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

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Message-ID: <0f533707-e9fc-4abd-95fb-bf22c322285d@huawei.com>
Date: Fri, 1 Nov 2024 17:03:46 +0300
From: Stepanov Anatoly <stepanov.anatoly@...wei.com>
To: Michal Hocko <mhocko@...e.com>
CC: Gutierrez Asier <gutierrez.asier@...wei-partners.com>,
	<akpm@...ux-foundation.org>, <david@...hat.com>, <ryan.roberts@....com>,
	<baohua@...nel.org>, <willy@...radead.org>, <peterx@...hat.com>,
	<hannes@...xchg.org>, <hocko@...nel.org>, <roman.gushchin@...ux.dev>,
	<shakeel.butt@...ux.dev>, <muchun.song@...ux.dev>, <cgroups@...r.kernel.org>,
	<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
	<alexander.kozhevnikov@...wei-partners.com>, <guohanjun@...wei.com>,
	<weiyongjun1@...wei.com>, <wangkefeng.wang@...wei.com>,
	<judy.chenhui@...wei.com>, <yusongping@...wei.com>, <artem.kuzin@...wei.com>,
	<kang.sun@...wei.com>, <nikita.panov@...wei-partners.com>
Subject: Re: [RFC PATCH 0/3] Cgroup-based THP control

On 11/1/2024 4:50 PM, Michal Hocko wrote:
> On Fri 01-11-24 16:39:07, Stepanov Anatoly wrote:
>> On 11/1/2024 4:28 PM, Michal Hocko wrote:
>>> On Fri 01-11-24 16:24:55, Stepanov Anatoly wrote:
>>>> On 11/1/2024 4:15 PM, Michal Hocko wrote:
>>>>> On Fri 01-11-24 14:54:27, Stepanov Anatoly wrote:
>>>>>> On 11/1/2024 10:35 AM, Michal Hocko wrote:
>>>>>>> On Thu 31-10-24 17:37:12, Stepanov Anatoly wrote:
>>>>>>>> If we consider the inheritance approach (prctl + launcher), it's fine until we need to change
>>>>>>>> THP mode property for several tasks at once, in this case some batch-change approach needed.
>>>>>>>
>>>>>>> I do not follow. How is this any different from a single process? Or do
>>>>>>> you mean to change the mode for an already running process?
>>>>>>>
>>>>>> yes, for already running set of processes
>>>>>
>>>>
>>>>> Why is that preferred over setting the policy upfront?
>>>> Setting the policy in advance is fine, as the first step to do.
>>>> But we might not know in advance
>>>> which exact policy is the most beneficial for one set of apps or another.
>>
>>>
>>> How do you plan to find that out when the application is running
>>> already?
>> For example, if someone willing to compare some DB server performance with THP-off vs THP-on,
>> and DB server restart isn't an option.

> 
> So you essentially expect user tell you that they want THP and you want
> to make that happen on fly, correct? It is not like there is an actual
> monitoring and dynamic policing.
For a user/sysadmin this scenario is almost the same as experimenting with 
global THP settings, but with explicit THP usage, less THP overuse by some random apps,
so more predictable.

> 
> If that is the case then I am not really convinced this is a worthwhile
> to support TBH. I can see that a workload knows in advance that they
> benefit from THP but I am much more dubious about "learning during the
> runtime" is a real life thing. I might be wrong of course but if
> somebody has performance monitoring that is able to identify performance
> bottlenecks based on specific workload then applying THP on the whole
> group of proceesses seems like a very crude way to deal with that. I

> could see a case for madvice_process(MADV_COLLAPSE) to deal with
> specific memory hotspots though.
Yes, we have something like this in mind.

-- 
Anatoly Stepanov, Huawei