Re: [RFC][PATCH 1/2] memcg: res_counter hierarchy

[Balbir Singh <balbir@linux.vnet.ibm.com>]

KAMEZAWA Hiroyuki wrote:
> This patch tries to implements _simple_ 'hierarchy policy' in res_counter.
> 
> While several policy of hierarchy can be considered, this patch implements
> simple one 
>    - the parent includes, over-commits the child
>    - there are no shared resource

I am not sure if this is desirable. The concept of a hierarchy applies really
well when there are shared resources.

>    - dynamic hierarchy resource usage management in the kernel is not necessary
> 

Could you please elaborate as to why? I am not sure I understand your point

> works as following.
> 
>  1. create a child. set default child limits to be 0.
>  2. set limit to child.
>     2-a. before setting limit to child, prepare enough room in parent.
>     2-b. increase 'usage' of parent by child's limit.

The problem with this is that you are forcing the parent will run into a reclaim
loop even if the child is not using the assigned limit to it.

>  3. the child sets its limit to the val moved from the parent.
>     the parent remembers what amount of resource is to the children.
> 

All of this needs to be dynamic

>  Above means that
> 	- a directory's usage implies the sum of all sub directories +
>           own usage.
> 	- there are no shared resource between parent <-> child.
> 
>  Pros.
>   - simple and easy policy.
>   - no hierarchy overhead.
>   - no resource share among child <-> parent. very suitable for multilevel
>     resource isolation.

Sharing is an important aspect of hierachies. I am not convinced of this
approach. Did you look at the patches I sent out? Was there something
fundamentally broken in them?

[snip]

-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL