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

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 30 Apr 2025 16:26:41 +0800 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.
> 
> 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.

It's a significant change and I'm thinking that broader performance
testing across a broader range of machines is needed before we can
confidently upstream such a change.

Also, it's presumably a small net loss on single-CPU machines (do these
exist any more?).  Is it hard to disable this feature on such machines?

>  
> +static bool swap_sched_async_compress(struct folio *folio)
> +{
> +	struct swap_info_struct *sis = swp_swap_info(folio->swap);
> +	int nid = numa_node_id();
> +	pg_data_t *pgdat = NODE_DATA(nid);
> +
> +	if (unlikely(!pgdat->kcompressd))
> +		return false;
> +
> +	if (!current_is_kswapd())
> +		return false;
> +
> +	if (!folio_test_anon(folio))
> +		return false;

Are you sure the above three tests are really needed?