Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751125Ab1FHEK1 (ORCPT ); Wed, 8 Jun 2011 00:10:27 -0400 Received: from fgwmail5.fujitsu.co.jp ([192.51.44.35]:37075 "EHLO fgwmail5.fujitsu.co.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750797Ab1FHEK0 (ORCPT ); Wed, 8 Jun 2011 00:10:26 -0400 X-SecurityPolicyCheck-FJ: OK by FujitsuOutboundMailChecker v1.3.1 Date: Wed, 8 Jun 2011 13:03:15 +0900 From: KAMEZAWA Hiroyuki To: Greg Thelen Cc: Vivek Goyal , 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: <20110608130315.0a365dbb.kamezawa.hiroyu@jp.fujitsu.com> In-Reply-To: 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> <20110608091815.fdef924d.kamezawa.hiroyu@jp.fujitsu.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=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5693 Lines: 124 On Tue, 7 Jun 2011 21:02:21 -0700 Greg Thelen wrote: > On Tue, Jun 7, 2011 at 5:18 PM, KAMEZAWA Hiroyuki > wrote: > > 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. > > Agreed. > > > 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 > > Not quite. The proposed patches do not set the "1" bit (css_id of > root is 1). mem_cgroup_balance_dirty_pages() (from patch 10/12) > introduces the following balancing loop: > + /* balance entire ancestry of current's mem. */ > + for (; mem_cgroup_has_dirty_limit(mem); mem = > parent_mem_cgroup(mem)) { > > The loop terminates when mem_cgroup_has_dirty_limit() is called for > the root cgroup. The bitmap is set in the body of the loop. So the > root cgroup's bit (bit 1) will never be set in the bitmap. However, I > think the effect is the same. The proposed changes in this patch > (11/12) have background writeback first checking if the system is over > limit and if yes, then b_dirty inodes from any cgroup written. This > means that a small system background limit with an over-{fg or > bg}-limit cgroup could cause other cgroups that are not over their > limit to have their inodes written back. In an system-over-limit > situation normal system-wide bdi writeback is used (writing inodes in > b_dirty order). For those who want isolation, a simple rule to avoid > this is to ensure that that sum of all cgroup background_limits is > less than the system background limit. > Hmm, should we add the rule ? How about disallowing to set dirty_ratio bigger than system's one ? Thanks, -Kame -- 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/