linux-kernel - Re: [PATCH 0/7] mm/zswap: optimize the scalability of zswap rb-tree

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Date:   Thu, 7 Dec 2023 23:18:09 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     Nhat Pham <nphamcs@...il.com>
Cc:     Vitaly Wool <vitaly.wool@...sulko.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Seth Jennings <sjenning@...hat.com>,
        Dan Streetman <ddstreet@...e.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yosry Ahmed <yosryahmed@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/7] mm/zswap: optimize the scalability of zswap rb-tree

On 2023/12/7 11:13, Chengming Zhou wrote:
> On 2023/12/7 04:08, Nhat Pham wrote:
>> On Wed, Dec 6, 2023 at 1:46 AM Chengming Zhou
>> <zhouchengming@...edance.com> wrote:
>>> When testing the zswap performance by using kernel build -j32 in a tmpfs
>>> directory, I found the scalability of zswap rb-tree is not good, which
>>> is protected by the only spinlock. That would cause heavy lock contention
>>> if multiple tasks zswap_store/load concurrently.
>>>
>>> So a simple solution is to split the only one zswap rb-tree into multiple
>>> rb-trees, each corresponds to SWAP_ADDRESS_SPACE_PAGES (64M). This idea is
>>> from the commit 4b3ef9daa4fc ("mm/swap: split swap cache into 64MB trunks").
>>>
>>> Although this method can't solve the spinlock contention completely, it
>>> can mitigate much of that contention.
>>
>> By how much? Do you have any stats to estimate the amount of
>> contention and the reduction by this patch?
> 
> Actually, I did some test using the linux-next 20231205 yesterday.
> 
> Testcase: memory.max = 2G, zswap enabled, make -j32 in tmpfs.
> 
> 			20231205	+patchset
> 1. !shrinker_enabled:   156s		126s
> 2.  shrinker_enabled:   79s		70s
> 
> I think your zswap shrinker fix patch can solve !shrinker_enabled case.
> 
> So will test again today using the new mm-unstable branch.
> 

Updated test data based on today's mm-unstable branch:

			mm-unstable	+patchset
1. !shrinker_enabled:	86s		74s
2.  shrinker_enabled:	63s		61s

Shows much less optimization for the shrinker_enabled case, but still
much optimization for the !shrinker_enabled case.

Thanks!