Re: [PATCH 3/5] superblock: introduce per-sb cache shrinker infrastructure

Nick Piggin
2010-05-27T04:22:34Z

On Thu, May 27, 2010 at 02:07:04PM +1000, Dave Chinner wrote:
> On Thu, May 27, 2010 at 12:19:05PM +1000, Nick Piggin wrote:
> > On Thu, May 27, 2010 at 09:12:14AM +1000, Dave Chinner wrote:
> > > On Thu, May 27, 2010 at 02:41:16AM +1000, Nick Piggin wrote:
> > > > > + count = ((sb->s_nr_dentry_unused + sb->s_nr_inodes_unused) / 100)
> > > > > + * sysctl_vfs_cache_pressure;
> > > >
> > > > Do you think truncating in the divisions is at all a problem? It
> > > > probably doesn't matter much I suppose.
> > >
> > > Same code as currently exists. IIRC, the reasoning is that if we've
> > > got less that 100 objects to reclaim, then we're unlikely to be able
> > > to free up any memory from the caches, anyway.
> >
> > Yeah, which is why I stop short of saying you should change it in
> > this patch.
> >
> > But I think we should ensure things can get reclaimed eventually.
> > 100 objects could be 100 slabs, which could be anything from
> > half a meg to half a dozen. Multiplied by each of the caches.
> > Could be significant in small systems.
>
> True, but usually there are busy objects in the dentry and inode
> slabs, so it shouldn't be a significant issue. We can probably
> address such problems if they can be demonstrated to be an issue in
> a separate patch set....

I didn't want to say it is a problem with your patchset, I just
thought of it when reviewing.