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SubjectRe: [patch] mm, vmscan: avoid thrashing anon lru when free + file is low
Hello David,

On Mon, Apr 17, 2017 at 05:06:20PM -0700, David Rientjes wrote:
> The purpose of the code that commit 623762517e23 ("revert 'mm: vmscan: do
> not swap anon pages just because free+file is low'") reintroduces is to
> prefer swapping anonymous memory rather than trashing the file lru.
>
> If all anonymous memory is unevictable, however, this insistance on

"unevictable" means hot workingset, not (mlocked and increased refcount
by some driver)?
I got confused.

> SCAN_ANON ends up thrashing that lru instead.

Sound reasonable.

>
> Check that enough evictable anon memory is actually on this lruvec before
> insisting on SCAN_ANON. SWAP_CLUSTER_MAX is used as the threshold to
> determine if only scanning anon is beneficial.

Why do you use SWAP_CLUSTER_MAX instead of (high wmark + free) like
file-backed pages?
As considering anonymous pages have more probability to become workingset
because they are are mapped, IMO, more {strong or equal} condition than
file-LRU would be better to prevent anon LRU thrashing.

>
> Otherwise, fallback to balanced reclaim so the file lru doesn't remain
> untouched.
>
> Signed-off-by: David Rientjes <rientjes@google.com>
> ---
> mm/vmscan.c | 41 +++++++++++++++++++++++------------------
> 1 file changed, 23 insertions(+), 18 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2186,26 +2186,31 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
> * anon pages. Try to detect this based on file LRU size.

Please update this comment, too.

> */
> if (global_reclaim(sc)) {
> - unsigned long pgdatfile;
> - unsigned long pgdatfree;
> - int z;
> - unsigned long total_high_wmark = 0;
> -
> - pgdatfree = sum_zone_node_page_state(pgdat->node_id, NR_FREE_PAGES);
> - pgdatfile = node_page_state(pgdat, NR_ACTIVE_FILE) +
> - node_page_state(pgdat, NR_INACTIVE_FILE);
> -
> - for (z = 0; z < MAX_NR_ZONES; z++) {
> - struct zone *zone = &pgdat->node_zones[z];
> - if (!managed_zone(zone))
> - continue;
> + anon = lruvec_lru_size(lruvec, LRU_ACTIVE_ANON, sc->reclaim_idx) +
> + lruvec_lru_size(lruvec, LRU_INACTIVE_ANON, sc->reclaim_idx);
> + if (likely(anon >= SWAP_CLUSTER_MAX)) {

With high_wmark, we can do this.

if (global_reclaim(sc)) {
pgdatfree = xxx;
pgdatfile = xxx;
total_high_wmark = xxx;

if (pgdatfile + pgdatfree <= total_high_wmark) {
pgdatanon = xxx;
if (pgdatanon + pgdatfree > total_high_wmark) {
scan_balance = SCAN_ANON;
goto out;
}
}
}


> + unsigned long total_high_wmark = 0;
> + unsigned long pgdatfile;
> + unsigned long pgdatfree;
> + int z;
> +
> + pgdatfree = sum_zone_node_page_state(pgdat->node_id,
> + NR_FREE_PAGES);
> + pgdatfile = node_page_state(pgdat, NR_ACTIVE_FILE) +
> + node_page_state(pgdat, NR_INACTIVE_FILE);
> +
> + for (z = 0; z < MAX_NR_ZONES; z++) {
> + struct zone *zone = &pgdat->node_zones[z];
> + if (!managed_zone(zone))
> + continue;
>
> - total_high_wmark += high_wmark_pages(zone);
> - }
> + total_high_wmark += high_wmark_pages(zone);
> + }
>
> - if (unlikely(pgdatfile + pgdatfree <= total_high_wmark)) {
> - scan_balance = SCAN_ANON;
> - goto out;
> + if (unlikely(pgdatfile + pgdatfree <= total_high_wmark)) {
> + scan_balance = SCAN_ANON;
> + goto out;
> + }
> }
> }
>
>
> --
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