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

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Message-ID: <87lewrxsv1.fsf@linux.ibm.com>
Date:   Wed, 30 Mar 2022 22:47:22 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     "Huang, Ying" <ying.huang@...el.com>,
        Jagdish Gediya <jvgediya@...ux.ibm.com>
Cc:     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

"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.

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