Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753372Ab0HWHPn (ORCPT ); Mon, 23 Aug 2010 03:15:43 -0400 Received: from mga03.intel.com ([143.182.124.21]:2265 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752105Ab0HWHPk (ORCPT ); Mon, 23 Aug 2010 03:15:40 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.56,256,1280732400"; d="scan'208";a="315726168" Date: Mon, 23 Aug 2010 15:15:35 +0800 From: Wu Fengguang To: Con Kolivas Cc: Neil Brown , KOSAKI Motohiro , Andrew Morton , "linux-kernel@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "linux-mm@kvack.org" , "riel@redhat.com" , "david@fromorbit.com" , "hch@lst.de" , "axboe@kernel.dk" Subject: Re: [PATCH] writeback: remove the internal 5% low bound on dirty_ratio Message-ID: <20100823071534.GA24566@localhost> References: <20100820032506.GA6662@localhost> <20100823144248.15fbb700@notabene> <20100823062359.GA19586@localhost> <201008231630.40892.kernel@kolivas.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <201008231630.40892.kernel@kolivas.org> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4115 Lines: 91 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, Fengguang --- --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -542,8 +536,8 @@ static void balance_dirty_pages(struct address_space *mapping, * 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/