From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4451A00A.2030606@yahoo.com.au> Date: Fri, 28 Apr 2006 14:54:34 +1000 From: Nick Piggin MIME-Version: 1.0 Subject: Re: Lockless page cache test results References: <20060426135310.GB5083@suse.de> <20060426095511.0cc7a3f9.akpm@osdl.org> <20060426174235.GC5002@suse.de> <20060426111054.2b4f1736.akpm@osdl.org> <44505B59.1060308@yahoo.com.au> In-Reply-To: Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Linus Torvalds Cc: Andrew Morton , Jens Axboe , linux-kernel@vger.kernel.org, npiggin@suse.de, linux-mm@kvack.org List-ID: 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. Send instant messages to your online friends http://au.messenger.yahoo.com -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org