linux-kernel - Re: [PATCH] mm: zsmalloc: share slab caches for all zsmalloc zpools

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Date: Wed, 5 Jun 2024 11:17:01 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan@...nel.org>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	David Rientjes <rientjes@...gle.com>,
	Christoph Lameter <cl@...ux.com>,
	Erhard Furtner <erhard_f@...lbox.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: zsmalloc: share slab caches for all zsmalloc zpools

On (24/06/04 17:53), Yosry Ahmed wrote:
> Zswap creates multiple zpools to improve concurrency. Each zsmalloc
> zpool creates its own 'zs_handle' and 'zspage' slab caches. Currently we
> end up with 32 slab caches of each type.
> 
> Since each slab cache holds some free objects, we end up with a lot of
> free objects distributed among the separate zpool caches. Slab caches
> are designed to handle concurrent allocations by using percpu
> structures, so having a single instance of each cache should be enough,
> and avoids wasting more memory than needed due to fragmentation.
> 
> Additionally, having more slab caches than needed unnecessarily slows
> down code paths that iterate slab_caches.
> 
> In the results reported by Eric in [1], the amount of unused slab memory
> in these caches goes down from 242808 bytes to 29216 bytes (-88%). This
> is calculated by (num_objs - active_objs) * objsize for each 'zs_handle'
> and 'zspage' cache. Although this patch did not help with the allocation
> failure reported by Eric with zswap + zsmalloc, I think it is still
> worth merging on its own.
> 
> [1]https://lore.kernel.org/lkml/20240604134458.3ae4396a@yea/
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>

Makes perfect sense, thanks.

Reviewed-by: Sergey Senozhatsky <senozhatsky@...omium.org>