On Wed,  1 Oct 2008 13:31:01 +0100
Andy Whitcroft <apw@shadowen.org> wrote:

quoted text
> 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) ?

Thanks,
-Kame

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