Re: [PATCH v6 0/9] memcg: per cgroup dirty page accounting

[Vivek Goyal <vgoyal@redhat.com>]

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@redhat.com> 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

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