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References: <20240117-zswap-xarray-v1-0-6daa86c08fae@kernel.org> <CAKEwX=PB0=-ecJwv1=JgnmRJJdPw5f0jnRp7jwusVG9pd=C5sA@mail.gmail.com>
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From: Chris Li <chriscli@google.com>
Date: Thu, 18 Jan 2024 21:14:14 -0800
Message-ID: <CAF8kJuP_ViuB08=uUWv-dM=FrofsYNuPY8hJQ7Ps5o7j5vzfWQ@mail.gmail.com>
Subject: Re: [PATCH 0/2] RFC: zswap tree use xarray instead of RB tree
To: Nhat Pham <nphamcs@gmail.com>
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 Thu, Jan 18, 2024 at 10:01=E2=80=AFAM Nhat Pham <nphamcs@gmail.com> wrot=
e:
>
> On Wed, Jan 17, 2024 at 7:06=E2=80=AFPM Chris Li <chrisl@kernel.org> wrot=
e:
> >
> > 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 :)

Ack.

Chris