Re: [PATCH v3] mm: zswap: shrink until can accept

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, May 26, 2023 at 08:32:27PM +0200, Domenico Cerasuolo 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
> 
> V2:
> - loop against == -EAGAIN rather than != -EINVAL and also break the loop
> on MAX_RECLAIM_RETRIES (thanks Yosry)
> - cond_resched() to ensure that the loop doesn't burn the cpu (thanks
> Vitaly)
> 
> V3:
> - fix wrong loop break, should continue on !ret (thanks Johannes)
> 
> Fixes: 45190f01dd40 ("mm/zswap.c: add allocation hysteresis if pool limit is hit")
> Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>