Re: Lockless page cache test results

[Nick Piggin]

Linus Torvalds wrote:
> 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.

Oh that's true, I forgot. So the many small files case will often have
as much d_lock activity as tree_lock.

> 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.

Yes I see. Perhaps a seqlock could do the trick (hmm, there already is one),
however we still have to increment the refcount, so there'll always be a
shared cacheline.

--
SUSE Labs, Novell Inc.
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