Re: [PATCH 3/3] mm, lru_gen: try to prefetch next page when canning LRU

[Yu Zhao <yuzhao@google.com>]

On Fri, Dec 22, 2023 at 3:24 AM Kairui Song <ryncsn@gmail.com> wrote:
>
> From: Kairui Song <kasong@tencent.com>
>
> Prefetch for inactive/active LRU have been long exiting, apply the same
> optimization for MGLRU.

I seriously doubt that prefetch helps in this case.

Willy, any thoughts on this? Thanks.

> Tested in a 4G memcg on a EPYC 7K62 with:
>
>   memcached -u nobody -m 16384 -s /tmp/memcached.socket \
>     -a 0766 -t 16 -B binary &
>
>   memtier_benchmark -S /tmp/memcached.socket \
>     -P memcache_binary -n allkeys \
>     --key-minimum=1 --key-maximum=16000000 -d 1024 \
>     --ratio=1:0 --key-pattern=P:P -c 2 -t 16 --pipeline 8 -x 6
>
> Average result of 18 test runs:
>
> Before:           44017.78 Ops/sec
> After patch 1-3:  44890.50 Ops/sec (+1.8%)
>
> Signed-off-by: Kairui Song <kasong@tencent.com>
> ---
>  mm/vmscan.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index af1266129c1b..1e9d69e18443 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3764,10 +3764,12 @@ static bool inc_min_seq(struct lruvec *lruvec, int type, bool can_swap)
>                         VM_WARN_ON_ONCE_FOLIO(folio_is_file_lru(folio) != type, folio);
>                         VM_WARN_ON_ONCE_FOLIO(folio_zonenum(folio) != zone, folio);
>
> -                       if (unlikely(list_is_first(&folio->lru, head)))
> +                       if (unlikely(list_is_first(&folio->lru, head))) {
>                                 prev = NULL;
> -                       else
> +                       } else {
>                                 prev = lru_to_folio(&folio->lru);
> +                               prefetchw(&prev->flags);
> +                       }
>
>                         new_gen = folio_inc_gen(lruvec, folio, false, &batch);
>                         lru_gen_try_inc_bulk(lrugen, folio, bulk_gen, new_gen, type, zone, &batch);
> @@ -4434,10 +4436,12 @@ static int scan_folios(struct lruvec *lruvec, struct scan_control *sc,
>                         VM_WARN_ON_ONCE_FOLIO(folio_zonenum(folio) != zone, folio);
>
>                         scanned += delta;
> -                       if (unlikely(list_is_first(&folio->lru, head)))
> +                       if (unlikely(list_is_first(&folio->lru, head))) {
>                                 prev = NULL;
> -                       else
> +                       } else {
>                                 prev = lru_to_folio(&folio->lru);
> +                               prefetchw(&prev->flags);
> +                       }
>
>                         if (sort_folio(lruvec, folio, sc, tier, bulk_gen, &batch))
>                                 sorted += delta;
> --
> 2.43.0
>