From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Thu, 27 Apr 2006 08:12:28 -0700 (PDT) From: Linus Torvalds Subject: Re: Lockless page cache test results In-Reply-To: <44505B59.1060308@yahoo.com.au> Message-ID: References: <20060426135310.GB5083@suse.de> <20060426095511.0cc7a3f9.akpm@osdl.org> <20060426174235.GC5002@suse.de> <20060426111054.2b4f1736.akpm@osdl.org> <44505B59.1060308@yahoo.com.au> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org Return-Path: To: Nick Piggin Cc: Andrew Morton , Jens Axboe , linux-kernel@vger.kernel.org, npiggin@suse.de, linux-mm@kvack.org List-ID: 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 -- 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