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

[Kirill Tkhai]

On 09.01.2019 20:10, Michal Hocko wrote:
> 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.

Yes, I mean it's clean and ready to drop.

I understand the problem, so in case of people worry about reclaim speed
increasing, this does not mean we should completely forget this way. This
means we possible may find a compromise, which is suitable for everybody.

>>> 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.

In general, I think a some time useful design is not a Bible, that nobody
is allowed to change. We should not limit us in something, in case of this
has a sense and may be useful. This is just a note in general.