Messages in this thread | | | Date | Fri, 28 Apr 2006 14:54:34 +1000 | From | Nick Piggin <> | Subject | Re: Lockless page cache test results |
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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.
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