On Thu, 27 Apr 2006, Nick Piggin wrote:
Of course, with small files, the actual filename lookup is likely to be the
real limiter.
Although that's lockless so it scales. find_get_page will overtake it
at some point.
filename lookup is only lockless for independent files. You end up getting the "dentry->d_lock" for a successful lookup in the lookup path, so if you have multiple threads looking up the same files (or - MUCH more commonly - directories), you're not going to be lockless.
I don't know how we could improve it. I've several times thought that we _should_ be able to do the directory lookups under the rcu read lock and never touch their d_count or d_lock at all, but the locking against directory renaming depends very intimately on d_lock.
It is _possible_ that we should be able to handle it purely with just memory ordering rather than depending on d_lock. That would be wonderful.
Of course, we do actually scale pretty damn well already. I'm just saying that it's not perfect.
See __d_lookup() for details.