[Devel] Re: [PATCH v2 04/11] kmem accounting basic infrastructure

[Ying Han]

On Thu, Aug 16, 2012 at 8:25 AM, Michal Hocko <mhocko at suse.cz> wrote:
> On Wed 15-08-12 12:50:55, Ying Han wrote:
>> On Tue, Aug 14, 2012 at 9:21 AM, Michal Hocko <mhocko at suse.cz> wrote:
>> > On Thu 09-08-12 17:01:12, Glauber Costa wrote:
>> >> This patch adds the basic infrastructure for the accounting of the slab
>> >> caches. To control that, the following files are created:
>> >>
>> >>  * memory.kmem.usage_in_bytes
>> >>  * memory.kmem.limit_in_bytes
>> >>  * memory.kmem.failcnt
>> >>  * memory.kmem.max_usage_in_bytes
>> >>
>> >> They have the same meaning of their user memory counterparts. They
>> >> reflect the state of the "kmem" res_counter.
>> >>
>> >> The code is not enabled until a limit is set. This can be tested by the
>> >> flag "kmem_accounted". This means that after the patch is applied, no
>> >> behavioral changes exists for whoever is still using memcg to control
>> >> their memory usage.
>> >>
>> >> We always account to both user and kernel resource_counters. This
>> >> effectively means that an independent kernel limit is in place when the
>> >> limit is set to a lower value than the user memory. A equal or higher
>> >> value means that the user limit will always hit first, meaning that kmem
>> >> is effectively unlimited.
>> >
>> > Well, it contributes to the user limit so it is not unlimited. It just
>> > falls under a different limit and it tends to contribute less. This can
>> > be quite confusing.  I am still not sure whether we should mix the two
>> > things together. If somebody wants to limit the kernel memory he has to
>> > touch the other limit anyway.  Do you have a strong reason to mix the
>> > user and kernel counters?
>>
>> The reason to mix the two together is a compromise of the two use
>> cases we've heard by far. In google, we only need one limit which
>> limits u & k, and the reclaim kicks in when the total usage hits the
>> limit.
>>
>> > My impression was that kernel allocation should simply fail while user
>> > allocations might reclaim as well. Why should we reclaim just because of
>> > the kernel allocation (which is unreclaimable from hard limit reclaim
>> > point of view)?
>>
>> Some of kernel objects are reclaimable if we have per-memcg shrinker.
>
> Agreed and I think we need that before this is merged as I state in
> other email.
>
>> > I also think that the whole thing would get much simpler if those two
>> > are split. Anyway if this is really a must then this should be
>> > documented here.
>>
>> What would be the use case you have in your end?
>
> I do not have any specific unfortunately but I would like to prevent us
> from closing other possible. I realize this sounds hand wavy and that is
> why I do not want to block this work but I think we should give it some
> time before this gets merged.

Agreed that we don't want to rush merge anything.

On the other hand, I was trying to understand your concern of the k &
u+k counter. After reading your previous replies,
I think I understand your concern of missing the target shrinker. I
posted the patch and please take a look :)

Meanwhile, can you help to clarify other concerns in your mind on
having the two counters? Please ignore me if you answered
the question somewhere and just give me the pointer.

--Ying

>
>> --Ying
> --
> Michal Hocko
> SUSE Labs