Re: [PATCH][BUGFIX] memcg fix zone congestion

Andrew Morton
2011-05-13T06:17:37Z

On Fri, 13 May 2011 12:10:30 +0900 KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

>
> ZONE_CONGESTED should be a state of global memory reclaim.
> If not, a busy memcg sets this and give unnecessary throttoling in
> wait_iff_congested() against memory recalim in other contexts. This makes
> system performance bad.
>
> I'll think about "memcg is congested!" flag is required or not, later.
> But this fix is required 1st.
>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> ---
> mm/vmscan.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> Index: mmotm-May11/mm/vmscan.c
> ===================================================================
> --- mmotm-May11.orig/mm/vmscan.c
> +++ mmotm-May11/mm/vmscan.c
> @@ -941,7 +941,8 @@ keep_lumpy:
> * back off and wait for congestion to clear because further reclaim
> * will encounter the same problem
> */
> - if (nr_dirty == nr_congested && nr_dirty != 0)
> + if (scanning_global_lru(sc) &&
> + nr_dirty == nr_congested && nr_dirty != 0)
> zone_set_flag(zone, ZONE_CONGESTED);
>

nit: which is more probable? nr_dirty==nr_congested or
scanning_global_lru(sc)?

If the user is actually _using_ memcg then

--- a/mm/vmscan.c~a
+++ a/mm/vmscan.c
@@ -937,7 +937,7 @@ keep_lumpy:
* back off and wait for congestion to clear because further reclaim
* will encounter the same problem
*/
- if (nr_dirty == nr_congested && nr_dirty != 0)
+ if (nr_dirty == nr_congested && scanning_global_lru(sc) && nr_dirty)
zone_set_flag(zone, ZONE_CONGESTED);

free_page_list(&free_pages);

is more efficient. If the user isn't using memcg then your patch is faster?