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

[Barry Song <21cnbao@gmail.com>]

On Sat, Mar 8, 2025 at 1:02 AM Qun-Wei Lin <qun-wei.lin@mediatek.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.
>
> 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.
>
> 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.

Sounds great. However, we also have a time window where folios under
writeback are kept, whereas previously, writeback was done synchronously
without your patch. This may temporarily increase memory usage until the
kept folios are re-scanned.

So, you’ve observed that folio_rotate_reclaimable() runs shortly while the
async thread completes compression? Then the kept folios are shortly
re-scanned?

>
> Patch 1:
> - Introduces 2 new feature flags, BLK_FEAT_READ_SYNCHRONOUS and
>   SWP_READ_SYNCHRONOUS_IO.
>
> Patch 2:
> - Implemented the core functionality of Kcompressd and made necessary
>   modifications to the zram driver to support it.
>
> In our handheld devices, we found that applying this mechanism under high
> memory pressure scenarios can increase the rate of pgsteal_anon per second
> by over 260% compared to the situation with only kswapd.

Sounds really great.

What compression algorithm is being used? I assume that after switching to a
different compression algorithms, the benefits will change significantly. For
example, Zstd might not show as much improvement.
How was the CPU usage ratio between page scan/unmap and compression
observed before applying this patch?

>
> Qun-Wei Lin (2):
>   mm: Split BLK_FEAT_SYNCHRONOUS and SWP_SYNCHRONOUS_IO into separate
>     read and write flags
>   kcompressd: Add Kcompressd for accelerated zram compression
>
>  drivers/block/brd.c             |   3 +-
>  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   |  21 +-
>  drivers/nvdimm/btt.c            |   3 +-
>  drivers/nvdimm/pmem.c           |   5 +-
>  include/linux/blkdev.h          |  24 ++-
>  include/linux/swap.h            |  31 +--
>  mm/memory.c                     |   4 +-
>  mm/page_io.c                    |   6 +-
>  mm/swapfile.c                   |   7 +-
>  13 files changed, 446 insertions(+), 37 deletions(-)
>  create mode 100644 drivers/block/zram/kcompressd.c
>  create mode 100644 drivers/block/zram/kcompressd.h
>
> --
> 2.45.2
>

Thanks
Barry