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

[Barry Song]

On Sat, Mar 8, 2025 at 12:03 PM Nhat Pham <nphamcs@xxxxxxxxx> wrote:
>
> On Fri, Mar 7, 2025 at 4:02 AM Qun-Wei Lin <qun-wei.lin@xxxxxxxxxxxx> 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