From: Jan Kara Subject: Re: [PATCHv4 3/5] ext4: remove unnecessary superblock dirtying Date: Tue, 10 Jul 2012 14:52:38 +0200 Message-ID: <20120710125238.GF13539@quack.suse.cz> References: <1341404514-13660-1-git-send-email-dedekind1@gmail.com> <1341404514-13660-4-git-send-email-dedekind1@gmail.com> <20120704131124.GA29954@quack.suse.cz> <1341916536.2963.61.camel@sauron> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Jan Kara , Theodore Tso , Linux FS Maling List , Linux Kernel Maling List , Ext4 Mailing List To: Artem Bityutskiy Return-path: Received: from cantor2.suse.de ([195.135.220.15]:39499 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752681Ab2GJMwk (ORCPT ); Tue, 10 Jul 2012 08:52:40 -0400 Content-Disposition: inline In-Reply-To: <1341916536.2963.61.camel@sauron> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Tue 10-07-12 13:35:36, Artem Bityutskiy wrote: > On Wed, 2012-07-04 at 15:11 +0200, Jan Kara wrote: > > On Wed 04-07-12 15:21:52, Artem Bityutskiy wrote: > > > From: Artem Bityutskiy > > > > > > This patch changes the '__ext4_handle_dirty_super()' function which is used > > > by ext4 to update the superblock via the journal in the following cases: > > > > > > 1. When creating the first large file on a file system without > > > EXT4_FEATURE_RO_COMPAT_LARGE_FILE feature. > > > 2. When re-sizing the file-system. > > > 3. When creating an xattr on a file-system without the > > > EXT4_FEATURE_COMPAT_EXT_ATTR feature. > > > 4. When adding or deleting an orphan (because we update the 's_last_orphan' > > > superblock field). > > > > > > This function, however, falls back to just marking the superblock as dirty > > > if the file-system has no journal. This means that we delay the actual > > > superblock I/O submission by 5 seconds (roughly speaking). Namely, the > > > 'sync_supers()' kernel thread will call 'ext4_write_super()' later, where > > > we actually will submit the superblock down to the media. > > > > > > However: > > > 1. For cases 1-3 it does not add any value to delay the I/O submission. These > > > events are rare and we may just commit submit the superblock for > > > asynchronous I/O right away. > > > 2. For case 4 - similarly, not terribly frequent event in most of workloads. > > > It should be good enough to just submit asynchronous superblock write-out. > > Well, it happens for every inode being truncated / deleted to it can be > > rather frequent. That's why I wanted to have now == 1 case everywhere - > > i.e. just recompute the checksum and do mark_buffer_dirty(). I'd just > > remove the 'now' test in this patch and then in patch 5 remove the now > > argument from the function and callers as you did. > > I am a bit confused. > > It seems you consider that 'ext4_commit_super()' is a considerably > slower than just marking the buffer as dirty right away. But I do not > really understand why - all it does - it just updates a couple of > superblock fields and then marks the buffer as dirty (I assume sync == > 0). So from my POW they are almost the same. And when csum is enabled - > re-calculating csum will probably be the longest part. Well, the part you might be missing is: ext4_free_blocks_count_set(es, EXT4_C2B(EXT4_SB(sb), percpu_counter_sum_positive( &EXT4_SB(sb)->s_freeclusters_counter))); es->s_free_inodes_count = cpu_to_le32(percpu_counter_sum_positive( &EXT4_SB(sb)->s_freeinodes_counter)); percpu_counter_sum() *is* rather expensive. At least for big machines. Also just marking the buffer dirty more corresponds to what we do when journalling. > More important is that we dirty the superblock on every deletion - this > mean that with my change we will re-calculate checsum on every deletion > and I am not sure it is nice. Ideally, we should be able to calculate > the checksum just before sending the buffer to the IO queue... Yes, that would be nice but it's not easy to do currently... Honza -- Jan Kara SUSE Labs, CR