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

[Waiman Long]

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:
> >
> > [ÂÂÂ 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?

Cheers,
Longman