On Wed, Mar 16, 2011 at 6:13 AM, Johannes Weiner <hannes@xxxxxxxxxxx> 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). > > 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 = &memcg_mapping inode->i_mapping = 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 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
