Re: [RFC][PATCH 1/6] memcg: free all at rmdir

[Balbir Singh]

Andrew Morton wrote:
> On Thu, 13 Nov 2008 05:53:49 +0530
> Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> wrote:
> 
>> Andrew Morton wrote:
>>> On Wed, 12 Nov 2008 12:26:56 +0900
>>> KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
>>>
>>>> +5.1 on_rmdir
>>>> +set behavior of memcg at rmdir (Removing cgroup) default is "drop".
>>>> +
>>>> +5.1.1 drop
>>>> +       #echo on_rmdir drop > memory.attribute
>>>> +       This is default. All pages on the memcg will be freed.
>>>> +       If pages are locked or too busy, they will be moved up to the parent.
>>>> +       Useful when you want to drop (large) page caches used in this memcg.
>>>> +       But some of in-use page cache can be dropped by this.
>>>> +
>>>> +5.1.2 keep
>>>> +       #echo on_rmdir keep > memory.attribute
>>>> +       All pages on the memcg will be moved to its parent.
>>>> +       Useful when you don't want to drop page caches used in this memcg.
>>>> +       You can keep page caches from some library or DB accessed by this
>>>> +       memcg on memory.
>>> Would it not be more useful to implement a per-memcg version of
>>> /proc/sys/vm/drop_caches?  (One without drop_caches' locking bug,
>>> hopefully).
>>>
>>> If we do this then we can make the above "keep" behaviour non-optional,
>>> and the operator gets to choose whether or not to drop the caches
>>> before doing the rmdir.
>>>
>>> Plus, we get a new per-memcg drop_caches capability.  And it's a nicer
>>> interface, and it doesn't have the obvious races which on_rmdir has,
>>> etc.
>>>
>> Andrew, I suspect that will not be easy, since we don't track address spaces
>> that belong to a particular memcg. If page cache ends up being shared across
>> memcg's, dropping them would impact both mem cgroups.
>>
> 
> walk the LRUs?

We do that for the force_empty() interface we have. Although we don't
differentiate between cache and RSS at the moment.

-- 
	Balbir
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