linux-kernel - Re: [v2 RFC PATCH 0/9] Another Approach to Use PMEM as NUMA Node

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Date:   Thu, 18 Apr 2019 09:24:35 -0700
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Keith Busch <keith.busch@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        mgorman@...hsingularity.net, riel@...riel.com, hannes@...xchg.org,
        akpm@...ux-foundation.org, dan.j.williams@...el.com,
        fengguang.wu@...el.com, fan.du@...el.com, ying.huang@...el.com,
        ziy@...dia.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [v2 RFC PATCH 0/9] Another Approach to Use PMEM as NUMA Node



On 4/17/19 10:51 AM, Michal Hocko wrote:
> On Wed 17-04-19 10:26:05, Yang Shi wrote:
>> On 4/17/19 9:39 AM, Michal Hocko wrote:
>>> On Wed 17-04-19 09:37:39, Keith Busch wrote:
>>>> On Wed, Apr 17, 2019 at 05:39:23PM +0200, Michal Hocko wrote:
>>>>> On Wed 17-04-19 09:23:46, Keith Busch wrote:
>>>>>> On Wed, Apr 17, 2019 at 11:23:18AM +0200, Michal Hocko wrote:
>>>>>>> On Tue 16-04-19 14:22:33, Dave Hansen wrote:
>>>>>>>> Keith Busch had a set of patches to let you specify the demotion order
>>>>>>>> via sysfs for fun.  The rules we came up with were:
>>>>>>> I am not a fan of any sysfs "fun"
>>>>>> I'm hung up on the user facing interface, but there should be some way a
>>>>>> user decides if a memory node is or is not a migrate target, right?
>>>>> Why? Or to put it differently, why do we have to start with a user
>>>>> interface at this stage when we actually barely have any real usecases
>>>>> out there?
>>>> The use case is an alternative to swap, right? The user has to decide
>>>> which storage is the swap target, so operating in the same spirit.
>>> I do not follow. If you use rebalancing you can still deplete the memory
>>> and end up in a swap storage. If you want to reclaim/swap rather than
>>> rebalance then you do not enable rebalancing (by node_reclaim or similar
>>> mechanism).
>> I'm a little bit confused. Do you mean just do *not* do reclaim/swap in
>> rebalancing mode? If rebalancing is on, then node_reclaim just move the
>> pages around nodes, then kswapd or direct reclaim would take care of swap?
> Yes, that was the idea I wanted to get through. Sorry if that was not
> really clear.
>
>> If so the node reclaim on PMEM node may rebalance the pages to DRAM node?
>> Should this be allowed?
> Why it shouldn't? If there are other vacant Nodes to absorb that memory
> then why not use it?
>
>> I think both I and Keith was supposed to treat PMEM as a tier in the reclaim
>> hierarchy. The reclaim should push inactive pages down to PMEM, then swap.
>> So, PMEM is kind of a "terminal" node. So, he introduced sysfs defined
>> target node, I introduced N_CPU_MEM.
> I understand that. And I am trying to figure out whether we really have
> to tream PMEM specially here. Why is it any better than a generic NUMA
> rebalancing code that could be used for many other usecases which are
> not PMEM specific. If you present PMEM as a regular memory then also use
> it as a normal memory.

This also makes some sense. We just look at PMEM from different point of 
view. Taking into account the performance disparity may outweigh 
treating it as a normal memory in this patchset.

A ridiculous idea, may we have two modes? One for "rebalancing", the 
other for "demotion"?