Re: [PATCH -v2] mm: do batched scans for mem_cgroup

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

Hi, Wu.

On Thu, Aug 20, 2009 at 1:05 PM, Wu Fengguang<fengguang.wu@intel.com> wrote:
> On Thu, Aug 20, 2009 at 11:13:47AM +0800, KAMEZAWA Hiroyuki wrote:
>> On Thu, 20 Aug 2009 10:49:29 +0800
>> Wu Fengguang <fengguang.wu@intel.com> wrote:
>>
>> > For mem_cgroup, shrink_zone() may call shrink_list() with nr_to_scan=1,
>> > in which case shrink_list() _still_ calls isolate_pages() with the much
>> > larger SWAP_CLUSTER_MAX.  It effectively scales up the inactive list
>> > scan rate by up to 32 times.
>> >
>> > For example, with 16k inactive pages and DEF_PRIORITY=12, (16k >> 12)=4.
>> > So when shrink_zone() expects to scan 4 pages in the active/inactive
>> > list, it will be scanned SWAP_CLUSTER_MAX=32 pages in effect.
>> >
>> > The accesses to nr_saved_scan are not lock protected and so not 100%
>> > accurate, however we can tolerate small errors and the resulted small
>> > imbalanced scan rates between zones.
>> >
>> > This batching won't blur up the cgroup limits, since it is driven by
>> > "pages reclaimed" rather than "pages scanned". When shrink_zone()
>> > decides to cancel (and save) one smallish scan, it may well be called
>> > again to accumulate up nr_saved_scan.
>> >
>> > It could possibly be a problem for some tiny mem_cgroup (which may be
>> > _full_ scanned too much times in order to accumulate up nr_saved_scan).
>> >
>> > CC: Rik van Riel <riel@redhat.com>
>> > CC: Minchan Kim <minchan.kim@gmail.com>
>> > CC: Balbir Singh <balbir@linux.vnet.ibm.com>
>> > CC: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
>> > CC: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>> > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
>> > ---
>>
>> Hmm, how about this ?
>> ==
>> Now, nr_saved_scan is tied to zone's LRU.
>> But, considering how vmscan works, it should be tied to reclaim_stat.
>>
>> By this, memcg can make use of nr_saved_scan information seamlessly.
>
> Good idea, full patch updated with your signed-off-by :)
>
> Thanks,
> Fengguang
> ---
> mm: do batched scans for mem_cgroup
>
> For mem_cgroup, shrink_zone() may call shrink_list() with nr_to_scan=1,
> in which case shrink_list() _still_ calls isolate_pages() with the much
> larger SWAP_CLUSTER_MAX.  It effectively scales up the inactive list
> scan rate by up to 32 times.

Yes. It can scan 32 times pages in only inactive list, not active list.

> For example, with 16k inactive pages and DEF_PRIORITY=12, (16k >> 12)=4.
> So when shrink_zone() expects to scan 4 pages in the active/inactive
> list, it will be scanned SWAP_CLUSTER_MAX=32 pages in effect.

Active list scan would be scanned in 4,  inactive list  is 32.

>
> The accesses to nr_saved_scan are not lock protected and so not 100%
> accurate, however we can tolerate small errors and the resulted small
> imbalanced scan rates between zones.

Yes.

> This batching won't blur up the cgroup limits, since it is driven by
> "pages reclaimed" rather than "pages scanned". When shrink_zone()
> decides to cancel (and save) one smallish scan, it may well be called
> again to accumulate up nr_saved_scan.

You mean nr_scan_try_batch logic ?
But that logic works for just global reclaim?
Now am I missing something?

Could you elaborate more? :)

> It could possibly be a problem for some tiny mem_cgroup (which may be
> _full_ scanned too much times in order to accumulate up nr_saved_scan).
>
> CC: Rik van Riel <riel@redhat.com>
> CC: Minchan Kim <minchan.kim@gmail.com>
> CC: Balbir Singh <balbir@linux.vnet.ibm.com>
> CC: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
>  include/linux/mmzone.h |    6 +++++-
>  mm/page_alloc.c        |    2 +-
>  mm/vmscan.c            |   20 +++++++++++---------
>  3 files changed, 17 insertions(+), 11 deletions(-)
>
> --- linux.orig/include/linux/mmzone.h   2009-07-30 10:45:15.000000000 +0800
> +++ linux/include/linux/mmzone.h        2009-08-20 11:51:08.000000000 +0800
> @@ -269,6 +269,11 @@ struct zone_reclaim_stat {
>         */
>        unsigned long           recent_rotated[2];
>        unsigned long           recent_scanned[2];
> +
> +       /*
> +        * accumulated for batching
> +        */
> +       unsigned long           nr_saved_scan[NR_LRU_LISTS];
>  };
>
>  struct zone {
> @@ -323,7 +328,6 @@ struct zone {
>        spinlock_t              lru_lock;
>        struct zone_lru {
>                struct list_head list;
> -               unsigned long nr_saved_scan;    /* accumulated for batching */
>        } lru[NR_LRU_LISTS];
>
>        struct zone_reclaim_stat reclaim_stat;
> --- linux.orig/mm/vmscan.c      2009-08-20 11:48:46.000000000 +0800
> +++ linux/mm/vmscan.c   2009-08-20 12:00:55.000000000 +0800
> @@ -1521,6 +1521,7 @@ static void shrink_zone(int priority, st
>        enum lru_list l;
>        unsigned long nr_reclaimed = sc->nr_reclaimed;
>        unsigned long swap_cluster_max = sc->swap_cluster_max;
> +       struct zone_reclaim_stat *reclaim_stat = get_reclaim_stat(zone, sc);
>        int noswap = 0;
>
>        /* If we have no swap space, do not bother scanning anon pages. */
> @@ -1540,12 +1541,9 @@ static void shrink_zone(int priority, st
>                        scan >>= priority;
>                        scan = (scan * percent[file]) / 100;
>                }
> -               if (scanning_global_lru(sc))
> -                       nr[l] = nr_scan_try_batch(scan,
> -                                                 &zone->lru[l].nr_saved_scan,
> -                                                 swap_cluster_max);
> -               else
> -                       nr[l] = scan;
> +               nr[l] = nr_scan_try_batch(scan,
> +                                         &reclaim_stat->nr_saved_scan[l],
> +                                         swap_cluster_max);
>        }
>
>        while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
> @@ -2128,6 +2126,7 @@ static void shrink_all_zones(unsigned lo
>  {
>        struct zone *zone;
>        unsigned long nr_reclaimed = 0;
> +       struct zone_reclaim_stat *reclaim_stat;
>
>        for_each_populated_zone(zone) {
>                enum lru_list l;
> @@ -2144,11 +2143,14 @@ static void shrink_all_zones(unsigned lo
>                                                l == LRU_ACTIVE_FILE))
>                                continue;
>
> -                       zone->lru[l].nr_saved_scan += (lru_pages >> prio) + 1;
> -                       if (zone->lru[l].nr_saved_scan >= nr_pages || pass > 3) {
> +                       reclaim_stat = get_reclaim_stat(zone, sc);
> +                       reclaim_stat->nr_saved_scan[l] +=
> +                                               (lru_pages >> prio) + 1;
> +                       if (reclaim_stat->nr_saved_scan[l]
> +                                               >= nr_pages || pass > 3) {
>                                unsigned long nr_to_scan;
>
> -                               zone->lru[l].nr_saved_scan = 0;
> +                               reclaim_stat->nr_saved_scan[l] = 0;
>                                nr_to_scan = min(nr_pages, lru_pages);
>                                nr_reclaimed += shrink_list(l, nr_to_scan, zone,
>                                                                sc, prio);
> --- linux.orig/mm/page_alloc.c  2009-08-20 11:57:54.000000000 +0800
> +++ linux/mm/page_alloc.c       2009-08-20 11:58:39.000000000 +0800
> @@ -3716,7 +3716,7 @@ static void __paginginit free_area_init_
>                zone_pcp_init(zone);
>                for_each_lru(l) {
>                        INIT_LIST_HEAD(&zone->lru[l].list);
> -                       zone->lru[l].nr_saved_scan = 0;
> +                       zone->reclaim_stat.nr_saved_scan[l] = 0;
>                }
>                zone->reclaim_stat.recent_rotated[0] = 0;
>                zone->reclaim_stat.recent_rotated[1] = 0;
>
> --
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>



-- 
Kind regards,
Minchan Kim

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