linux-kernel - Re: [PATCH RFC 1/8] dcache: show count of hash buckets in sysctl fs.dentry-state

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Date:   Fri, 8 May 2020 22:38:10 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     Waiman Long <longman@...hat.com>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH RFC 1/8] dcache: show count of hash buckets in sysctl
 fs.dentry-state



On 08/05/2020 22.05, Waiman Long wrote:
> On 5/8/20 12:16 PM, Konstantin Khlebnikov wrote:
>> On 08/05/2020 17.49, Waiman Long wrote:
>>> On 5/8/20 8:23 AM, Konstantin Khlebnikov wrote:
>>>> Count of buckets is required for estimating average length of hash chains.
>>>> Size of hash table depends on memory size and printed once at boot.
>>>>
>>>> Let's expose nr_buckets as sixth number in sysctl fs.dentry-state
>>>
>>> The hash bucket count is a constant determined at boot time. Is there a need to use up one dentry_stat entry for that? Besides one can 
>>> get it by looking up the kernel dmesg log like:
>>>
>>> [    0.055212] Dentry cache hash table entries: 8388608 (order: 14, 67108864 bytes)
>>
>> Grepping logs since boot time is a worst API ever.
>>
>> dentry-state shows count of dentries in various states.
>> It's very convenient to show count of buckets next to it,
>> because this number defines overall scale. 
> 
> I am not against using the last free entry for that. My only concern is when we want to expose another internal dcache data point via 
> dentry-state, we will have to add one more number to the array which can cause all sort of compatibility problem. So do we want to use the 
> last free slot for a constant that can be retrieved from somewhere else?

I see no problem in adding more numbers into sysctl.
Especially into such rarely used.
This interface is designed for that.

Also fields 'age_limit' and 'want_pages' are unused since kernel 2.2.0

> 
> Cheers,
> Longman
>