Re: [PATCH] mm/vmscan.c: drop_slab_node with task's memcg

[Neil Sun <neilsun@yunify.com>]

On 2021/4/6 19:39, Michal Hocko wrote:
> On Tue 06-04-21 19:30:22, Neil Sun wrote:
>> On 2021/4/6 15:21, Michal Hocko wrote:
>>>
>>> You are changing semantic of the existing user interface. This knob has
>>> never been memcg aware and it is supposed to have a global impact. I do
>>> not think we can simply change that without some users being surprised
>>> or even breaking them.
>>
>> Yes, do you think add new interface to sysfs is a good way? such as
>> /sys/fs/cgroup/memory/lxc/i-vbe1u8o7/memory.kmem.drop_caches
> 
> There were other attempts to add a memcg specific alternative to
> drop_caches. A lack of a strong usecase has been a reason that no such
> attempt has been merged until now. drop_caches is a problematic
> interface because it is really coarse and people have learned to (ab)use
> it to workaround problem rather than fix them properly.
> 
> What is your usecase?
> 

We have some lxc containers running on the server, when mysqld running 
backup jobs in the container, page cache will grow up and eat up all 
unused memory in the container, then some new jobs come, we can see that 
tasks are busy on allocing memory with reclaiming, so we want to drop 
page cache after mysql backup job for individual container, it will 
speed up allocing memory when new jobs come.

This patch only drop slab cache but not page cache, this can be the
first step if people really need this interface.

---
Neil