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

[Minchan Kim <minchan.kim@gmail.com>]

Hi, Mel.

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.

>
> -               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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-- 
Kind regards,
Minchan Kim

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