Re: [PATCH 1/2] mm/zswap: batch the writeback IO of consecutive entries

[Nhat Pham]

On Wed, Oct 7, 2026 at 5:36 PM Alexandre Ghiti <alex@xxxxxxxx> wrote:
>
> Commit 8f29aa226f82 ("mm/swap: introduce struct swap_io_ctx") introduced a
> simple way to batch the swap writes of folios with consecutive slots into a
> single bio. zswap writeback does not use it: zswap_writeback_entry()
> submits a context of its own for every entry, so every entry written back
> becomes its own IO.
>
> So let the two functions that walk the zswap LRU, zswap_shrinker_scan()
> and shrink_memcg(), own the context and submit it once the walk is done.
>
> 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:

Number looks good to me!

>
> Alone:
>
>                                 unbatched       batched
>   build time (s)                1066 +- 9     1061 +- 2   -0.5%
>   write requests (k)            748 +- 47     596 +- 26   -20.4%
>   pages per write request    1.07 +- 0.01  1.30 +- 0.01   +21.2%
>   iocost debt (s)             15.0 +- 2.7   12.1 +- 1.2   -19.5%
>   iocost wait (s)              7.5 +- 1.2    7.2 +- 1.1   -3.6%
>   IO full pressure (s)        19.0 +- 1.8   18.2 +- 0.9   -4.2%
>
> Next to the writer:
>
>                                 unbatched       batched
>   build time (s)               2269 +- 50    2203 +- 62   -2.9%
>   write requests (k)            714 +- 31     605 +- 24   -15.2%
>   pages per write request    1.08 +- 0.00  1.30 +- 0.02   +20.3%

The improvement is pretty big, but still only 1.3 :(

Is this because writeback entries are not contiguous in physical swap
space? Physical swap allocation algorithm has a bias for allocating
contiguous slots, but by writeback time we might have already punched
a bunch of holes (refaults etc.).

Ironically, vswap might help a bit here, because we defer physical
swap space allocation to right before writeback time... But who knows
:/

Anyway, I'll take the 20% :)

>   iocost debt (s)               685 +- 25     646 +- 26   -5.7%
>   iocost wait (s)               655 +- 26     632 +- 23   -3.6%
>   IO full pressure (s)          655 +- 31     624 +- 30   -4.7%
>
> Suggested-by: Nhat Pham <nphamcs@xxxxxxxxx>
> Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>

Looks good to me FWIW:

Acked-by: Nhat Pham <nphamcs@xxxxxxxxx>