On Wed, Sep 25, 2024 at 02:43:15PM GMT, Dave Chinner wrote: > On Tue, Sep 24, 2024 at 10:13:01PM -0400, Kent Overstreet wrote: > > On Wed, Sep 25, 2024 at 11:00:10AM GMT, Dave Chinner wrote: > > > > Eh? Of course it'd have to be coherent, but just checking if an inode is > > > > present in the VFS cache is what, 1-2 cache misses? Depending on hash > > > > table fill factor... > > > > > > Sure, when there is no contention and you have CPU to spare. But the > > > moment the lookup hits contention problems (i.e. we are exceeding > > > the cache lookup scalability capability), we are straight back to > > > running a VFS cache speed instead of uncached speed. > > > > The cache lookups are just reads; they don't introduce scalability > > issues, unless they're contending with other cores writing to those > > cachelines - checking if an item is present in a hash table is trivial > > to do locklessly. > > Which was not something the VFS inode cache did until a couple of > months ago. Just because something is possible/easy today, it > doesn't mean it was possible or viable 15-20 years ago. We've been doing lockless hash table lookups for a lot longer than that. > > But pulling an inode into and then evicting it from the inode cache > > entails a lot more work - just initializing a struct inode is > > nontrivial, and then there's the (multiple) shared data structures you > > have to manipulate. > > Yes, but to avoid this we'd need to come up with a mechanism that is > generally safe for most filesystems, not just bcachefs. > > I mean, if you can come up with a stat() mechanism that is safe > enough for us read straight out the XFS buffer cache for inode cache > misses, then we'll switch over to using it ASAP. So add a per buffer lock to your buffer cache, and check for vfs cache residency under the lock. Whether bulkstat is worth adding that lock, for a filesystem that doesn't need it? That I can't say, it's a tradeoff. > That's your challenge - if you want bcachefs to be able to do this, > then you have to make sure the infrastructure required works for > other filesystems just as safely, too. So... you think bcachefs should be limited to the capabilities of what other filesystems can do? Interesting position...
