On Thu 09-06-11 17:00:26, Michal Hocko wrote:

On Thu 02-06-11 22:25:29, Ying Han wrote:

quoted

On Thu, Jun 2, 2011 at 2:55 PM, Ying Han [off-list ref] wrote:

quoted

On Tue, May 31, 2011 at 11:25 PM, Johannes Weiner [off-list ref] wrote:

quoted

Currently, soft limit reclaim is entered from kswapd, where it selects

[...]

quoted

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diff --git a/mm/vmscan.c b/mm/vmscan.c
index c7d4b44..0163840 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c

@@ -1988,9 +1988,13 @@ static void shrink_zone(int priority, struct zone *zone,
               unsigned long reclaimed = sc->nr_reclaimed;
               unsigned long scanned = sc->nr_scanned;
               unsigned long nr_reclaimed;
+               int epriority = priority;
+
+               if (mem_cgroup_soft_limit_exceeded(root, mem))
+                       epriority -= 1;

Here we grant the ability to shrink from all the memcgs, but only
higher the priority for those exceed the soft_limit. That is a design
change
for the "soft_limit" which giving a hint to which memcgs to reclaim
from first under global memory pressure.

Basically, we shouldn't reclaim from a memcg under its soft_limit
unless we have trouble reclaim pages from others. 

Agreed.

quoted

Something like the following makes better sense:

diff --git a/mm/vmscan.c b/mm/vmscan.c
index bdc2fd3..b82ba8c 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c

@@ -1989,6 +1989,8 @@ restart:
        throttle_vm_writeout(sc->gfp_mask);
 }

+#define MEMCG_SOFTLIMIT_RECLAIM_PRIORITY       2
+
 static void shrink_zone(int priority, struct zone *zone,
                                struct scan_control *sc)
 {

@@ -2001,13 +2003,13 @@ static void shrink_zone(int priority, struct zone *zone,
                unsigned long reclaimed = sc->nr_reclaimed;
                unsigned long scanned = sc->nr_scanned;
                unsigned long nr_reclaimed;
-               int epriority = priority;

-               if (mem_cgroup_soft_limit_exceeded(root, mem))
-                       epriority -= 1;
+               if (!mem_cgroup_soft_limit_exceeded(root, mem) &&
+                               priority > MEMCG_SOFTLIMIT_RECLAIM_PRIORITY)
+                       continue;

yes, this makes sense but I am not sure about the right(tm) value of the
MEMCG_SOFTLIMIT_RECLAIM_PRIORITY. 2 sounds too low. 

There is also another problem. I have just realized that this code path
is shared with the cgroup direct reclaim. We shouldn't care about soft
limit in such a situation. It would be just a wasting of cycles. So we
have to:

if (current_is_kswapd() && 
	!mem_cgroup_soft_limit_exceeded(root, mem) && 
	priority > MEMCG_SOFTLIMIT_RECLAIM_PRIORITY)
	continue;

Maybe the condition would have to be more complex for per-cgroup
background reclaim, though.

You would do quite a
lot of loops 
(DEFAULT_PRIORITY-MEMCG_SOFTLIMIT_RECLAIM_PRIORITY) * zones * memcg_count
without any progress (assuming that all of them are under soft limit
which doesn't sound like a totally artificial configuration) until you
allow reclaiming from groups that are under soft limit. Then, when you
finally get to reclaiming, you scan rather aggressively.

Maybe something like 3/4 of DEFAULT_PRIORITY? You would get 3 times
over all (unbalanced) zones and all cgroups that are above the limit
(scanning max{1/4096+1/2048+1/1024, 3*SWAP_CLUSTER_MAX} of the LRUs for
each cgroup) which could be enough to collect the low hanging fruit.

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

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