Re: [PATCH 4/4] capture pages freed during direct reclaim forallocation by the reclaimer

[Andy Whitcroft]

On Thu, Oct 02, 2008 at 04:24:14PM +0900, KAMEZAWA Hiroyuki wrote:
> On Wed,  1 Oct 2008 13:31:01 +0100
> Andy Whitcroft <apw@xxxxxxxxxxxx> 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.

-apw
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