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Date: Wed, 20 Mar 2024 06:14:44 -0400
From: Johannes Weiner <hannes@cmpxchg.org>
To: Chris Li <chrisl@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org, Yosry Ahmed <yosryahmed@google.com>,
	Nhat Pham <nphamcs@gmail.com>,
	"Matthew Wilcox (Oracle)" <willy@infradead.org>,
	Chengming Zhou <zhouchengming@bytedance.com>,
	Barry Song <v-songbaohua@oppo.com>
Subject: Re: [PATCH v7] zswap: replace RB tree with xarray
Message-ID: <20240320101444.GC294822@cmpxchg.org>
References: <20240319-zswap-xarray-v7-1-e9a03a049e86@kernel.org>
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In-Reply-To: <20240319-zswap-xarray-v7-1-e9a03a049e86@kernel.org>

On Tue, Mar 19, 2024 at 10:52:26PM -0700, Chris Li 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 The "struct zswap_tree" has been replaced by "struct xarray".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-unstable-a824831a082f     xarray v7
> user       3547.264			3541.509
> sys        531.176                      526.111
> real       200.752                      201.334
> 
> ---
> Reviewed-by: Nhat Pham <nphamcs@gmail.com>
> 
> Signed-off-by: Chris Li <chrisl@kernel.org>

Excellent! With the checkpatch issues fixed,

Acked-by: Johannes Weiner <hannes@cmpxchg.org>