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

From	Michal Hocko <mhocko@kernel.org>
Newsgroups	linux.kernel
Subject	Re: [patch v2] mm, vmscan: avoid thrashing anon lru when free + file is low
Date	2017-05-02 10:10 +0200
Message-ID	<tCB2F-3YK-9@gated-at.bofh.it> (permalink)
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On Mon 01-05-17 14:34:21, David Rientjes wrote:
[...]
> @@ -2204,8 +2204,17 @@ 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;
> +			/*
> +			 * Force SCAN_ANON if there are enough inactive
> +			 * anonymous pages on the LRU in eligible zones.
> +			 * Otherwise, the small LRU gets thrashed.
> +			 */
> +			if (!inactive_list_is_low(lruvec, false, sc, false) &&
> +			    lruvec_lru_size(lruvec, LRU_INACTIVE_ANON, sc->reclaim_idx)
> +					>> sc->priority) {
> +				scan_balance = SCAN_ANON;
> +				goto out;
> +			}

I have already asked and my questions were ignored. So let me ask again
and hopefuly not get ignored this time. So Why do we need a different
criterion on anon pages than file pages? 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;
	}

Also it would help to describe the workload which can trigger this
behavior so that we can compare numbers before and after this patch.
-- 
Michal Hocko
SUSE Labs

Thread

[patch v2] mm, vmscan: avoid thrashing anon lru when free + file is  low David Rientjes <rientjes@google.com> - 2017-05-01 23:40 +0200
  Re: [patch v2] mm, vmscan: avoid thrashing anon lru when free + file  is low Michal Hocko <mhocko@kernel.org> - 2017-05-02 10:10 +0200
    Re: [patch v2] mm, vmscan: avoid thrashing anon lru when free + file  is low David Rientjes <rientjes@google.com> - 2017-05-02 22:50 +0200
      Re: [patch v2] mm, vmscan: avoid thrashing anon lru when free + file  is low Michal Hocko <mhocko@kernel.org> - 2017-05-03 08:20 +0200
        Re: [patch v2] mm, vmscan: avoid thrashing anon lru when free + file  is low Michal Hocko <mhocko@kernel.org> - 2017-05-03 09:10 +0200
          Re: [patch v2] mm, vmscan: avoid thrashing anon lru when free + file  is low Michal Hocko <mhocko@kernel.org> - 2017-05-03 11:00 +0200
            Re: [patch v2] mm, vmscan: avoid thrashing anon lru when free + file  is low David Rientjes <rientjes@google.com> - 2017-05-04 01:00 +0200
              Re: [patch v2] mm, vmscan: avoid thrashing anon lru when free + file  is low Michal Hocko <mhocko@kernel.org> - 2017-05-04 13:50 +0200

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