From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail172.messagelabs.com (mail172.messagelabs.com [216.82.254.3]) by kanga.kvack.org (Postfix) with ESMTP id D8E158D0039 for ; Wed, 16 Mar 2011 17:19:55 -0400 (EDT) Received: from wpaz5.hot.corp.google.com (wpaz5.hot.corp.google.com [172.24.198.69]) by smtp-out.google.com with ESMTP id p2GLJlk6031983 for ; Wed, 16 Mar 2011 14:19:48 -0700 Received: from qyk2 (qyk2.prod.google.com [10.241.83.130]) by wpaz5.hot.corp.google.com with ESMTP id p2GLIRHx016763 (version=TLSv1/SSLv3 cipher=RC4-SHA bits=128 verify=NOT) for ; Wed, 16 Mar 2011 14:19:46 -0700 Received: by qyk2 with SMTP id 2so2000449qyk.16 for ; Wed, 16 Mar 2011 14:19:46 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <20110316131324.GM2140@cmpxchg.org> References: <1299869011-26152-1-git-send-email-gthelen@google.com> <20110311171006.ec0d9c37.akpm@linux-foundation.org> <20110314202324.GG31120@redhat.com> <20110315184839.GB5740@redhat.com> <20110316131324.GM2140@cmpxchg.org> From: Greg Thelen Date: Wed, 16 Mar 2011 14:19:26 -0700 Message-ID: Subject: Re: [PATCH v6 0/9] memcg: per cgroup dirty page accounting Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Sender: owner-linux-mm@kvack.org List-ID: To: Johannes Weiner 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 , KAMEZAWA Hiroyuki , Daisuke Nishimura , Minchan Kim , Ciju Rajan K , David Rientjes , Wu Fengguang , Chad Talbott , Justin TerAvest On Wed, Mar 16, 2011 at 6:13 AM, Johannes Weiner wrote= : > On Tue, Mar 15, 2011 at 02:48:39PM -0400, Vivek Goyal wrote: >> On Mon, Mar 14, 2011 at 07:41:13PM -0700, Greg Thelen wrote: >> > On Mon, Mar 14, 2011 at 1:23 PM, Vivek Goyal wrote= : >> > > On Mon, Mar 14, 2011 at 11:29:17AM -0700, Greg Thelen wrote: >> > > >> > > [..] >> > >> > We could just crawl the memcg's page LRU and bring things under c= ontrol >> > >> > that way, couldn't we? =A0That would fix it. =A0What were the rea= sons for >> > >> > not doing this? >> > >> >> > >> My rational for pursuing bdi writeback was I/O locality. =A0I have = heard that >> > >> per-page I/O has bad locality. =A0Per inode bdi-style writeback sho= uld have better >> > >> locality. >> > >> >> > >> My hunch is the best solution is a hybrid which uses a) bdi writeba= ck with a >> > >> target memcg filter and b) using the memcg lru as a fallback to ide= ntify the bdi >> > >> that needed writeback. =A0I think the part a) memcg filtering is li= kely something >> > >> like: >> > >> =A0http://marc.info/?l=3Dlinux-kernel&m=3D129910424431837 >> > >> >> > >> The part b) bdi selection should not be too hard assuming that page= -to-mapping >> > >> locking is doable. >> > > >> > > Greg, >> > > >> > > IIUC, option b) seems to be going through pages of particular memcg = and >> > > mapping page to inode and start writeback on particular inode? >> > >> > Yes. >> > >> > > If yes, this might be reasonably good. In the case when cgroups are = not >> > > sharing inodes then it automatically maps one inode to one cgroup an= d >> > > once cgroup is over limit, it starts writebacks of its own inode. >> > > >> > > In case inode is shared, then we get the case of one cgroup writting >> > > back the pages of other cgroup. Well I guess that also can be handel= ed >> > > by flusher thread where a bunch or group of pages can be compared wi= th >> > > the cgroup passed in writeback structure. I guess that might hurt us >> > > more than benefit us. >> > >> > Agreed. =A0For now just writing the entire inode is probably fine. >> > >> > > IIUC how option b) works then we don't even need option a) where an = N level >> > > deep cache is maintained? >> > >> > Originally I was thinking that bdi-wide writeback with memcg filter >> > was a good idea. =A0But this may be unnecessarily complex. =A0Now I am >> > agreeing with you that option (a) may not be needed. =A0Memcg could >> > queue per-inode writeback using the memcg lru to locate inodes >> > (lru->page->inode) with something like this in >> > [mem_cgroup_]balance_dirty_pages(): >> > >> > =A0 while (memcg_usage() >=3D memcg_fg_limit) { >> > =A0 =A0 inode =3D memcg_dirty_inode(cg); =A0/* scan lru for a dirty pa= ge, then >> > grab mapping & inode */ >> > =A0 =A0 sync_inode(inode, &wbc); >> > =A0 } >> > >> > =A0 if (memcg_usage() >=3D memcg_bg_limit) { >> > =A0 =A0 queue per-memcg bg flush work item >> > =A0 } >> >> I think even for background we shall have to implement some kind of logi= c >> where inodes are selected by traversing memcg->lru list so that for >> background write we don't end up writting too many inodes from other >> root group in an attempt to meet the low background ratio of memcg. >> >> So to me it boils down to coming up a new inode selection logic for >> memcg which can be used both for background as well as foreground >> writes. This will make sure we don't end up writting pages from the >> inodes we don't want to. > > Originally for struct page_cgroup reduction, I had the idea of > introducing something like > > =A0 =A0 =A0 =A0struct memcg_mapping { > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0struct address_space *mapping; > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0struct mem_cgroup *memcg; > =A0 =A0 =A0 =A0}; > > hanging off page->mapping to make memcg association no longer per-page > and save the pc->memcg linkage (it's not completely per-inode either, > multiple memcgs can still refer to a single inode). > > We could put these descriptors on a per-memcg list and write inodes > from this list during memcg-writeback. > > We would have the option of extending this structure to contain hints > as to which subrange of the inode is actually owned by the cgroup, to > further narrow writeback to the right pages - iff shared big files > become a problem. > > Does that sound feasible? If I understand your memcg_mapping proposal, then each inode could have a collection of memcg_mapping objects representing the set of memcg that were charged for caching pages of the inode's data. When a new file page is charged to a memcg, then the inode's set of memcg_mapping would be scanned to determine if current's memcg is already in the memcg_mapping set. If this is the first page for the memcg within the inode, then a new memcg_mapping would be allocated and attached to the inode. The memcg_mapping may be reference counted and would be deleted when the last inode page for a particular memcg is uncharged. page->mapping =3D &memcg_mapping inode->i_mapping =3D collection of memcg_mapping, grows/shrinks with [un]= charge Am I close? I still have to think though the various use cases, but I wanted to make sure I had the basic idea. -- 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