Re: [PATCH 2/2] page-allocator: Maintain rolling count of pages to free from the PCP

[Mel Gorman <mel@csn.ul.ie>]

On Sat, Aug 29, 2009 at 12:04:48AM +0900, Minchan Kim wrote:
> 
> On Fri, Aug 28, 2009 at 5:44 PM, Mel Gorman<mel@csn.ul.ie> wrote:
> > When round-robin freeing pages from the PCP lists, empty lists may be
> > encountered. In the event one of the lists has more pages than another,
> > there may be numerous checks for list_empty() which is undesirable. This
> > patch maintains a count of pages to free which is incremented when empty
> > lists are encountered. The intention is that more pages will then be freed
> > from fuller lists than the empty ones reducing the number of empty list
> > checks in the free path.
> >
> > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> > ---
> >  mm/page_alloc.c |   23 ++++++++++++++---------
> >  1 files changed, 14 insertions(+), 9 deletions(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 65eedb5..9b86977 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -536,32 +536,37 @@ static void free_pcppages_bulk(struct zone *zone, int count,
> >                                        struct per_cpu_pages *pcp)
> >  {
> >        int migratetype = 0;
> > +       int batch_free = 0;
> >
> >        spin_lock(&zone->lock);
> >        zone_clear_flag(zone, ZONE_ALL_UNRECLAIMABLE);
> >        zone->pages_scanned = 0;
> >
> >        __mod_zone_page_state(zone, NR_FREE_PAGES, count);
> > -       while (count--) {
> > +       while (count) {
> >                struct page *page;
> >                struct list_head *list;
> >
> >                /*
> > -                * Remove pages from lists in a round-robin fashion. This spinning
> > -                * around potentially empty lists is bloody awful, alternatives that
> > -                * don't suck are welcome
> > +                * Remove pages from lists in a round-robin fashion. A batch_free
> > +                * count is maintained that is incremented when an empty list is
> > +                * encountered. This is so more pages are freed off fuller lists
> > +                * instead of spinning excessively around empty lists
> >                 */
> >                do {
> > +                       batch_free++;
> >                        if (++migratetype == MIGRATE_PCPTYPES)
> >                                migratetype = 0;
> >                        list = &pcp->lists[migratetype];
> >                } while (list_empty(list));
> 
> How about increasing the weight by batch_free ?
> 
> batch_free = 1 << (batch_free - 1);
> 
> It's assumed that if batch_free is big, it means
> there are contiguous empty lists.
> Then it is likely to need more time to refill empty lists than
> one list refill. So I think it can decrease spinning empty list
> a little more.
> 

Maybe. As it is, the list spinning is not too bad and significant
amounts of time are no longer being spent in there. I've taken note to
follow-up and investigate your suggestion to see if it works out and if
it makes a difference.

Thanks

> >
> > -               page = list_entry(list->prev, struct page, lru);
> > -               /* have to delete it as __free_one_page list manipulates */
> > -               list_del(&page->lru);
> > -               trace_mm_page_pcpu_drain(page, 0, migratetype);
> > -               __free_one_page(page, zone, 0, migratetype);
> > +               do {
> > +                       page = list_entry(list->prev, struct page, lru);
> > +                       /* must delete as __free_one_page list manipulates */
> > +                       list_del(&page->lru);
> > +                       __free_one_page(page, zone, 0, migratetype);
> > +                       trace_mm_page_pcpu_drain(page, 0, migratetype);
> > +               } while (--count && --batch_free && !list_empty(list));
> >        }
> >        spin_unlock(&zone->lock);
> >  }
> > --
> > 1.6.3.3
> >
> > --
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> > the body to majordomo@kvack.org.  For more info on Linux MM,
> > see: http://www.linux-mm.org/ .
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> >
> 
> 
> 
> -- 
> Kind regards,
> Minchan Kim
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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