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Date: Thu, 18 Jan 2024 10:11:23 -0500
From: Johannes Weiner <hannes@cmpxchg.org>
To: Chengming Zhou <zhouchengming@bytedance.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Yosry Ahmed <yosryahmed@google.com>, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org, Chris Li <chriscli@google.com>,
	Nhat Pham <nphamcs@gmail.com>
Subject: Re: [PATCH 2/2] mm/zswap: split zswap rb-tree
Message-ID: <20240118151123.GH939255@cmpxchg.org>
References: <20240117-b4-zswap-lock-optimize-v1-0-23f6effe5775@bytedance.com>
 <20240117-b4-zswap-lock-optimize-v1-2-23f6effe5775@bytedance.com>
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In-Reply-To: <20240117-b4-zswap-lock-optimize-v1-2-23f6effe5775@bytedance.com>

On Wed, Jan 17, 2024 at 09:23:19AM +0000, Chengming Zhou wrote:
> Each swapfile has one rb-tree to search the mapping of swp_entry_t to
> zswap_entry, that use a spinlock to protect, which can cause heavy lock
> contention if multiple tasks zswap_store/load concurrently.
> 
> Optimize the scalability problem by splitting the zswap rb-tree into
> multiple rb-trees, each corresponds to SWAP_ADDRESS_SPACE_PAGES (64M),
> just like we did in the swap cache address_space splitting.
> 
> Although this method can't solve the spinlock contention completely, it
> can mitigate much of that contention. Below is the results of kernel build
> in tmpfs with zswap shrinker enabled:
> 
>      linux-next  zswap-lock-optimize
> real 1m9.181s    1m3.820s
> user 17m44.036s  17m40.100s
> sys  7m37.297s   4m54.622s
> 
> So there are clearly improvements.
> 
> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>

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

One minor nit:

> @@ -265,6 +266,10 @@ static bool zswap_has_pool;
>  * helpers and fwd declarations
>  **********************************/
>  
> +#define swap_zswap_tree(entry)					\
> +	(&zswap_trees[swp_type(entry)][swp_offset(entry)	\
> +		>> SWAP_ADDRESS_SPACE_SHIFT])

Make this a static inline function instead?