Re: [PATCH 0/2] Improve Zram by separating compression context from kswapd

[Barry Song <baohua@kernel.org>]

On Thu, Mar 13, 2025 at 4:09 PM Sergey Senozhatsky
<senozhatsky@chromium.org> wrote:
>
> On (25/03/12 11:11), Minchan Kim wrote:
> > On Fri, Mar 07, 2025 at 08:01:02PM +0800, Qun-Wei Lin wrote:
> > > This patch series introduces a new mechanism called kcompressd to
> > > improve the efficiency of memory reclaiming in the operating system. The
> > > main goal is to separate the tasks of page scanning and page compression
> > > into distinct processes or threads, thereby reducing the load on the
> > > kswapd thread and enhancing overall system performance under high memory
> > > pressure conditions.
> > >
> > > Problem:
> > >  In the current system, the kswapd thread is responsible for both
> > >  scanning the LRU pages and compressing pages into the ZRAM. This
> > >  combined responsibility can lead to significant performance bottlenecks,
> > >  especially under high memory pressure. The kswapd thread becomes a
> > >  single point of contention, causing delays in memory reclaiming and
> > >  overall system performance degradation.
> >
> > Isn't it general problem if backend for swap is slow(but synchronous)?
> > I think zram need to support asynchrnous IO(can do introduce multiple
> > threads to compress batched pages) and doesn't declare it's
> > synchrnous device for the case.
>
> The current conclusion is that kcompressd will sit above zram,
> because zram is not the only compressing swap backend we have.

also. it is not good to hack zram to be aware of if it is kswapd
, direct reclaim , proactive reclaim and block device with
mounted filesystem.

so i am thinking sth as below

page_io.c

if (sync_device or zswap_enabled())
   schedule swap_writepage to a separate per-node thread

btw,  ran the current patchset with one thread(not default 4)
on phones and saw 50%+ allocstall reduction. so the idea
looks like a good direction to go.

Thanks
Barry