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From: Nhat Pham <nphamcs@gmail.com>
Date: Thu, 18 Jan 2024 10:01:21 -0800
Message-ID: <CAKEwX=PB0=-ecJwv1=JgnmRJJdPw5f0jnRp7jwusVG9pd=C5sA@mail.gmail.com>
Subject: Re: [PATCH 0/2] RFC: zswap tree use xarray instead of RB tree
To: Chris Li <chrisl@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org, 
	linux-mm@kvack.org, =?UTF-8?B?V2VpIFh177+8?= <weixugc@google.com>, 
	Yu Zhao <yuzhao@google.com>, Greg Thelen <gthelen@google.com>, 
	Chun-Tse Shao <ctshao@google.com>, =?UTF-8?Q?Suren_Baghdasaryan=EF=BF=BC?= <surenb@google.com>, 
	Yosry Ahmed <yosryahmed@google.com>, Brain Geffon <bgeffon@google.com>, 
	Minchan Kim <minchan@kernel.org>, Michal Hocko <mhocko@suse.com>, 
	Mel Gorman <mgorman@techsingularity.net>, Huang Ying <ying.huang@intel.com>, 
	Johannes Weiner <hannes@cmpxchg.org>, Kairui Song <kasong@tencent.com>, 
	Zhongkun He <hezhongkun.hzk@bytedance.com>, Kemeng Shi <shikemeng@huaweicloud.com>, 
	Barry Song <v-songbaohua@oppo.com>, "Matthew Wilcox (Oracle)" <willy@infradead.org>, 
	"Liam R. Howlett" <Liam.Howlett@oracle.com>, Joel Fernandes <joel@joelfernandes.org>, 
	Chengming Zhou <zhouchengming@bytedance.com>
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On Wed, Jan 17, 2024 at 7:06=E2=80=AFPM Chris Li <chrisl@kernel.org> wrote:
>
> The RB tree shows some contribution to the swap fault
> long tail latency due to two factors:
> 1) RB tree requires re-balance from time to time.
> 2) The zswap RB tree has a tree level spin lock protecting
> the tree access.
>
> The swap cache is using xarray. The break down the swap
> cache access does not have the similar long time as zswap
> RB tree.
>
> Moving the zswap entry to xarray enable read side
> take read RCU lock only.
>
> The first patch adds the xarray alongside the RB tree.
> There is some debug check asserting the xarray agrees with
> the RB tree results.
>
> The second patch removes the zwap RB tree.
>
> I expect to merge the zswap rb tree spin lock with the xarray
> lock in the follow up changes.
>
> I can surely use some help in reviewing and testing.
>
> Signed-off-by: Chris Li <chrisl@kernel.org>
> ---
> Chris Li (2):
>       mm: zswap.c: add xarray tree to zswap

While I think it is pretty neat to keep the rbtree around to check if
the results agree during development stages, in the final version
please squash the patches. One patch is enough :)

>       mm: zswap.c: remove RB tree
>
>  mm/zswap.c | 120 ++++++++++++++++++++++++++++++-------------------------=
------
>  1 file changed, 59 insertions(+), 61 deletions(-)
> ---
> base-commit: d7ba3d7c3bf13e2faf419cce9e9bdfc3a1a50905
> change-id: 20240104-zswap-xarray-716260e541e3
>
> Best regards,
> --
> Chris Li <chrisl@kernel.org>
>