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

[Andrew Morton <akpm@linux-foundation.org>]

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?

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