From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wu Fengguang Subject: Re: Likley stupid question on "throttle_vm_writeout" Date: Tue, 10 Nov 2009 21:08:18 +0800 Message-ID: <20091110130818.GA6229@localhost> References: <799070.68490.qm@web32608.mail.mud.yahoo.com> <1257780393.4108.343.camel@laptop> <20091110020858.GA5749@localhost> <707547.6272.qm@web32605.mail.mud.yahoo.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Peter Zijlstra , "linux-kernel@vger.kernel.org" , "Myklebust, Trond" , Peter Staubach , linux-fsdevel@vger.kernel.org To: Martin Knoblauch Return-path: Received: from mga03.intel.com ([143.182.124.21]:64135 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753212AbZKJNIa (ORCPT ); Tue, 10 Nov 2009 08:08:30 -0500 Content-Disposition: inline In-Reply-To: <707547.6272.qm@web32605.mail.mud.yahoo.com> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Tue, Nov 10, 2009 at 08:01:47PM +0800, Martin Knoblauch wrote: > ----- Original Message ---- > > > From: Wu Fengguang > > To: Peter Zijlstra > > Cc: Martin Knoblauch ; linux-kernel@vger.kernel.org > > Sent: Tue, November 10, 2009 3:08:58 AM > > Subject: Re: Likley stupid question on "throttle_vm_writeout" > > > > On Mon, Nov 09, 2009 at 04:26:33PM +0100, Peter Zijlstra wrote: > > > On Mon, 2009-11-09 at 07:15 -0800, Martin Knoblauch wrote: > > > > Hi, (please CC me on replies) > > > > > > > > I have a likely stupid question on the function "throttle_vm_writeout". > > Looking at the code I find: > > > > > > > > if (global_page_state(NR_UNSTABLE_NFS) + > > > > global_page_state(NR_WRITEBACK) <= dirty_thresh) > > > > break; > > > > congestion_wait(WRITE, HZ/10); > > > > > > > > Shouldn't the NR_FILE_DIRTY pages be considered as well? > > > > > > Ha, you just trod onto a piece of ugly I'd totally forgotten about ;-) > > > > > > The intent of throttle_vm_writeout() is to limit the total pages in > > > writeout and to wait for them to go-away. > > > > Like this: > > > > vmscan fast => large NR_WRITEBACK => throttle vmscan based on it > > > > > Everybody hates the function, nobody managed to actually come up with > > > anything better. > > > > btw, here is another reason to limit NR_WRITEBACK: I saw many > > throttle_vm_writeout() waits if there is no wait queue to limit > > NR_WRITEBACK (eg. NFS). In that case the (steadily) big NR_WRITEBACK > > is _not_ caused by fast vmscan.. > > > > That is exactely what made me look again into the code. My observation is that when doing something like: > > dd if=/dev/zero of=fast-local-disk bs=1M count=15000 > > most of the "dirty" pages are in NR_FILE_DIRTY with some relatively small amount (10% or so) in NR_WRITEBACK. If I do: > > dd if=/dev/zero of=some-nfs-mount bs=1M count=15000 > > NR_WRITEBACK almost immediatelly goes up to dirty_ratio, with > NR_UNSTABLE_NFS small. Over time NR_UNSTABLE_NFS grows, but is > always lower than NR_WRITEBACK (maybe 40/60). This is interesting, though I don't see explicit NFS code to limit NR_UNSTABLE_NFS. Maybe there are some implicit rules. > But don't ask what happens if I do both in parallel.... The local > IO really slows to a crawl and sometimes the system just becomes > very unresponsive. Have we heard that before? :-) You may be the first reporter as far as I can tell :) > Somehow I have the impression that NFS writeout is able to > absolutely dominate the dirty pages to an extent that the system is > unusable. This is why I want to limit NR_WRITEBACK for NFS: [PATCH] NFS: introduce writeback wait queue http://lkml.org/lkml/2009/10/3/198 Thanks, Fengguang