From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751598AbaJOXQ2 (ORCPT ); Wed, 15 Oct 2014 19:16:28 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:38090 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751130AbaJOXQ0 (ORCPT ); Wed, 15 Oct 2014 19:16:26 -0400 Date: Wed, 15 Oct 2014 16:16:25 -0700 From: Andrew Morton To: Jamie Liu Cc: Johannes Weiner , Mel Gorman , Greg Thelen , Hugh Dickins , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: vmscan: count only dirty pages as congested Message-Id: <20141015161625.cad6bf6ffc5655fb843c8965@linux-foundation.org> In-Reply-To: References: <1413403115-1551-1-git-send-email-jamieliu@google.com> <20141015130544.380aca0acfcb1413459520b0@linux-foundation.org> X-Mailer: Sylpheed 3.2.0beta5 (GTK+ 2.24.10; 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 On Wed, 15 Oct 2014 16:07:42 -0700 Jamie Liu wrote: > On Wed, Oct 15, 2014 at 1:05 PM, Andrew Morton > wrote: > > On Wed, 15 Oct 2014 12:58:35 -0700 Jamie Liu wrote: > > > >> shrink_page_list() counts all pages with a mapping, including clean > >> pages, toward nr_congested if they're on a write-congested BDI. > >> shrink_inactive_list() then sets ZONE_CONGESTED if nr_dirty == > >> nr_congested. Fix this apples-to-oranges comparison by only counting > >> pages for nr_congested if they count for nr_dirty. > >> > >> ... > >> > >> --- a/mm/vmscan.c > >> +++ b/mm/vmscan.c > >> @@ -875,7 +875,8 @@ static unsigned long shrink_page_list(struct list_head *page_list, > >> * end of the LRU a second time. > >> */ > >> mapping = page_mapping(page); > >> - if ((mapping && bdi_write_congested(mapping->backing_dev_info)) || > >> + if (((dirty || writeback) && mapping && > >> + bdi_write_congested(mapping->backing_dev_info)) || > >> (writeback && PageReclaim(page))) > >> nr_congested++; > > > > What are the observed runtime effects of this change? > > wait_iff_congested() only waits if ZONE_CONGESTED is set (and at least > one BDI is still congested). Modulo concurrent changes to BDI > congestion status: > > After this change, the probability that a given shrink_inactive_list() > sets ZONE_CONGESTED increases monotonically with the fraction of dirty > pages on the LRU, to 100% if all dirty pages are backed by a > write-congested BDI. This is in line with what appears to intended, > judging by the comment: > > /* > * Tag a zone as congested if all the dirty pages scanned were > * backed by a congested BDI and wait_iff_congested will stall. > */ > if (nr_dirty && nr_dirty == nr_congested) > set_bit(ZONE_CONGESTED, &zone->flags); > > Before this change, the probability that a given > shrink_inactive_list() sets ZONE_CONGESTED varies erratically. Because > the ZONE_CONGESTED condition is nr_dirty && nr_dirty == nr_congested, > the probability peaks when the fraction of dirty pages is equal to the > fraction of file pages backed by congested BDIs. So under some > circumstances, an increase in the fraction of dirty pages or in the > fraction of congested pages can actually result in an *decreased* > probability that reclaim will stall for writeback congestion, and vice > versa; which is both counterintuitive and counterproductive. (top-posting repaired. Please don't do that!) OK, I buy all that. But has any runtime testing been performed to confirm this and to quantify the effects?