From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail203.messagelabs.com (mail203.messagelabs.com [216.82.254.243]) by kanga.kvack.org (Postfix) with ESMTP id 757BC6B007B for ; Wed, 8 Jun 2011 00:02:46 -0400 (EDT) Received: from wpaz1.hot.corp.google.com (wpaz1.hot.corp.google.com [172.24.198.65]) by smtp-out.google.com with ESMTP id p5842g5N014548 for ; Tue, 7 Jun 2011 21:02:42 -0700 Received: from qwb7 (qwb7.prod.google.com [10.241.193.71]) by wpaz1.hot.corp.google.com with ESMTP id p5842Dff010064 (version=TLSv1/SSLv3 cipher=RC4-SHA bits=128 verify=NOT) for ; Tue, 7 Jun 2011 21:02:41 -0700 Received: by qwb7 with SMTP id 7so61495qwb.40 for ; Tue, 07 Jun 2011 21:02:41 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <20110608091815.fdef924d.kamezawa.hiroyu@jp.fujitsu.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> <20110608091815.fdef924d.kamezawa.hiroyu@jp.fujitsu.com> From: Greg Thelen Date: Tue, 7 Jun 2011 21:02:21 -0700 Message-ID: Subject: Re: [PATCH v8 11/12] writeback: make background writeback cgroup aware Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Sender: owner-linux-mm@kvack.org List-ID: To: KAMEZAWA Hiroyuki 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 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 cg= roup >> > >> 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, >> > >> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 = =A0 =A0 =A0 struct writeback_control *wbc) >> > >> =A0{ >> > >> =A0 =A0 =A0 =A0 =A0unsigned long background_thresh, dirty_thresh; >> > >> >> > >> =A0 =A0 =A0 =A0 =A0global_dirty_limits(&background_thresh, &dirty_t= hresh); >> > >> >> > >> - =A0 =A0 =A0 =A0return (global_page_state(NR_FILE_DIRTY) + >> > >> - =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0global_page_state(NR_UNSTABLE_NFS)= > background_thresh); >> > >> + =A0 =A0 =A0 =A0if (global_page_state(NR_FILE_DIRTY) + >> > >> + =A0 =A0 =A0 =A0 =A0 =A0global_page_state(NR_UNSTABLE_NFS) > backg= round_thresh) { >> > >> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0wbc->for_cgroup =3D 0; >> > >> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0return true; >> > >> + =A0 =A0 =A0 =A0} >> > >> + >> > >> + =A0 =A0 =A0 =A0wbc->for_cgroup =3D 1; >> > >> + =A0 =A0 =A0 =A0wbc->shared_inodes =3D 1; >> > >> + =A0 =A0 =A0 =A0return mem_cgroups_over_bground_dirty_thresh(); >> > >> =A0} >> > > >> > > 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. =A0If the system is over its background limit then the previo= us >> > cgroup-unaware background writeback occurs. =A0I think of the system >> > limits as those of the root cgroup. =A0If the system is over the globa= l >> > limit than all cgroups are eligible for writeback. =A0In this situatio= n >> > 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 pro= blem >> > > 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 a= ny >> > memcg are over limit; assuming none are over limit, then check global >> > limits. =A0Assuming that the system is over its background limit and s= ome >> > cgroups are also over their limits, then the over limit cgroups would >> > first be written possibly getting the system below its limit. =A0Does = this >> > address your concern? >> >> Do you treat root group also as any other cgroup? If no, then above logi= c >> 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 y= ou 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 =3D 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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org