linux-kernel - Re: [PATCH v2 2/2] mm/zswap: split zswap rb-tree

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Date: Mon, 22 Jan 2024 11:49:05 -0800
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Chengming Zhou <zhouchengming@...edance.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Nhat Pham <nphamcs@...il.com>, 
	Chris Li <chriscli@...gle.com>, linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH v2 2/2] mm/zswap: split zswap rb-tree

On Fri, Jan 19, 2024 at 3:22 AM Chengming Zhou
<zhouchengming@...edance.com> 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.

If/when you respin this, can you mention that testing was done with a
single swapfile? I assume the improvements will be less with multiple
swapfiles as lock contention should be better.

>
> Acked-by: Johannes Weiner <hannes@...xchg.org>
> Acked-by: Nhat Pham <nphamcs@...il.com>

I think the diff in zswap_swapoff() should be much simpler with the
tree(s) cleanup removed. Otherwise LGTM.

Acked-by: Yosry Ahmed <yosryahmed@...gle.com>