From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758181Ab1FHAZV (ORCPT ); Tue, 7 Jun 2011 20:25:21 -0400 Received: from fgwmail6.fujitsu.co.jp ([192.51.44.36]:55170 "EHLO fgwmail6.fujitsu.co.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753593Ab1FHAZQ (ORCPT ); Tue, 7 Jun 2011 20:25:16 -0400 X-SecurityPolicyCheck-FJ: OK by FujitsuOutboundMailChecker v1.3.1 Date: Wed, 8 Jun 2011 09:18:15 +0900 From: KAMEZAWA Hiroyuki To: Vivek Goyal Cc: Greg Thelen , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, containers@lists.osdl.org, linux-fsdevel@vger.kernel.org, Andrea Righi , Balbir Singh , Daisuke Nishimura , Minchan Kim , Johannes Weiner , Ciju Rajan K , David Rientjes , Wu Fengguang , Dave Chinner Subject: Re: [PATCH v8 11/12] writeback: make background writeback cgroup aware Message-Id: <20110608091815.fdef924d.kamezawa.hiroyu@jp.fujitsu.com> In-Reply-To: <20110607210540.GB30919@redhat.com> References: <1307117538-14317-1-git-send-email-gthelen@google.com> <1307117538-14317-12-git-send-email-gthelen@google.com> <20110607193835.GD26965@redhat.com> <20110607210540.GB30919@redhat.com> Organization: FUJITSU Co. LTD. X-Mailer: Sylpheed 3.1.0 (GTK+ 2.10.14; i686-pc-mingw32) 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 Tue, 7 Jun 2011 17:05:40 -0400 Vivek Goyal wrote: > On Tue, Jun 07, 2011 at 01:43:08PM -0700, Greg Thelen wrote: > > Vivek Goyal writes: > > > > > On Fri, Jun 03, 2011 at 09:12:17AM -0700, Greg Thelen wrote: > > >> When the system is under background dirty memory threshold but a cgroup > > >> is over its background dirty memory threshold, then only writeback > > >> inodes associated with the over-limit cgroup(s). > > >> > > > > > > [..] > > >> -static inline bool over_bground_thresh(void) > > >> +static inline bool over_bground_thresh(struct bdi_writeback *wb, > > >> + struct writeback_control *wbc) > > >> { > > >> unsigned long background_thresh, dirty_thresh; > > >> > > >> global_dirty_limits(&background_thresh, &dirty_thresh); > > >> > > >> - return (global_page_state(NR_FILE_DIRTY) + > > >> - global_page_state(NR_UNSTABLE_NFS) > background_thresh); > > >> + if (global_page_state(NR_FILE_DIRTY) + > > >> + global_page_state(NR_UNSTABLE_NFS) > background_thresh) { > > >> + wbc->for_cgroup = 0; > > >> + return true; > > >> + } > > >> + > > >> + wbc->for_cgroup = 1; > > >> + wbc->shared_inodes = 1; > > >> + return mem_cgroups_over_bground_dirty_thresh(); > > >> } > > > > > > Hi Greg, > > > > > > So all the logic of writeout from mem cgroup works only if system is > > > below background limit. The moment we cross background limit, looks > > > like we will fall back to existing way of writting inodes? > > > > Correct. If the system is over its background limit then the previous > > cgroup-unaware background writeback occurs. I think of the system > > limits as those of the root cgroup. If the system is over the global > > limit than all cgroups are eligible for writeback. In this situation > > the current code does not distinguish between cgroups over or under > > their dirty background limit. > > > > Vivek Goyal writes: > > > If yes, then from design point of view it is little odd that as long > > > as we are below background limit, we share the bdi between different > > > cgroups. The moment we are above background limit, we fall back to > > > algorithm of sharing the disk among individual inodes and forget > > > about memory cgroups. Kind of awkward. > > > > > > This kind of cgroup writeback I think will atleast not solve the problem > > > for CFQ IO controller, as we fall back to old ways of writting back inodes > > > the moment we cross dirty ratio. > > > > It might make more sense to reverse the order of the checks in the > > proposed over_bground_thresh(): the new version would first check if any > > memcg are over limit; assuming none are over limit, then check global > > limits. Assuming that the system is over its background limit and some > > cgroups are also over their limits, then the over limit cgroups would > > first be written possibly getting the system below its limit. Does this > > address your concern? > > Do you treat root group also as any other cgroup? If no, then above logic > can lead to issue of starvation of root group inode. Or unfair writeback. > So I guess it will be important to treat root group same as other groups. > As far as I can say, you should not place programs onto ROOT cgroups if you need performance isolation. >>From the code, I think if the system hits dirty_ratio, "1" bit of bitmap should be set and background writeback can work for ROOT cgroup seamlessly. Thanks, -Kame