linux-kernel - Re: [PATCH] mm: migrate: set demotion targets differently

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Date:   Thu, 31 Mar 2022 08:32:33 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc:     Jagdish Gediya <jvgediya@...ux.ibm.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        baolin.wang@...ux.alibaba.com, dave.hansen@...ux.intel.com
Subject: Re: [PATCH] mm: migrate: set demotion targets differently

"Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com> writes:

> "Huang, Ying" <ying.huang@...el.com> writes:
>
>> Hi, Jagdish,
>>
>> Jagdish Gediya <jvgediya@...ux.ibm.com> writes:
>>
>
> ...
>
>>> e.g. with below NUMA topology, where node 0 & 1 are
>>> cpu + dram nodes, node 2 & 3 are equally slower memory
>>> only nodes, and node 4 is slowest memory only node,
>>>
>>> available: 5 nodes (0-4)
>>> node 0 cpus: 0 1
>>> node 0 size: n MB
>>> node 0 free: n MB
>>> node 1 cpus: 2 3
>>> node 1 size: n MB
>>> node 1 free: n MB
>>> node 2 cpus:
>>> node 2 size: n MB
>>> node 2 free: n MB
>>> node 3 cpus:
>>> node 3 size: n MB
>>> node 3 free: n MB
>>> node 4 cpus:
>>> node 4 size: n MB
>>> node 4 free: n MB
>>> node distances:
>>> node   0   1   2   3   4
>>>   0:  10  20  40  40  80
>>>   1:  20  10  40  40  80
>>>   2:  40  40  10  40  80
>>>   3:  40  40  40  10  80
>>>   4:  80  80  80  80  10
>>>
>>> The existing implementation gives below demotion targets,
>>>
>>> node    demotion_target
>>>  0              3, 2
>>>  1              4
>>>  2              X
>>>  3              X
>>>  4		X
>>>
>>> With this patch applied, below are the demotion targets,
>>>
>>> node    demotion_target
>>>  0              3, 2
>>>  1              3, 2
>>>  2              3
>>>  3              4
>>>  4		X
>>
>> For such machine, I think the perfect demotion order is,
>>
>> node    demotion_target
>>  0              2, 3
>>  1              2, 3
>>  2              4
>>  3              4
>>  4              X
>
> I guess the "equally slow nodes" is a confusing definition here. Now if the
> system consists of 2 1GB equally slow memory and the firmware doesn't want to
> differentiate between them, firmware can present a single NUMA node
> with 2GB capacity? The fact that we are finding two NUMA nodes is a hint
> that there is some difference between these two memory devices. This is
> also captured by the fact that the distance between 2 and 3 is 40 and not 10.

Do you have more information about this?

Best Regards,
Huang, Ying

> For that specific topology where the distance between 2 and 3 is 40 and 2
> and 4 is 80, the demotion target derived by the new code is better
> right? 
>
> ...
>
>
> -aneesh