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Date:   Thu, 14 Apr 2022 14:18:00 +0530
From:   Jagdish Gediya <jvgediya@linux.ibm.com>
To:     "ying.huang@intel.com" <ying.huang@intel.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        akpm@linux-foundation.org, aneesh.kumar@linux.ibm.com,
        baolin.wang@linux.alibaba.com, dave.hansen@linux.intel.com
Subject: Re: [PATCH v2 1/5] mm: demotion: Set demotion list differently
Message-ID: <YlffwGJg/RWCYja7@li-6e1fa1cc-351b-11b2-a85c-b897023bb5f3.ibm.com>
References: <20220413092206.73974-1-jvgediya@linux.ibm.com>
 <20220413092206.73974-2-jvgediya@linux.ibm.com>
 <4288cbad49690d7c4eb2b1d705eea01221ab8328.camel@intel.com>
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On Thu, Apr 14, 2022 at 03:09:42PM +0800, ying.huang@intel.com wrote:
> On Wed, 2022-04-13 at 14:52 +0530, Jagdish Gediya wrote:
> > Sharing used_targets between multiple nodes in a single
> > pass limits some of the opportunities for demotion target
> > sharing.
> > 
> > Don't share the used targets between multiple nodes in a
> > single pass, instead accumulate all the used targets in
> > source nodes shared by all pass, and reset 'used_targets'
> > to source nodes while finding demotion targets for any new
> > node.
> > 
> > This results into some more opportunities to share demotion
> > targets between multiple source nodes, 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              4
> >  3              4
> >  4              X
> > 
> > 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
> > 
> 
> With the [PATCH v1], you have describe the demotion order changes for
> the following system, I guess there's no change with [PATCH v2]?

Yes, there is no change with v2.

> 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
> 
> And, what is the demotion order for the following system with [PATCH
> v2]?
> 
> Node 0 & 2 are cpu + dram nodes and node 1 are slow
> memory node near node 0,
> 
> available: 3 nodes (0-2)
> 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 distances:
> node   0   1   2
>   0:  10  40  20
>   1:  40  10  80
>   2:  20  80  10

node 1 is demotion target for both node 0 and node 2 with this patch.
node 1 is demotion target only for node 0 with existing implementation,
however if node 1 is near to node 2 instead of node 0, still existing
implementation will give node 1 as demotion target only for node 0 which
is not the correct behavior.

for both the scenario, with this patch applied, node 1 will be demotion
target for both 0 and 2.

> Best Regards,
> Huang, Ying
> 
> 
> [snip]
> 
> 
> 
Best regards,
Jagdish