Re: [PATCH v1 0/3] mm: zswap: global shrinker fix and proactive shrink

[Nhat Pham <nphamcs@gmail.com>]

On Sat, Jun 8, 2024 at 8:53 AM Takero Funaki <flintglass@gmail.com> wrote:
>
> This series addresses two issues and introduces a minor improvement in
> zswap global shrinker:

By the way, what is your current setup?

This global shrinker loop should only be run when the global pool
limit is hit. That *never* happens to us in production, even with the
zswap shrinker disabled.

The default pool limit is 20% of memory, which is quite a lot,
especially if anonymous memory is well-compressed and/or has a lot of
zero pages (which do not count towards the limit).

>
> 1. Fix the memcg iteration logic that breaks iteration on offline memcgs.
> 2. Fix the error path that aborts on expected error codes.
> 3. Add proactive shrinking at 91% full, for 90% accept threshold.
>
> These patches need to be applied in this order to avoid potential loops
> caused by the first issue. Patch 3 can be applied independently, but the
> two issues must be resolved to ensure the shrinker can evict pages.
>
> Previously, the zswap pool could be filled with old pages that the
> shrinker failed to evict, leading to zswap rejecting new pages. With
> this series applied, the shrinker will continue to evict pages until the
> pool reaches the accept_thr_percent threshold proactively, as
> documented, and maintain the pool to keep recent pages.
>
> As a side effect of changes in the hysteresis logic, zswap will no
> longer reject pages under the max pool limit.
>
> With this series, reclaims smaller than the proative shrinking amount
> finish instantly and trigger background shrinking. Admins can check if
> new pages are buffered by zswap by monitoring the pool_limit_hit
> counter.
>
> Changes since v0:
> mm: zswap: fix global shrinker memcg iteration
> - Drop and reacquire spinlock before skipping a memcg.
> - Add some comment to clarify the locking mechanism.
> mm: zswap: proactive shrinking before pool size limit is hit
> - Remove unneeded check before scheduling work.
> - Change shrink start threshold to accept_thr_percent + 1%.
>
> Now it starts shrinking at accept_thr_percent + 1%. Previously, the
> threshold was at the midpoint of 100% to accept_threshold.
>
> If a workload needs 10% space to buffer the average reclaim amount, with
> the previous patch, it required setting the accept_thr_percent to 80%.
> For 50%, it became 0%, which is not acceptable and unclear for admins.
> We can use the accept percent as the shrink threshold directly but that
> sounds shrinker is called too frequently around the accept threshold.  I
> added 1% as a minimum gap to the shrink threshold.
>
> ----
>
> Takero Funaki (3):
>   mm: zswap: fix global shrinker memcg iteration
>   mm: zswap: fix global shrinker error handling logic
>   mm: zswap: proactive shrinking before pool size limit is hit
>
>  Documentation/admin-guide/mm/zswap.rst |  17 ++-
>  mm/zswap.c                             | 172 ++++++++++++++++++-------
>  2 files changed, 136 insertions(+), 53 deletions(-)
>
> --
> 2.43.0
>