Re: Lockless page cache test results

[Linus Torvalds <torvalds@osdl.org>]

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.

			Linus

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