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

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Date:   Wed, 19 Apr 2017 09:04:25 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch] mm, vmscan: avoid thrashing anon lru when free + file is
 low

On Tue 18-04-17 14:32:56, David Rientjes wrote:
[...]
> If the suggestion is checking
> NR_ACTIVE_ANON + NR_INACTIVE_ANON > total_high_wmark pages, it would be a 
> separate heurstic to address a problem that I'm not having :)  My issue is 
> specifically when NR_ACTIVE_FILE + NR_INACTIVE_FILE < total_high_wmark, 
> NR_ACTIVE_ANON + NR_INACTIVE_ANON is very large, but all not on this 
> lruvec's evictable lrus.

Hmm, why are those pages not moved to the unevictable LRU lists?

> This is the reason why I chose lruvec_lru_size() rather than per-node 
> statistics.  The argument could also be made for the file lrus in the 
> get_scan_count() heuristic that forces SCAN_ANON, but I have not met such 
> an issue (yet).  I could follow-up with that change or incorporate it into 
> a v2 of this patch if you'd prefer.
> 
> In other words, I want get_scan_count() to not force SCAN_ANON and 
> fallback to SCAN_FRACT, absent other heuristics, if the amount of 
> evictable anon is below a certain threshold for this lruvec.  I 
> arbitrarily chose SWAP_CLUSTER_MAX to be conservative, but I could easily 
> compare to total_high_wmark as well, although I would consider that more 
> aggressive.
> 
> So we're in global reclaim, our file lrus are below thresholds, but we 
> don't want to force SCAN_ANON for all lruvecs if there's not enough to 
> reclaim from evictable anon.  Do you have a suggestion for how to 
> implement this logic other than this patch?

I agree that forcing SCAN_ANON without looking at the ANON lru size is
not optimal but I would rather see the same criterion for both anon and
file. get_scan_count is full of magic heuristics which tend to break for
different workloads. Let's not add another magic on top please.
-- 
Michal Hocko
SUSE Labs