Re: [PATCH v6] zswap: replace RB tree with xarray

[Nhat Pham]

On Wed, Mar 13, 2024 at 12:31 AM Chris Li <chrisl@xxxxxxxxxx> wrote:
>
> Very deep RB tree requires rebalance at times. That
> contributes to the zswap fault latencies. Xarray does not
> need to perform tree rebalance. Replacing RB tree to xarray
> can have some small performance gain.
>
> One small difference is that xarray insert might fail with
> ENOMEM, while RB tree insert does not allocate additional
> memory.
>
> The zswap_entry size will reduce a bit due to removing the
> RB node, which has two pointers and a color field. Xarray
> store the pointer in the xarray tree rather than the
> zswap_entry. Every entry has one pointer from the xarray
> tree. Overall, switching to xarray should save some memory,
> if the swap entries are densely packed.
>
> Notice the zswap_rb_search and zswap_rb_insert always
> followed by zswap_rb_erase. Use xa_erase and xa_store
> directly. That saves one tree lookup as well.
>
> Remove zswap_invalidate_entry due to no need to call
> zswap_rb_erase any more. Use zswap_free_entry instead.
>
> The "struct zswap_tree" has been replaced by "struct xarray".
> The tree spin lock has transferred to the xarray lock.
>
> Run the kernel build testing 10 times for each version, averages:
> (memory.max=2GB, zswap shrinker and writeback enabled,
> one 50GB swapfile, 24 HT core, 32 jobs)
>
> mm-9a0181a3710eb             xarray v5
> user       3532.385                     3535.658
> sys        536.231                      530.083
> real       200.431                      200.176
>
> ---
>
>
> Reviewed-by: Barry Song <baohua@xxxxxxxxxx>
> Reviewed-by: Chengming Zhou <chengming.zhou@xxxxxxxxx>
> Acked-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> Signed-off-by: Chris Li <chrisl@xxxxxxxxxx>

Apologies for the delayed review :)

LGTM FWIW. Looks like you're sending a fixlet to address Johannes'
comments, but I assume it won't change too much, so feel free to
include:

Reviewed-by: Nhat Pham <nphamcs@xxxxxxxxx>