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Date: Wed, 30 Mar 2022 14:37:35 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Jagdish Gediya <jvgediya@...ux.ibm.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
akpm@...ux-foundation.org, aneesh.kumar@...ux.ibm.com,
dave.hansen@...ux.intel.com, ying.huang@...el.com
Subject: Re: [PATCH] mm: migrate: set demotion targets differently
On 3/29/2022 10:04 PM, Jagdish Gediya wrote:
> On Tue, Mar 29, 2022 at 08:26:05PM +0800, Baolin Wang wrote:
> Hi Baolin,
>> Hi Jagdish,
>>
>> On 3/29/2022 7:52 PM, Jagdish Gediya wrote:
>>> The current implementation to identify the demotion
>>> targets limits some of the opportunities to share
>>> the demotion targets between multiple source nodes.
>>>
>>> Implement a logic to identify the loop in the demotion
>>> targets such that all the possibilities of demotion can
>>> be utilized. Don't share the used targets between all
>>> the nodes, instead create the used targets from scratch
>>> for each individual node based on for what all node this
>>> node is a demotion target. This helps to share the demotion
>>> targets without missing any possible way of demotion.
>>>
>>> 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
>>
>> Node 2 and node 3 both are slow memory and have same distance, why node 2
>> should demote cold memory to node 3? They should have the same target
>> demotion node 4, which is the slowest memory node, right?
>>
> Current demotion target finding algorithm works based on best distance, as distance between node 2 & 3 is 40 and distance between node 2 & 4 is 80, node 2 demotes to node 3.
If node 2 can demote to node 3, which means node 3's memory is colder
than node 2, right? The accessing time of node 3 should be larger than
node 2, then we can demote colder memory to node 3 from node 2.
But node 2 and node 3 are same memory type and have same distance, the
accessing time of node 2 and node 3 should be same too, so why add so
many page migration between node 2 and node 3? I'm still not sure the
benefits.
Huang Ying and Dave, how do you think about this demotion targets?
>>>
>>> e.g. with below NUMA topology, where node 0, 1 & 2 are
>>> cpu + dram nodes and node 3 is slow memory node,
>>>
>>> available: 4 nodes (0-3)
>>> 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: 4 5
>>> node 2 size: n MB
>>> node 2 free: n MB
>>> node 3 cpus:
>>> node 3 size: n MB
>>> node 3 free: n MB
>>> node distances:
>>> node 0 1 2 3
>>> 0: 10 20 20 40
>>> 1: 20 10 20 40
>>> 2: 20 20 10 40
>>> 3: 40 40 40 10
>>>
>>> The existing implementation gives below demotion targets,
>>>
>>> node demotion_target
>>> 0 3
>>> 1 X
>>> 2 X
>>> 3 X
>>>
>>> With this patch applied, below are the demotion targets,
>>>
>>> node demotion_target
>>> 0 3
>>> 1 3
>>> 2 3
>>> 3 X
>>
>> Sounds reasonable.
>>
>>>
>>> with below NUMA topology, where node 0 & 2 are cpu + dram
>>> nodes and node 1 & 3 are slow memory nodes,
>>>
>>> available: 4 nodes (0-3)
>>> node 0 cpus: 0 1
>>> node 0 size: n MB
>>> node 0 free: n MB
>>> node 1 cpus:
>>> node 1 size: n MB
>>> node 1 free: n MB
>>> node 2 cpus: 2 3
>>> node 2 size: n MB
>>> node 2 free: n MB
>>> node 3 cpus:
>>> node 3 size: n MB
>>> node 3 free: n MB
>>> node distances:
>>> node 0 1 2 3
>>> 0: 10 40 20 80
>>> 1: 40 10 80 80
>>> 2: 20 80 10 40
>>> 3: 80 80 40 10
>>>
>>> The existing implementation gives below demotion targets,
>>>
>>> node demotion_target
>>> 0 3
>>> 1 X
>>> 2 3
>>> 3 X
>>
>> If I understand correctly, this is not true. The demotion route should be as
>> below with existing implementation:
>> node 0 ---> node 1
>> node 1 ---> X
>> node 2 ---> node 3
>> node 3 ---> X
>>
> Its typo, It should be 0 -> 1, Will correct it in v2.
>>>
>>> With this patch applied, below are the demotion targets,
>>>
>>> node demotion_target
>>> 0 1
>>> 1 3
>>> 2 3
>>> 3 X
>>>
>>> As it can be seen above, node 3 can be demotion target for node
>>> 1 but existing implementation doesn't configure it that way. It
>>> is better to move pages from node 1 to node 3 instead of moving
>>> it from node 1 to swap.
>>
>> Which means node 3 is the slowest memory node?
>>
> Node 1 and 3 are equally slower but 1 is near to 0 and 3 is near to 2. Basically you can think of it like node 1 is slow memory logical node near to node 0 and node 3 is slow memory logical node near to node 2.
OK.
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