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

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Date:   Thu, 7 Dec 2023 11:25:13 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     Chris Li <chriscli@...gle.com>,
        Yosry Ahmed <yosryahmed@...gle.com>,
        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>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH 0/7] mm/zswap: optimize the scalability of zswap rb-tree

On 2023/12/7 08:43, Chris Li wrote:
> Hi Nhat and Yosry,
> 
> On Wed, Dec 6, 2023 at 12:42 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>>
>> On Wed, Dec 6, 2023 at 9:24 AM Nhat Pham <nphamcs@...il.com> wrote:
>>>
>>> + Chris Li
>>>
>>> Chris, I vaguely remember from our last conversation that you have
>>> some concurrent efforts to use xarray here right?
> 
> Yes, I do have the zswap xarray for older versions of the kernel. The
> recent mm-unstable tree has  a lot of zswap related updates. Give me 2
> days to refresh and post it. The zswap invalid entry and the reference
> count change is causing a good portion of the code to be updated. That
> is the price to pay keeping out of tree patches. My fault is not
> getting to it sooner.
> 
>>
>> If I recall correctly, the xarray already reduces the lock contention
>> as lookups are lockless, but Chris knows more here. As you mentioned
> 
> Yes. To be exact, xarray can use spin lock (same as current RB tree)
> or take RCU read lock on the lookup path (depending on how you call
> the xarray API). Not completely lockless but the RCU read lock should
> have less lock contention than normal spinlock. +Matthew
> 

Great! Lockless lookup in zswap_load() should reduce spinlock contention.
And multiple trees (multiple xarrays) can further reduce the contention
on the concurrent zswap_store() side. So it's complementary IMHO.

>> in a different email, it would be nice to get some data so that we can
>> compare different solutions.
> 
> Yes, it is certainly welcome to see more data points. If I recall
> correctly, the zswap xarray array makes the lookup similar to the swap
> cache lookup. It has a noticeable difference in the long tail end.
> 

Right, I post some data from yesterday in another reply.
Will test again and update the data since Nhat's zswap shrinker fix patch
has been merged into mm-unstable today.

Thanks!