER by temporarily collecting the pages we are releasing
    > > onto a local free list. Instead of freeing them to the main buddy lists,
    > > pages are collected and coelesced on this per direct reclaimer free list.
    > > Pages which are freed by other processes are also considered, where they
    > > coelesce with a page already under capture they will be moved to the
    > > capture list. When pressure has been applied to a zone we then consult
    > > the capture list and if there is an appropriatly sized page available
    > > it is taken immediatly and the remainder returned to the free pool.
    > > Capture is only enabled when the reclaimer's allocation order exceeds
    > > ALLOC_COSTLY_ORDER as free pages below this order should naturally occur
    > > in large numbers following regular reclaim.
    > >
    > > Thanks go to Mel Gorman for numerous discussions during the development
    > > of this patch and for his repeated reviews.
    > >
    > Hmm.. is this routine better than
    > mm/memory_hotplug.c::do_migrate_range(start_pfn, end_pfn) ?

    Are you suggesting that it might be more adventageous to try and migrate
    things out of this area as part of reclaim? If so then I tend to agree,
    though that would be a good idea generally with or without capture.

    /me adds it to his todo list to test that out.


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