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

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Date: Tue, 23 Jan 2024 15:46:25 +0800
From: Chengming Zhou <zhouchengming@...edance.com>
To: Yosry Ahmed <yosryahmed@...gle.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 2024/1/23 03:49, Yosry Ahmed wrote:
> 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.
> 

Ok. Not sure how much improvement, may do some tests later.

>>
>> 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>

Right, thanks!