Re: [rfc][patch] dynamic resizing dentry hash using RCU

[Nick Piggin <npiggin@suse.de>]

On Sat, Feb 24, 2007 at 01:07:23PM +0900, KAMEZAWA Hiroyuki wrote:
> On Fri, 23 Feb 2007 16:37:43 +0100
> Nick Piggin <npiggin@suse.de> wrote:
> 
> > +static void dcache_hash_resize(unsigned int new_shift);
> > +static void mod_nr_dentry(int mod)
> > +{
> > +	unsigned long dentry_size;
> > +
> > +	dentry_stat.nr_dentry += mod;
> > +
> > +	dentry_size = 1 << dentry_hash->shift;
> > +	if (unlikely(dentry_stat.nr_dentry > dentry_size+(dentry_size>>1)))
> > +		dcache_hash_resize(dentry_hash->shift+1);
> > +	else if (unlikely(dentry_stat.nr_dentry < (dentry_size>>1)))
> > +		dcache_hash_resize(dentry_hash->shift-1);
> > +}
> > +
> 
> Do we need to do this kind of resizing in implicit automatic way ?
> I think it's good to show contention rate by /proc and add sysctl for
> resize this.

Well having the kernel automatically adjust to the workload is better
than a sysctl. But it could well be the case that these simple heuristics
are bad (though they're fine for my system).

But a manual override is a good idea as well.