On Wed, Mar 16, 2011 at 02:13:24PM +0100, 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 <vgoyal@xxxxxxxxxx> 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 control > > > >> > that way, couldn't we? That would fix it. What were the reasons for > > > >> > not doing this? > > > >> > > > >> My rational for pursuing bdi writeback was I/O locality. I have heard that > > > >> per-page I/O has bad locality. Per inode bdi-style writeback should have better > > > >> locality. > > > >> > > > >> My hunch is the best solution is a hybrid which uses a) bdi writeback with a > > > >> target memcg filter and b) using the memcg lru as a fallback to identify the bdi > > > >> that needed writeback. I think the part a) memcg filtering is likely something > > > >> like: > > > >> http://marc.info/?l=linux-kernel&m=129910424431837 > > > >> > > > >> 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 and > > > > 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 handeled > > > > by flusher thread where a bunch or group of pages can be compared with > > > > the cgroup passed in writeback structure. I guess that might hurt us > > > > more than benefit us. > > > > > > Agreed. For 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. But this may be unnecessarily complex. Now I am > > > agreeing with you that option (a) may not be needed. Memcg 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(): > > > > > > while (memcg_usage() >= memcg_fg_limit) { > > > inode = memcg_dirty_inode(cg); /* scan lru for a dirty page, then > > > grab mapping & inode */ > > > sync_inode(inode, &wbc); > > > } > > > > > > if (memcg_usage() >= memcg_bg_limit) { > > > queue per-memcg bg flush work item > > > } > > > > I think even for background we shall have to implement some kind of logic > > 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 > > struct memcg_mapping { > struct address_space *mapping; > struct mem_cgroup *memcg; > }; > > 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). So page->mapping will basically be a list where multiple memcg_mappings are hanging? That will essentially tell what memory cgroups own pages in this inode? And similary every cgroup will have a list where these memcg_mapping are hanging allowing to trace which memcg is doing IO on which inodes? > > 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? May be. I am really not an expert in this area. IIUC, this sounds more like a solution to quickly come up with a list of inodes one should be writting back. One could also come up with this kind of list by going through memcg->lru list also (approximate). So this can be an improvement over going through memcg->lru instead go through memcg->mapping_list. Thanks Vivek -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
