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

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Date:   Fri, 8 Dec 2023 23:41:33 +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 2023/12/8 02:15, Nhat Pham wrote:
> On Thu, Dec 7, 2023 at 7:18 AM Chengming Zhou
> <zhouchengming@...edance.com> wrote:
>>
>> 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!
> 
> I'm gonna assume this is build time since it makes the zswap shrinker
> look pretty good :)
> I think this just means some of the gains between this patchset and
> the zswap shrinker overlaps. But on the positive note:
> 
> a) Both are complementary, i.e enable both (bottom right corner) gives
> us the best result.

Right, both optimizations are complementary, to make zswap perform better :)

> b) Each individual change improves the runtime. If you disable the
> shrinker, then this patch helps tremendously, so we're onto something.
> c) The !shrinker_enabled is no longer *too* bad - once again, thanks
> for noticing the regression and help me fix it! In fact, every cell
> improves compared to the last run. Woohoo!

It's my pleasure! Thanks!