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

Waiman Long
2020-05-08T20:00:16Z

On 5/8/20 3:38 PM, Konstantin Khlebnikov wrote:
>
>
> 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

Well, I got rebuke the last time I want to reuse one of
age_limit/want_pages entry for negative dentry count because of the
potential of breaking some really old applications or tools. Changing
dentry-state to output one more number can potentially break
compatibility too. That is why I am questioning if it is a good idea to
use up the last free slot.

Cheers,
Longman