Re: [PATCH 1/2] mm: compaction: try harder to isolate free pages

[Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>]

On Friday 06 April 2012 08:40:56 Minchan Kim wrote:
> On Fri, Apr 6, 2012 at 1:32 AM, Bartlomiej Zolnierkiewicz <
> b.zolnierkie@samsung.com> wrote:
> 
> > In isolate_freepages() check each page in a pageblock
> > instead of checking only first pages of pageblock_nr_pages
> > intervals (suitable_migration_target(page) is called before
> > isolate_freepages_block() so if page is "unsuitable" whole
> > pageblock_nr_pages pages will be ommited from the check).
> > It greatly improves possibility of finding free pages to
> > isolate during compaction_alloc() phase.
> >
> 
> I doubt how this can help keeping free pages.
> Now, compaction works by pageblock_nr_pages unit so although you work by
> per page, all pages in a block would have same block type.
> It means we can't pass suitable_migration_target. No?

suitable_migration_target() only checks first page of pageblock_nr_pages
block (1024 normal 4KiB pages in my test case cause there is no hugepage
support on ARM) and pages in pageblock_nr_pages block can have different
types otherwise I would not see improvement from this patch.

Best regards,
--
Bartlomiej Zolnierkiewicz
Samsung Poland R&D Center

> > Cc: Mel Gorman <mgorman@suse.de>
> > Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
> > Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
> > ---
> >  mm/compaction.c |    5 ++---
> >  1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/compaction.c b/mm/compaction.c
> > index d9ebebe..bc77135 100644
> > --- a/mm/compaction.c
> > +++ b/mm/compaction.c
> > @@ -65,7 +65,7 @@ static unsigned long isolate_freepages_block(struct zone
> > *zone,
> >
> >        /* Get the last PFN we should scan for free pages at */
> >        zone_end_pfn = zone->zone_start_pfn + zone->spanned_pages;
> > -       end_pfn = min(blockpfn + pageblock_nr_pages, zone_end_pfn);
> > +       end_pfn = min(blockpfn + 1, zone_end_pfn);
> >
> >        /* Find the first usable PFN in the block to initialse page cursor
> > */
> >        for (; blockpfn < end_pfn; blockpfn++) {
> > @@ -160,8 +160,7 @@ static void isolate_freepages(struct zone *zone,
> >         * pages on cc->migratepages. We stop searching if the migrate
> >         * and free page scanners meet or enough free pages are isolated.
> >         */
> > -       for (; pfn > low_pfn && cc->nr_migratepages > nr_freepages;
> > -                                       pfn -= pageblock_nr_pages) {
> > +       for (; pfn > low_pfn && cc->nr_migratepages > nr_freepages; pfn--)
> > {
> >                unsigned long isolated;
> >
> >                if (!pfn_valid(pfn))

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