Re: [PATCH] mm: Add Kcompressd for accelerated memory compression

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Apr 30, 2025 at 04:26:41PM +0800, Qun-Wei Lin wrote:
> This patch series introduces a new mechanism called kcompressd to
> improve the efficiency of memory reclaiming in the operating system.
> 
> Problem:
>   In the current system, the kswapd thread is responsible for both scanning
>   the LRU pages and handling memory compression tasks (such as those
>   involving ZSWAP/ZRAM, if enabled). 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.
> 
> Solution:
>   Introduced kcompressd to handle asynchronous compression during memory
>   reclaim, improving efficiency by offloading compression tasks from
>   kswapd. This allows kswapd to focus on its primary task of page reclaim
>   without being burdened by the additional overhead of compression.
> 
> 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. Additionally, we
> observed a reduction of over 50% in page allocation stall occurrences,
> further demonstrating the effectiveness of kcompressd in alleviating memory
> pressure and improving system responsiveness.

Yes, I think parallelizing this work makes a lot of sense.

> Co-developed-by: Barry Song <21cnbao@gmail.com>
> Signed-off-by: Barry Song <21cnbao@gmail.com>
> Signed-off-by: Qun-Wei Lin <qun-wei.lin@mediatek.com>
> Reference: Re: [PATCH 0/2] Improve Zram by separating compression context from kswapd - Barry Song
>            https://lore.kernel.org/lkml/20250313093005.13998-1-21cnbao@gmail.com/
> ---
>  include/linux/mmzone.h |  6 ++++
>  mm/mm_init.c           |  1 +
>  mm/page_io.c           | 71 ++++++++++++++++++++++++++++++++++++++++++
>  mm/swap.h              |  6 ++++
>  mm/vmscan.c            | 25 +++++++++++++++
>  5 files changed, 109 insertions(+)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 6ccec1bf2896..93c9195a54ae 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -23,6 +23,7 @@
>  #include <linux/page-flags.h>
>  #include <linux/local_lock.h>
>  #include <linux/zswap.h>
> +#include <linux/kfifo.h>
>  #include <asm/page.h>
>  
>  /* Free memory management - zoned buddy allocator.  */
> @@ -1398,6 +1399,11 @@ typedef struct pglist_data {
>  
>  	int kswapd_failures;		/* Number of 'reclaimed == 0' runs */
>  
> +#define KCOMPRESS_FIFO_SIZE 256
> +	wait_queue_head_t kcompressd_wait;
> +	struct task_struct *kcompressd;
> +	struct kfifo kcompress_fifo;

The way you implemented this adds time-and-space overhead even on
systems that don't have any sort of swap compression enabled.

That seems unnecessary. There is an existing method for asynchronous
writeback, and pageout() is naturally fully set up to handle this.

IMO the better way to do this is to make zswap_store() (and
zram_bio_write()?) asynchronous. Make those functions queue the work
and wake the compression daemon, and then have the daemon call
folio_end_writeback() / bio_endio() when it's done with it.