Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753026Ab0HSLyM (ORCPT ); Thu, 19 Aug 2010 07:54:12 -0400 Received: from f0.cmpxchg.org ([85.214.51.133]:43860 "EHLO cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752987Ab0HSLyH (ORCPT ); Thu, 19 Aug 2010 07:54:07 -0400 Date: Thu, 19 Aug 2010 13:51:06 +0200 From: Johannes Weiner To: Wu Fengguang Cc: Matthew Wilcox , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Li Shaohua Subject: Re: [TESTCASE] Clean pages clogging the VM Message-ID: <20100819115106.GG1779@cmpxchg.org> References: <20100809133000.GB6981@wil.cx> <20100817195001.GA18817@linux.intel.com> <20100818141308.GD1779@cmpxchg.org> <20100818160613.GE9431@localhost> <20100818160731.GA15002@localhost> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100818160731.GA15002@localhost> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4122 Lines: 92 On Thu, Aug 19, 2010 at 12:07:31AM +0800, Wu Fengguang wrote: > On Thu, Aug 19, 2010 at 12:06:13AM +0800, Wu Fengguang wrote: > > On Wed, Aug 18, 2010 at 04:13:08PM +0200, Johannes Weiner wrote: > > > Hi Matthew, > > > > > > On Tue, Aug 17, 2010 at 03:50:01PM -0400, Matthew Wilcox wrote: > > > > > > > > No comment on this? Was it just that I posted it during the VM summit? > > > > > > I have not forgotten about it. I just have a hard time reproducing > > > those extreme stalls you observed. > > > > > > Running that test on a 2.5GHz machine with 2G of memory gives me > > > stalls of up to half a second. The patchset I am experimenting with > > > gets me down to peaks of 70ms, but it needs further work. > > > > > > Mapped file pages get two rounds on the LRU list, so once the VM > > > starts scanning, it has to go through all of them twice and can only > > > reclaim them on the second encounter. > > > > > > At that point, since we scan without making progress, we start waiting > > > for IO, which is not happening in this case, so we sit there until a > > > timeout expires. > > > > Right, this could lead to some 1s stall. Shaohua and me also noticed > > this when investigating the responsiveness issues. And we are wondering > > if it makes sense to do congestion_wait() only when the bdi is really > > congested? There are no IO underway anyway in this case. I am currently trying to get rid of all the congestion_wait() in the VM. They are used for different purposes, so they need different replacement mechanisms. I saw Shaohua's patch to make congestion_wait() cleverer. But I really think that congestion is not a good predicate in the first place. Why would the VM care about IO _congestion_? It needs a bunch of pages to complete IO, whether the writing device is congested is not really useful information at this point, I think. > > > since I can not reproduce your observations, I don't know if this is > > > the (sole) source of the problem. Can I send you patches? > > > > Sure. Cool! > > > > On Mon, Aug 09, 2010 at 09:30:00AM -0400, Matthew Wilcox wrote: > > > > > > > > > > This testcase shows some odd behaviour from the Linux VM. > > > > > > > > > > It creates a 1TB sparse file, mmaps it, and randomly reads locations > > > > > in it. Due to the file being entirely sparse, the VM allocates new pages > > > > > and zeroes them. Initially, it runs very fast, taking on the order of > > > > > 2.7 to 4us per page fault. Eventually, the VM runs out of free pages, > > > > > and starts doing huge amounts of work trying to figure out which of > > > > > these clean pages to throw away. > > > > > > This is similar to one of my test cases for: > > > > > > 6457474 vmscan: detect mapped file pages used only once > > > 31c0569 vmscan: drop page_mapping_inuse() > > > dfc8d63 vmscan: factor out page reference checks > > > > > > because the situation was even worse before (see the series > > > description in dfc8d63). Maybe asking the obvious, but the kernel you > > > tested on did include those commits, right? > > > > > > And just to be sure, I sent you a test-patch to disable the used-once > > > detection on IRC the other day. Did you have time to run it yet? > > > Here it is again: > > > > > > diff --git a/mm/vmscan.c b/mm/vmscan.c > > > index 9c7e57c..c757bba 100644 > > > --- a/mm/vmscan.c > > > +++ b/mm/vmscan.c > > > @@ -584,6 +584,7 @@ static enum page_references page_check_references(struct page *page, > > > return PAGEREF_RECLAIM; > > > > > > if (referenced_ptes) { > > > + return PAGEREF_ACTIVATE; > > > > How come page activation helps? This is effectively disabling used-once detection and going back to the old VM behaviour. I don't think it helps, but this code is recent and directly related to the test-case. Maybe I/we missed something, it can't hurt to make sure, right? -- 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/