Re: [PATCH 2/2] mm/zswap: plug the writeback IO of an LRU walk

[Nhat Pham]

On Wed, Oct 7, 2026 at 5:37 PM Alexandre Ghiti <alex@xxxxxxxx> wrote:
>
> zswap writeback only appends an entry to the pending bio when its slot
> directly follows the previous one, and submits the pending bio
> otherwise.
>
> Plug the LRU walk so that the block layer can merge what the batch
> cannot.
>
> Kernel build (defconfig, -j4) in a 600M memory.max cgroup with the zswap
> shrinker enabled, swapping to an NVMe partition with iocost enabled. It
> runs alone, and next to a sibling fio random writer with 10 times its
> io.weight so that iocost throttles it. Mean +- stddev, counters from the
> build's cgroup:
>
> Alone:
>
>                                 unplugged       plugged
>   build time (s)                1061 +- 2     1057 +- 2   -0.4%
>   write requests (k)            622 +- 34     579 +- 21   -6.8%
>   pages per write request    1.30 +- 0.02  1.36 +- 0.05   +4.9%
>   iocost debt (s)             14.0 +- 2.5   11.0 +- 1.7   -21.6%
>   iocost wait (s)              7.7 +- 1.8    7.1 +- 1.1   -7.9%
>   IO full pressure (s)        18.3 +- 1.4   17.5 +- 1.3   -4.5%
>
> Next to the writer:
>
>                                 unplugged       plugged
>   build time (s)               2188 +- 45    2104 +- 75   -3.8%
>   write requests (k)            594 +- 21     567 +- 24   -4.6%
>   pages per write request    1.30 +- 0.01  1.35 +- 0.02   +3.6%
>   iocost debt (s)               648 +- 22     645 +- 29   -0.5%
>   iocost wait (s)               624 +- 23     633 +- 29   +1.5%
>   IO full pressure (s)          615 +- 31     550 +- 44   -10.6%
>
> Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>

Acked-by: Nhat Pham <nphamcs@xxxxxxxxx>