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

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Message-ID: <bb2cb21b-0a5e-0154-2a7d-7e630b50aa4a@linux.com>
Date: Fri, 12 Apr 2024 11:16:10 -0700 (PDT)
From: "Christoph Lameter (Ampere)" <cl@...ux.com>
To: Jianfeng Wang <jianfeng.w.wang@...cle.com>
cc: Vlastimil Babka <vbabka@...e.cz>, 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: [PATCH] slub: limit number of slabs to scan in count_partial()

On Fri, 12 Apr 2024, Jianfeng Wang wrote:

>>> 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.
> For "dentry", the length of partial slabs is slightly above 250000. Then, I
> compare my approach of scanning N slabs from the list's head v.s. the original
> approach of scanning the full list. I do it by getting both results using
> the new and the original count_partial() and printing them in /proc/slabinfo.
>
> N = 10000
> my_result = 4741651
> org_result = 4744966
> diff = (org_result - my_result) / org_result = 0.00069 = 0.069 %
>
> Increasing N further to 25000 will only slight improve the accuracy:
> N = 15000 -> diff =  0.02 %
> N = 20000 -> diff =  0.01 %
> N = 25000 -> diff = -0.017 %
>
> Based on the measurement, I think the difference between the estimation and
> the real count is very limited (i.e. less than 0.1% for N = 10000). The
> benefit is significant: shorter execution time for get_slabinfo(); no more
> soft lockups or crashes caused by count_partial().

Wow. That is good. Maybe decrease N to 1000 instead?