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

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Message-ID: <CAGsJ_4ysL1xV=902oNM3vBfianF6F_iqDgyck6DGzFrZCtOprw@mail.gmail.com>
Date: Sat, 8 Mar 2025 18:41:33 +1300
From: Barry Song <21cnbao@...il.com>
To: Nhat Pham <nphamcs@...il.com>
Cc: Qun-Wei Lin <qun-wei.lin@...iatek.com>, Jens Axboe <axboe@...nel.dk>, 
	Minchan Kim <minchan@...nel.org>, Sergey Senozhatsky <senozhatsky@...omium.org>, 
	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 Sat, Mar 8, 2025 at 12:03 PM Nhat Pham <nphamcs@...il.com> wrote:
>
> On Fri, Mar 7, 2025 at 4:02 AM Qun-Wei Lin <qun-wei.lin@...iatek.com> 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.
>
> Please excuse my ignorance, but from your cover letter I still don't
> quite get what is the problem here? And how would decouple compression
> and scanning help?

My understanding is as follows:

When kswapd attempts to reclaim M anonymous folios and N file folios,
the process involves the following steps:

* t1: Time to scan and unmap anonymous folios
* t2: Time to compress anonymous folios
* t3: Time to reclaim file folios

Currently, these steps are executed sequentially, meaning the total time
required to reclaim M + N folios is t1 + t2 + t3.

However, Qun-Wei's patch enables t1 + t3 and t2 to run in parallel,
reducing the total time to max(t1 + t3, t2). This likely improves the
reclamation speed, potentially reducing allocation stalls.

I don’t have concrete data on this. Does Qun-Wei have detailed
performance data?

>
> >
> > 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,
>
> What bottleneck are we talking about? Is one stage slower than the other?
>
> >  especially under high memory pressure. The kswapd thread becomes a
> >  single point of contention, causing delays in memory reclaiming and
> >  overall system performance degradation.
> >
> > Target:
> >  The target of this invention is to improve the efficiency of memory
> >  reclaiming. By separating the tasks of page scanning and page
> >  compression into distinct processes or threads, the system can handle
> >  memory pressure more effectively.
>
> I'm not a zram maintainer, so I'm definitely not trying to stop this
> patch. But whatever problem zram is facing will likely occur with
> zswap too, so I'd like to learn more :)

Right, this is likely something that could be addressed more generally
for zswap and zram.

Thanks
Barry