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

[Glauber Costa]

On 08/15/2012 07:34 PM, Christoph Lameter wrote:
> On Wed, 15 Aug 2012, Glauber Costa wrote:
> 
>> On 08/15/2012 06:47 PM, Christoph Lameter wrote:
>>> On Wed, 15 Aug 2012, Michal Hocko wrote:
>>>
>>>>> That is not what the kernel does, in general. We assume that if he wants
>>>>> that memory and we can serve it, we should. Also, not all kernel memory
>>>>> is unreclaimable. We can shrink the slabs, for instance. Ying Han
>>>>> claims she has patches for that already...
>>>>
>>>> Are those patches somewhere around?
>>>
>>> You can already shrink the reclaimable slabs (dentries / inodes) via
>>> calls to the subsystem specific shrinkers. Did Ying Han do anything to
>>> go beyond that?
>>>
>> That is not enough for us.
>> We would like to make sure that the objects being discarded belong to
>> the memcg which is under pressure. We don't need to be perfect here, and
>> an occasional slip is totally fine. But if in general, shrinking from
>> memcg A will mostly wipe out objects from memcg B, we harmed the system
>> in return for nothing good.
> 
> How can you figure out which objects belong to which memcg? The ownerships
> of dentries and inodes is a dubious concept already.
> 

Remember we copy over the metadata and create copies of the caches
per-memcg. Therefore, a dentry belongs to a memcg if it was allocated
from the slab pertaining to that memcg.

It is not 100 % accurate, but it is good enough.