linux-kernel - Re: [patch v2] mm, vmscan: avoid thrashing anon lru when free + file is low

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Date:   Wed, 3 May 2017 09:06:56 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Minchan Kim <minchan@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch v2] mm, vmscan: avoid thrashing anon lru when free + file
 is low

On Wed 03-05-17 08:15:28, Michal Hocko wrote:
> On Tue 02-05-17 13:41:23, David Rientjes wrote:
> > On Tue, 2 May 2017, Michal Hocko wrote:
[...]
> > > I do agree that blindly
> > > scanning anon pages when file pages are low is very suboptimal but this
> > > adds yet another heuristic without _any_ numbers. Why cannot we simply
> > > treat anon and file pages equally? Something like the following
> > > 
> > > 	if (pgdatfile + pgdatanon + pgdatfree > 2*total_high_wmark) {
> > > 		scan_balance = SCAN_FILE;
> > > 		if (pgdatfile < pgdatanon)
> > > 			scan_balance = SCAN_ANON;
> > > 		goto out;
> > > 	}
> > > 
> > 
> > This would be substantially worse than the current code because it 
> > thrashes the anon lru when anon out numbers file pages rather than at the 
> > point we fall under the high watermarks for all eligible zones.  If you 
> > tested your suggestion, you could see gigabytes of memory left untouched 
> > on the file lru.  Anonymous memory is more probable to be part of the 
> > working set.
> 
> This was supposed to be more an example of a direction I was thinking,
> definitely not a final patch. I will think more to come up with a
> more complete proposal.

This is still untested but should be much closer to what I've had in
mind.
---
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 24efcc20af91..bcdad30f942d 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2174,8 +2174,14 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
 		}
 
 		if (unlikely(pgdatfile + pgdatfree <= total_high_wmark)) {
-			scan_balance = SCAN_ANON;
-			goto out;
+			unsigned long pgdatanon;
+
+			pgdatanon = node_page_state(pgdat, NR_ACTIVE_ANON) +
+				node_page_state(pgdat, NR_INACTIVE_ANON);
+			if (pgdatanon + pgdatfree > total_high_wmark) {
+				scan_balance = SCAN_ANON;
+				goto out;
+			}
 		}
 	}
 

-- 
Michal Hocko
SUSE Labs