Re: [RFC PATCH 0/3] Cgroup-based THP control

[Gutierrez Asier <gutierrez.asier@huawei-partners.com>]

On 10/30/2024 11:38 AM, Michal Hocko wrote:
> On Wed 30-10-24 16:33:08, gutierrez.asier@huawei-partners.com wrote:
>> From: Asier Gutierrez <gutierrez.asier@huawei-partners.com>
>>
>> Currently THP modes are set globally. It can be an overkill if only some
>> specific app/set of apps need to get benefits from THP usage. Moreover, various
>> apps might need different THP settings. Here we propose a cgroup-based THP
>> control mechanism.
>>
>> THP interface is added to memory cgroup subsystem. Existing global THP control
>> semantics is supported for backward compatibility. When THP modes are set
>> globally all the changes are propagated to memory cgroups. However, when a
>> particular cgroup changes its THP policy, the global THP policy in sysfs remains
>> the same.
> 
> Do you have any specific examples where this would be benefitial?

Now we're mostly focused on database scenarios (MySQL, Redis).  

The main idea is to avoid using a global THP setting that can potentially waste 
overall resource and have per cgroup granularity.

Besides THP are being beneficial for DB performance, we observe high THP 
"over-usage" by some unrelated apps/services, when "always" mode is enabled 
globally.

With cgroup-THP, we're able to specify exact "THP-users", and plan to introduce
an ability to limit the amount of THPs per-cgroup.

We suppose it should be beneficial for some container-based workloads, when 
certain containers can have different THP-policies, but haven't looked into 
this case yet.

>> New memcg files are exposed: memory.thp_enabled and memory.thp_defrag, which
>> have completely the same format as global THP enabled/defrag.
>>
>> Child cgroups inherit THP settings from parent cgroup upon creation. Particular
>> cgroup mode changes aren't propagated to child cgroups.
> 
> So this breaks hierarchical property, doesn't it? In other words if a
> parent cgroup would like to enforce a certain policy to all descendants
> then this is not really possible. 

The first idea was to have some flexibility when changing THP policies. 

I will submit a new patch set which will enforce the cgroup hierarchy and change all
the children recursively.

-- 
Asier Gutierrez
Huawei