linux-kernel - Re: [PATCH v5] zswap: replace RB tree with xarray

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Message-ID: <CAGsJ_4z1CTzBHLdpyyHpwhQeXmN7dXbsoHJ1zcXFS2x=D2Xedg@mail.gmail.com>
Date: Tue, 12 Mar 2024 06:52:21 +0800
From: Barry Song <21cnbao@...il.com>
To: Chris Li <chrisl@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, Yosry Ahmed <yosryahmed@...gle.com>, Nhat Pham <nphamcs@...il.com>, 
	Johannes Weiner <hannes@...xchg.org>, "Matthew Wilcox (Oracle)" <willy@...radead.org>, 
	Chengming Zhou <zhouchengming@...edance.com>, Barry Song <v-songbaohua@...o.com>
Subject: Re: [PATCH v5] zswap: replace RB tree with xarray

On Tue, Mar 12, 2024 at 6:26 AM Chris Li <chrisl@...nel.org> 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 The "struct zswap_tree" has been replaced by "struct xarray".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
>
> ---
>
>
> Signed-off-by: Chris Li <chrisl@...nel.org>
> ---
> Changes in v5:
> - Remove zswap_xa_insert(), call xa_store and xa_erase directly.
> - Remove zswap_reject_xarray_fail.
> - Link to v4: https://lore.kernel.org/r/20240304-zswap-xarray-v4-1-c4b45670cc30@kernel.org
>
> Changes in v4:
> - Remove zswap_xa_search_and_earse, use xa_erase directly.
> - Move charge of objcg after zswap_xa_insert.
> - Avoid erase old entry on insert fail error path.
> - Remove not needed swap_zswap_tree change
> - Link to v3: https://lore.kernel.org/r/20240302-zswap-xarray-v3-1-5900252f2302@kernel.org
>
> Changes in v3:
> - Use xa_cmpxchg instead of zswap_xa_search_and_delete in zswap_writeback_entry.
> - Use xa_store in zswap_xa_insert directly. Reduce the scope of spinlock.
> - Fix xa_store error handling for same page fill case.
> - Link to v2: https://lore.kernel.org/r/20240229-zswap-xarray-v2-1-e50284dfcdb1@kernel.org
>
> Changes in v2:
> - Replace struct zswap_tree with struct xarray.
> - Remove zswap_tree spinlock, use xarray lock instead.
> - Fold zswap_rb_erase() into zswap_xa_search_and_delete() and zswap_xa_insert().
> - Delete zswap_invalidate_entry(), use zswap_free_entry() instead.
> - Link to v1: https://lore.kernel.org/r/20240117-zswap-xarray-v1-0-6daa86c08fae@kernel.org
> ---
>  mm/zswap.c | 166 +++++++++++++++----------------------------------------------
>  1 file changed, 41 insertions(+), 125 deletions(-)

It appears to be a suitable figure. And LGTM,

Reviewed-by: Barry Song <baohua@...nel.org>

Thanks
Barry