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Message-ID: <d695354e-4e7c-e0cd-603b-b585d80d5caa@linux.ibm.com>
Date:   Wed, 8 Jun 2022 13:59:54 +0530
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Subject: Re: [PATCH v5 6/9] mm/demotion: Add support for removing node from
 demotion memory tiers
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To:     Ying Huang <ying.huang@intel.com>, linux-mm@kvack.org,
        akpm@linux-foundation.org
Cc:     Wei Xu <weixugc@google.com>, Greg Thelen <gthelen@google.com>,
        Yang Shi <shy828301@gmail.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Tim C Chen <tim.c.chen@intel.com>,
        Brice Goglin <brice.goglin@gmail.com>,
        Michal Hocko <mhocko@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Hesham Almatary <hesham.almatary@huawei.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Alistair Popple <apopple@nvidia.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Feng Tang <feng.tang@intel.com>,
        Jagdish Gediya <jvgediya@linux.ibm.com>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        David Rientjes <rientjes@google.com>
References: <20220603134237.131362-1-aneesh.kumar@linux.ibm.com>
 <20220603134237.131362-7-aneesh.kumar@linux.ibm.com>
 <a0a70b517bf1da920dca6082afa3fbc6aba77a67.camel@intel.com>
 <81956d2e-0bfe-78ba-5ad0-f6c388c2190e@linux.ibm.com>
 <06d04b6588b43ca010ec78ce0dee8127193f5562.camel@intel.com>
From:   Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>
In-Reply-To: <06d04b6588b43ca010ec78ce0dee8127193f5562.camel@intel.com>
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On 6/8/22 1:53 PM, Ying Huang wrote:
> On Wed, 2022-06-08 at 13:50 +0530, Aneesh Kumar K V wrote:
>> On 6/8/22 12:29 PM, Ying Huang wrote:
>>> On Fri, 2022-06-03 at 19:12 +0530, Aneesh Kumar K.V wrote:
>>>> This patch adds the special string "none" as a supported memtier value
>>>> that we can use to remove a specific node from being using as demotion target.
>>>>
>>>> For ex:
>>>> :/sys/devices/system/node/node1# cat memtier
>>>> 1
>>>> :/sys/devices/system/node/node1# cat ../../memtier/memtier1/nodelist
>>>> 1-3
>>>> :/sys/devices/system/node/node1# echo none > memtier
>>>> :/sys/devices/system/node/node1#
>>>> :/sys/devices/system/node/node1# cat memtier
>>>> :/sys/devices/system/node/node1# cat ../../memtier/memtier1/nodelist
>>>> 2-3
>>>> :/sys/devices/system/node/node1#
>>>
>>> Do you have a practical use case for this?  What kind of memory node
>>> needs to be removed from memory tiers demotion/promotion?
>>>
>>
>> This came up in our internal discussion. It was mentioned that there is
>> a need to skip some slow memory nodes from participating in demotion.
> 
> Again, can you provide a practical use case?  Why we shouldn't demote
> cold pages to these slow memory nodes?  How do we use these slow memory
> node?  These slow memory node is slower than disk?
> 

This was discussed in the context of memory borrowed from remote machine 
(aka OpenCAPI memory). In such case, we would have a memory only NUMA 
node which we want to avoid using for demotion.

-aneesh