linux-kernel - Re: [External] : Re: [PATCH] slub: limit number of slabs to scan in count

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Message-ID: <1207c5d7-8bb7-4574-b811-0cd5f7eaf33d@suse.cz>
Date: Fri, 12 Apr 2024 22:20:48 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Jianfeng Wang <jianfeng.w.wang@...cle.com>,
 "Christoph Lameter (Ampere)" <cl@...ux.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, penberg@...nel.org,
 rientjes@...gle.com, iamjoonsoo.kim@....com, akpm@...ux-foundation.org,
 junxiao.bi@...cle.com
Subject: Re: [External] : Re: [PATCH] slub: limit number of slabs to scan in
 count_partial()

On 4/12/24 7:29 PM, Jianfeng Wang wrote:
> 
> 
> On 4/12/24 12:48 AM, Vlastimil Babka wrote:
>> On 4/11/24 7:02 PM, Christoph Lameter (Ampere) wrote:
>>> On Thu, 11 Apr 2024, Jianfeng Wang wrote:
>>>
>>>> So, the fix is to limit the number of slabs to scan in
>>>> count_partial(), and output an approximated result if the list is too
>>>> long. Default to 10000 which should be enough for most sane cases.
>>>
>>>
>>> That is a creative approach. The problem though is that objects on the 
>>> partial lists are kind of sorted. The partial slabs with only a few 
>>> objects available are at the start of the list so that allocations cause 
>>> them to be removed from the partial list fast. Full slabs do not need to 
>>> be tracked on any list.
>>>
>>> The partial slabs with few objects are put at the end of the partial list 
>>> in the hope that the few objects remaining will also be freed which would 
>>> allow the freeing of the slab folio.
>>>
>>> So the object density may be higher at the beginning of the list.
>>>
>>> kmem_cache_shrink() will explicitly sort the partial lists to put the 
>>> partial pages in that order.
>>>
>>> Can you run some tests showing the difference between the estimation and 
>>> the real count?
> 
> Yes.
> On a server with one NUMA node, I create a case that uses many dentry objects.

Could you describe in more detail how do you make dentry cache to grow such
a large partial slabs list? Thanks.