Re: [PATCH RFC 0/3] mm: Reduce IO by improving algorithm of memcg pagecache pages eviction

[Michal Hocko]

On Wed 09-01-19 18:43:05, Kirill Tkhai wrote:
> Hi, Michal,
> 
> On 09.01.2019 17:11, Michal Hocko wrote:
> > On Wed 09-01-19 15:20:18, Kirill Tkhai wrote:
> >> On nodes without memory overcommit, it's common a situation,
> >> when memcg exceeds its limit and pages from pagecache are
> >> shrinked on reclaim, while node has a lot of free memory.
> > 
> > Yes, that is the semantic of the hard limit. If the system is not
> > overcommitted then the hard limit can be used to prevent unexpected
> > direct reclaim from unrelated activity.
> 
> According to Documentation/admin-guide/cgroup-v2.rst:
> 
>   memory.max
>         Memory usage hard limit.  This is the final protection
>         mechanism.  If a cgroup's memory usage reaches this limit and
>         can't be reduced, the OOM killer is invoked in the cgroup.
>         Under certain circumstances, the usage may go over the limit
>         temporarily.
> 
> There is nothing about direct reclaim in another memcg. I don't think
> we break something here.

Others in the thread have pointed that out already. What is a hard limit
in one memcg is an isolateion protection in another one. Especially when
the system is not overcommited.

> File pages are accounted to memcg, and this guarantees, that single
> memcg won't occupy all system memory by its unevictible page cache.
> But the suggested patchset follows the same way. Pages, which remain
> in pagecache, are easy-to-be-evicted, since they are not dirty and
> not under writeback. System can drop them fast and in foreseeable time.
> This is cardinal thing about the patchset: remained pages do not
> introduce principal burden on system memory or reclaim time.

What does prevent that the page cache is easily reclaimable? Aka clean
and ready to be dropped? Not to mention that even when the reclaim is
fast it is not free. Especially when you do not expect that because you
haven't reached your hard limit and the admin made sure that hard limits
do not overcommit.

[...]

> > But this also means that any hard limited memcg can fill up all the
> > memory and break the above assumption about the isolation from direct
> > reclaim. Not to mention the OOM or is there anything you do anything
> > about preventing that?
> 
> This is discussed thing. We may add such the pages into tail of LRU list
> instead of head. We may introduce one more separate list to link such
> the pages only, and fastly evict them in case of global reclaim. I don't
> think there is a problem.
>  
> > That beig said, I do not think we want to or even can change the
> > semantic of the hard limit and break existing setups.
> 
> Using the original description and the comments I gave in this message,
> could you please to clarify the way we break existing setups?

isolation as explained above.

> > I am still
> > interested to hear more about more detailed/specific usecases that might
> > benefit from this behavior. Why do those users even use hard limit at
> > all? To protect from anon memory leaks?
> 
> In multi-user machine people want to have size of available to container
> memory equal to the size, which they pay. So, hard limit is needed to prevent
> one container to occupy all system memory via slowly-evictible writeback
> pages, unevictible anon pages, etc. You can't fastly allocate a page,
> in case of many pages are under writeback, this operation is very slow.
> 
> (But unmapped pagecache pages introduced by patchset is another thing:
>  you just need to take not sleeping spinlock to call __delete_from_page_cache()
>  only. This is fast)
> 
> Multi-user machine may have more memory, than sum of all containers hard
> limit. This may be used as an optimization just to reduce disk IO. There
> is no contradiction to sane sense here. And it's not a rare situation.
> In our kernel we have cleancache driver for handling this situation, but
> cleancache is not the best solution like I wrote.
> 
> Not overcommited system is likely case for the patchset, while the below
> is a little less likely:

I beliave Johannes has explained that you are trying to use the hard
limit in a wrong way for something it is not designed for.

-- 
Michal Hocko
SUSE Labs