Re: [PATCH 08/11] vfs: inode cache conversion to hash-bl

Dave Chinner
2023-12-07T06:03:22Z

On Wed, Dec 06, 2023 at 11:58:44PM -0500, Kent Overstreet wrote:
> On Wed, Dec 06, 2023 at 05:05:37PM +1100, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@redhat.com>
> >
> > Scalability of the global inode_hash_lock really sucks for
> > filesystems that use the vfs inode cache (i.e. everything but XFS).
>
> Ages ago, we talked about (and I attempted, but ended up swearing at
> inode lifetime rules) - conversion to rhashtable instead, which I still
> believe would be preferable since that code is fully lockless (and
> resizeable, of course). But it turned out to be a much bigger project...

I don't think that the size of the has table is a big issue at the
moment. We already have RCU lookups for the inode cache
(find_inode_rcu() and find_inode_by_ino_rcu()) even before this
patchset, so we don't need rhashtable for that.

We still have to prevent duplicate inodes from being added to the cache
due to racing inserts, so I think we still need some form of
serialisation on the "lookup miss+insert" side. I've not thought
about it further than that - the hash-bl removes the existing
VFS contention points and the limitations move to
filesystem internal algorithms once again.

So until the filesystems can scale to much larger thread counts and
put the pressure back on the VFS inode cache scalability, I
don't see any need to try to do anything more complex or smarter...

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com