Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754287Ab0HZBhJ (ORCPT ); Wed, 25 Aug 2010 21:37:09 -0400 Received: from cantor.suse.de ([195.135.220.2]:57477 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751188Ab0HZBhH (ORCPT ); Wed, 25 Aug 2010 21:37:07 -0400 Date: Thu, 26 Aug 2010 11:36:49 +1000 From: Neil Brown To: Wu Fengguang Cc: Con Kolivas , Jan Kara , Rik van Riel , Peter Zijlstra , KOSAKI Motohiro , Andrew Morton , "linux-kernel@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "linux-mm@kvack.org" , "david@fromorbit.com" , "hch@lst.de" , "axboe@kernel.dk" Subject: Re: [PATCH] writeback: remove the internal 5% low bound on dirty_ratio Message-ID: <20100826113649.687b453a@notabene> In-Reply-To: <20100826012945.GA7859@localhost> References: <20100820032506.GA6662@localhost> <201008241620.54048.kernel@kolivas.org> <20100824071440.GA14598@localhost> <201008251840.00532.kernel@kolivas.org> <20100826012945.GA7859@localhost> X-Mailer: Claws Mail 3.7.6 (GTK+ 2.20.1; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 7654 Lines: 171 On Thu, 26 Aug 2010 09:29:45 +0800 Wu Fengguang wrote: > On Wed, Aug 25, 2010 at 04:40:00PM +0800, Con Kolivas wrote: > > On Tue, 24 Aug 2010 05:14:40 pm you wrote: > > > On Tue, Aug 24, 2010 at 02:20:54PM +0800, Con Kolivas wrote: > > > > On Mon, 23 Aug 2010 05:15:35 pm you wrote: > > > > > On Mon, Aug 23, 2010 at 02:30:40PM +0800, Con Kolivas wrote: > > > > > > On Mon, 23 Aug 2010 04:23:59 pm Wu Fengguang wrote: > > > > > > > On Mon, Aug 23, 2010 at 12:42:48PM +0800, Neil Brown wrote: > > > > > > > > On Fri, 20 Aug 2010 15:50:54 +1000 > > > > > > > > > > > > > > > > Con Kolivas wrote: > > > > > > > > > On Fri, 20 Aug 2010 02:13:25 pm KOSAKI Motohiro wrote: > > > > > > > > > > > The dirty_ratio was silently limited to >= 5%. This is not > > > > > > > > > > > a user expected behavior. Let's rip it. > > > > > > > > > > > > > > > > > > > > > > It's not likely the user space will depend on the old > > > > > > > > > > > behavior. So the risk of breaking user space is very low. > > > > > > > > > > > > > > > > > > > > > > CC: Jan Kara > > > > > > > > > > > CC: Neil Brown > > > > > > > > > > > Signed-off-by: Wu Fengguang > > > > > > > > > > > > > > > > > > > > Thank you. > > > > > > > > > > Reviewed-by: KOSAKI Motohiro > > > > > > > > > > > > > > > > > > > > > > > > > > > > I have tried to do this in the past, and setting this value to > > > > > > > > > 0 on some machines caused the machine to come to a complete > > > > > > > > > standstill with small writes to disk. It seemed there was some > > > > > > > > > kind of "minimum" amount of data required by the VM before > > > > > > > > > anything would make it to the disk and I never quite found out > > > > > > > > > where that blockade occurred. This was some time ago (3 years > > > > > > > > > ago) so I'm not sure if the problem has since been fixed in the > > > > > > > > > VM since then. I suggest you do some testing with this value > > > > > > > > > set to zero before approving this change. > > > > > > > > > > > > > > You are right, vm.dirty_ratio=0 will block applications for ever.. > > > > > > > > > > > > Indeed. And while you shouldn't set the lower limit to zero to avoid > > > > > > this problem, it doesn't answer _why_ this happens. What is this > > > > > > "minimum write" that blocks everything, will 1% be enough, and is it > > > > > > hiding another real bug somewhere in the VM? > > > > > > > > > > Good question. > > > > > This simple change will unblock the application even with > > > > > vm_dirty_ratio=0. > > > > > > > > > > # echo 0 > /proc/sys/vm/dirty_ratio > > > > > # echo 0 > /proc/sys/vm/dirty_background_ratio > > > > > # vmmon nr_dirty nr_writeback nr_unstable > > > > > > > > > > nr_dirty nr_writeback nr_unstable > > > > > 0 444 1369 > > > > > 37 37 326 > > > > > 0 0 37 > > > > > 74 772 694 > > > > > 0 0 19 > > > > > 0 0 1406 > > > > > 0 0 23 > > > > > 0 0 0 > > > > > 0 370 186 > > > > > 74 1073 1221 > > > > > 0 12 26 > > > > > 0 703 1147 > > > > > 37 0 999 > > > > > 37 37 1517 > > > > > 0 888 63 > > > > > 0 0 0 > > > > > 0 0 20 > > > > > 37 0 0 > > > > > 37 74 1776 > > > > > 0 0 8 > > > > > 37 629 333 > > > > > 0 12 19 > > > > > > > > > > Even with it, the 1% explicit bound still looks reasonable for me. > > > > > Who will want to set it to 0%? That would destroy IO inefficient. > > > > > > > > Thanks for your work in this area. I'll experiment with these later. > > > > There are low latency applications that would benefit with it set to > > > > zero. > > > > > > It might be useful to some users. Shall we give the rope to users, heh? > > > > > > Note that for these applications, they may well use > > > /proc/sys/vm/dirty_bytes for more fine grained control. That interface only > > > imposes a low limit of 2 pages. > > > > I don't see why there needs to be a limit. Users fiddling with sysctls should > > know what they're messing with, and there may well be a valid use out there > > somewhere for it. > > OK, the following patch gives users the full freedom. I tested 1 > single dirtier and 9 parallel dirtiers, the system remains alive, but > with much slower IO throughput. Maybe not all users care IO performance > in all situations? > > Thanks, > Fengguang > --- > writeback: remove the internal 5% low bound on dirty_ratio > > The dirty_ratio was silently limited in global_dirty_limits() to >= 5%. > This is not a user expected behavior. And it's inconsistent with > calc_period_shift(), which uses the plain vm_dirty_ratio value. > > Let's rip the internal bound. > > At the same time, fix balance_dirty_pages() to work with the > dirty_thresh=0 case. This allows applications to proceed when > dirty+writeback pages are all cleaned. And ">" fits with the name "exceeded" better than ">=" does. I think it is an aesthetic improvement as well as a functional one. Reviewed-by: NeilBrown Thanks, NeilBrown > > CC: Jan Kara > CC: Neil Brown > CC: Con Kolivas > CC: Rik van Riel > CC: Peter Zijlstra > CC: KOSAKI Motohiro > Signed-off-by: Wu Fengguang > --- > mm/page-writeback.c | 14 ++++---------- > 1 file changed, 4 insertions(+), 10 deletions(-) > > --- linux-next.orig/mm/page-writeback.c 2010-08-26 08:37:31.000000000 +0800 > +++ linux-next/mm/page-writeback.c 2010-08-26 08:37:55.000000000 +0800 > @@ -415,14 +415,8 @@ void global_dirty_limits(unsigned long * > > if (vm_dirty_bytes) > dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE); > - else { > - int dirty_ratio; > - > - dirty_ratio = vm_dirty_ratio; > - if (dirty_ratio < 5) > - dirty_ratio = 5; > - dirty = (dirty_ratio * available_memory) / 100; > - } > + else > + dirty = (vm_dirty_ratio * available_memory) / 100; > > if (dirty_background_bytes) > background = DIV_ROUND_UP(dirty_background_bytes, PAGE_SIZE); > @@ -542,8 +536,8 @@ static void balance_dirty_pages(struct a > * the last resort safeguard. > */ > dirty_exceeded = > - (bdi_nr_reclaimable + bdi_nr_writeback >= bdi_thresh) > - || (nr_reclaimable + nr_writeback >= dirty_thresh); > + (bdi_nr_reclaimable + bdi_nr_writeback > bdi_thresh) > + || (nr_reclaimable + nr_writeback > dirty_thresh); > > if (!dirty_exceeded) > break; -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/