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

[Waiman Long]

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:
> > > >
> > > > [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?
>
> 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