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

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Message-ID: <CAGsJ_4yw17rLJPgS6CNXfXNVQnv2pf0PnLSA4UVAR1sUWDhP5Q@mail.gmail.com>
Date: Thu, 13 Mar 2025 16:52:22 +1300
From: Barry Song <21cnbao@...il.com>
To: Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: Minchan Kim <minchan@...nel.org>, Qun-Wei Lin <qun-wei.lin@...iatek.com>, 
	Jens Axboe <axboe@...nel.dk>, Vishal Verma <vishal.l.verma@...el.com>, 
	Dan Williams <dan.j.williams@...el.com>, Dave Jiang <dave.jiang@...el.com>, 
	Ira Weiny <ira.weiny@...el.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Matthias Brugger <matthias.bgg@...il.com>, 
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>, Chris Li <chrisl@...nel.org>, 
	Ryan Roberts <ryan.roberts@....com>, "Huang, Ying" <ying.huang@...el.com>, 
	Kairui Song <kasong@...cent.com>, Dan Schatzberg <schatzberg.dan@...il.com>, 
	Al Viro <viro@...iv.linux.org.uk>, linux-kernel@...r.kernel.org, 
	linux-block@...r.kernel.org, nvdimm@...ts.linux.dev, linux-mm@...ck.org, 
	linux-arm-kernel@...ts.infradead.org, linux-mediatek@...ts.infradead.org, 
	Casper Li <casper.li@...iatek.com>, Chinwen Chang <chinwen.chang@...iatek.com>, 
	Andrew Yang <andrew.yang@...iatek.com>, James Hsu <james.hsu@...iatek.com>
Subject: Re: [PATCH 0/2] Improve Zram by separating compression context from kswapd

On Thu, Mar 13, 2025 at 4:09 PM Sergey Senozhatsky
<senozhatsky@...omium.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