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

[Greg Thelen]

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
  }

Does this look sensible?
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