Re: [PATCH] mm: zswap: shrink until can accept

[Vitaly Wool <vitaly.wool@konsulko.com>]

Hi Domenico,

On Wed, May 24, 2023 at 8:50 AM Domenico Cerasuolo
<cerasuolodomenico@gmail.com> wrote:
>
> This update addresses an issue with the zswap reclaim mechanism, which
> hinders the efficient offloading of cold pages to disk, thereby
> compromising the preservation of the LRU order and consequently
> diminishing, if not inverting, its performance benefits.
>
> The functioning of the zswap shrink worker was found to be inadequate,
> as shown by basic benchmark test. For the test, a kernel build was
> utilized as a reference, with its memory confined to 1G via a cgroup and
> a 5G swap file provided. The results are presented below, these are
> averages of three runs without the use of zswap:
>
> real 46m26s
> user 35m4s
> sys 7m37s
>
> With zswap (zbud) enabled and max_pool_percent set to 1 (in a 32G
> system), the results changed to:
>
> real 56m4s
> user 35m13s
> sys 8m43s
>
> written_back_pages: 18
> reject_reclaim_fail: 0
> pool_limit_hit:1478
>
> Besides the evident regression, one thing to notice from this data is
> the extremely low number of written_back_pages and pool_limit_hit.
>
> The pool_limit_hit counter, which is increased in zswap_frontswap_store
> when zswap is completely full, doesn't account for a particular
> scenario: once zswap hits his limit, zswap_pool_reached_full is set to
> true; with this flag on, zswap_frontswap_store rejects pages if zswap is
> still above the acceptance threshold. Once we include the rejections due
> to zswap_pool_reached_full && !zswap_can_accept(), the number goes from
> 1478 to a significant 21578266.
>
> Zswap is stuck in an undesirable state where it rejects pages because
> it's above the acceptance threshold, yet fails to attempt memory
> reclaimation. This happens because the shrink work is only queued when
> zswap_frontswap_store detects that it's full and the work itself only
> reclaims one page per run.
>
> This state results in hot pages getting written directly to disk,
> while cold ones remain memory, waiting only to be invalidated. The LRU
> order is completely broken and zswap ends up being just an overhead
> without providing any benefits.
>
> This commit applies 2 changes: a) the shrink worker is set to reclaim
> pages until the acceptance threshold is met and b) the task is also
> enqueued when zswap is not full but still above the threshold.
>
> Testing this suggested update showed much better numbers:
>
> real 36m37s
> user 35m8s
> sys 9m32s
>
> written_back_pages: 10459423
> reject_reclaim_fail: 12896
> pool_limit_hit: 75653
>
> Fixes: 45190f01dd40 ("mm/zswap.c: add allocation hysteresis if pool limit is hit")
> Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
> ---
>  mm/zswap.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 59da2a415fbb..2ee0775d8213 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -587,9 +587,13 @@ static void shrink_worker(struct work_struct *w)
>  {
>         struct zswap_pool *pool = container_of(w, typeof(*pool),
>                                                 shrink_work);
> +       int ret;
>
> -       if (zpool_shrink(pool->zpool, 1, NULL))
> -               zswap_reject_reclaim_fail++;
> +       do {
> +               ret = zpool_shrink(pool->zpool, 1, NULL);
> +               if (ret)
> +                       zswap_reject_reclaim_fail++;
> +       } while (!zswap_can_accept() && ret != -EINVAL);
>         zswap_pool_put(pool);
>  }

while I do agree with your points, I have a concern about this
shrinker logic change. The reason for not doing this as you do was
possible real time/responsiveness characteristics degrade. Have you
checked that it's not really the case?

Thanks,
Vitaly

> @@ -1188,7 +1192,7 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
>         if (zswap_pool_reached_full) {
>                if (!zswap_can_accept()) {
>                         ret = -ENOMEM;
> -                       goto reject;
> +                       goto shrink;
>                 } else
>                         zswap_pool_reached_full = false;
>         }
> --
> 2.34.1
>