On Fri, May 26, 2023 at 8:32 PM Domenico Cerasuolo
<cerasuolodomenico@xxxxxxxxx> 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@xxxxxxxxx>
Reviewed-by: Vitaly Wool <vitaly.wool@xxxxxxxxxxxx>
> ---
> mm/zswap.c | 17 ++++++++++++++---
> 1 file changed, 14 insertions(+), 3 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 59da2a415fbb..bcb82e09eb64 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -37,6 +37,7 @@
> #include <linux/workqueue.h>
>
> #include "swap.h"
> +#include "internal.h"
>
> /*********************************
> * statistics
> @@ -587,9 +588,19 @@ static void shrink_worker(struct work_struct *w)
> {
> struct zswap_pool *pool = container_of(w, typeof(*pool),
> shrink_work);
> + int ret, failures = 0;
>
> - 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++;
> + if (ret != -EAGAIN)
> + break;
> + if (++failures == MAX_RECLAIM_RETRIES)
> + break;
> + }
> + cond_resched();
> + } while (!zswap_can_accept());
> zswap_pool_put(pool);
> }
>
> @@ -1188,7 +1199,7 @@ 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
>
