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

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Message-ID: <CAF8kJuMegmMD3+YZfvsW3h1y1z1-kH7SiyJHPVnrSc89OZASuA@mail.gmail.com>
Date:   Wed, 6 Dec 2023 16:43:58 -0800
From:   Chris Li <chriscli@...gle.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Nhat Pham <nphamcs@...il.com>,
        Chengming Zhou <zhouchengming@...edance.com>,
        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

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

> 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.

Stay tuned.

Chris