From: Chris Mason Subject: Re: [PATCH] Improve buffered streaming write ordering Date: Mon, 06 Oct 2008 10:21:43 -0400 Message-ID: <1223302903.16546.58.camel@think.oraclecorp.com> References: <1222886451.9158.34.camel@think.oraclecorp.com> <20081001215239.ee2ae63f.akpm@linux-foundation.org> <1222950054.6745.18.camel@think.oraclecorp.com> <20081002181856.GB29613@skywalker> <20081002234309.GH30001@disturbed> <1223063155.13375.64.camel@think.oraclecorp.com> <20081006101605.GA15881@skywalker> Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Cc: Dave Chinner , Andrew Morton , linux-kernel , linux-fsdevel , ext4 To: "Aneesh Kumar K.V" Return-path: In-Reply-To: <20081006101605.GA15881@skywalker> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Mon, 2008-10-06 at 15:46 +0530, Aneesh Kumar K.V wrote: > On Fri, Oct 03, 2008 at 03:45:55PM -0400, Chris Mason wrote: > > On Fri, 2008-10-03 at 09:43 +1000, Dave Chinner wrote: > > > On Thu, Oct 02, 2008 at 11:48:56PM +0530, Aneesh Kumar K.V wrote: > > > > On Thu, Oct 02, 2008 at 08:20:54AM -0400, Chris Mason wrote: > > > > > On Wed, 2008-10-01 at 21:52 -0700, Andrew Morton wrote: > > > > > For a 4.5GB streaming buffered write, this printk inside > > > > > ext4_da_writepage shows up 37,2429 times in /var/log/messages. > > > > > > > > > > > > > Part of that can happen due to shrink_page_list -> pageout -> writepagee > > > > call back with lots of unallocated buffer_heads(blocks). > > > > > > Quite frankly, a simple streaming buffered write should *never* > > > trigger writeback from the LRU in memory reclaim. > > > > The blktrace runs on ext4 didn't show kswapd doing any IO. It isn't > > clear if this is because ext4 did the redirty trick or if kswapd didn't > > call writepage. > > > > -chris > > This patch actually reduced the number of extents for the below test > from 564 to 171. > For my array, this patch brings the number of ext4 extents down from over 4000 to 27. The throughput reported by dd goes up from ~80MB/s to 330MB/s, which means buffered IO is going as fast as O_DIRECT. Here's the graph: http://oss.oracle.com/~mason/bugs/writeback_ordering/ext4-aneesh.png The strange metadata writeback for the uninit block groups is gone. Looking at the patch, I think the ext4_writepages code should just make its own write_cache_pages. It's pretty hard to follow the code that is there for ext4 vs the code that is there to make write_cache_pages do what ext4 expects it to. -chris