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

[Sergey Senozhatsky]

On (25/03/08 18:41), Barry Song wrote:
> 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.

If compression kthread-s can run (have CPUs to be scheduled on).
This looks a bit like a bottleneck.  Is there anything that
guarantees forward progress?  Also, if compression kthreads
constantly preempt kswapd, then it might not be worth it to
have compression kthreads, I assume?

If we have a pagefault and need to map a page that is still in
the compression queue (not compressed and stored in zram yet, e.g.
dut to scheduling latency + slow compression algorithm) then what
happens?