Re: [PATCH 2/2] kcompressd: Add Kcompressd for accelerated zram compression

[Hillf Danton]

On Fri, 7 Mar 2025 15:13:12 -0800 Nhat Pham <nphamcs@xxxxxxxxx>
> On Fri, Mar 7, 2025 at 11:41 AM Barry Song <21cnbao@xxxxxxxxx> wrote:
> > On Sat, Mar 8, 2025 at 1:02 AM Qun-Wei Lin <qun-wei.lin@xxxxxxxxxxxx> wrote:
> > >
> > > Introduced Kcompressd to offload zram page compression, improving
> > > system efficiency by handling compression separately from memory
> > > reclaiming. Added necessary configurations and dependencies.
> > >
> > > Signed-off-by: Qun-Wei Lin <qun-wei.lin@xxxxxxxxxxxx>
> > > ---
> > >  drivers/block/zram/Kconfig      |  11 ++
> > >  drivers/block/zram/Makefile     |   3 +-
> > >  drivers/block/zram/kcompressd.c | 340 ++++++++++++++++++++++++++++++++
> > >  drivers/block/zram/kcompressd.h |  25 +++
> > >  drivers/block/zram/zram_drv.c   |  22 ++-
> > >  5 files changed, 397 insertions(+), 4 deletions(-)
> > >  create mode 100644 drivers/block/zram/kcompressd.c
> > >  create mode 100644 drivers/block/zram/kcompressd.h
> > >
> > > diff --git a/drivers/block/zram/Kconfig b/drivers/block/zram/Kconfig
> > > index 402b7b175863..f0a1b574f770 100644
> > > --- a/drivers/block/zram/Kconfig
> > > +++ b/drivers/block/zram/Kconfig
> > > @@ -145,3 +145,14 @@ config ZRAM_MULTI_COMP
> > >           re-compress pages using a potentially slower but more effective
> > >           compression algorithm. Note, that IDLE page recompression
> > >           requires ZRAM_TRACK_ENTRY_ACTIME.
> > > +
> > > +config KCOMPRESSD
> > > +       tristate "Kcompressd: Accelerated zram compression"
> > > +       depends on ZRAM
> > > +       help
> > > +         Kcompressd creates multiple daemons to accelerate the compression of pages
> > > +         in zram, offloading this time-consuming task from the zram driver.
> > > +
> > > +         This approach improves system efficiency by handling page compression separately,
> > > +         which was originally done by kswapd or direct reclaim.
> >
> > For direct reclaim, we were previously able to compress using multiple CPUs
> > with multi-threading.
> > After your patch, it seems that only a single thread/CPU is used for compression
> > so it won't necessarily improve direct reclaim performance?
> >
> > Even for kswapd, we used to have multiple threads like [kswapd0], [kswapd1],
> > and [kswapd2] for different nodes. Now, are we also limited to just one thread?
> > I also wonder if this could be handled at the vmscan level instead of the zram
> > level. then it might potentially help other sync devices or even zswap later.
> 
> Agree. A shared solution would be much appreciated. We can keep the
> kcompressd idea, but have it accept IO work from multiple sources
> (zram, zswap, whatever) through a shared API.
> 
> Otherwise we would need to reinvent the wheel multiple times :)
>
Could you explain what nr_kcompressd means, Qun-Wei, to quiesce barking lads?