Re: [PATCH v7 0/7] mm: Hot page tracking and promotion infrastructure

[Balbir Singh <balbirs@nvidia.com>]

On 5/6/26 13:43, Bharata B Rao wrote:
> On 06-May-26 3:47 AM, Balbir Singh wrote:
>> On 5/5/26 06:36, Matthew Wilcox wrote:
>>> On Mon, May 04, 2026 at 11:39:17AM +0530, Bharata B Rao wrote:
>>>> This is v7 of pghot, a hot-page tracking and promotion subsystem. The
>>>
>>> I continue to think we should not do this.
>>>
>>
>> I am unclear about the benefits of the patchset, I have not tested
>> it or reviewed the latest revision. My big concern was that top-tier
>> might not always be suitable.
> 
> So you are saying that we should have a capability to promote accessed pages
> from lower tier to an other tier that is not classified as top tier? Is that
> non-top tier node the one which generates accesses?
> 

Yes, a top tier node could be CPU less for example.

>>
>> I see that there are some numbers posted, but I find this weird
>> "After the graph creation, the processes are stopped and data is migrated
>> to CXL node 2 before continuing so that BFS phase starts accessing lower
>> tier memory." Why not allocate everything on CXL node 2?
> 
> In the ideal scenario, the benefit is to see if any pages that land up on lower
> tier get identified as hot and get promoted. That means we need to create an
> over-committed scenario where the pages get demoted first. I have provided

Why do the pages need to get demoted? Why not allocate them from the lower tier
to show that promotion upwards is helpful

> numbers from such cases in my previous versions. The problem with this case is
> that the base hot page promotion (NUMAB2) hasn't shown any benefit at all with
> my micro-benchmark - Ref:
> https://lore.kernel.org/linux-mm/868004d8-bb8e-4800-9fdd-ade48e95fe3b@amd.com/
> 
> Same has been observed with redis-memtier benchmark -
> https://lore.kernel.org/linux-mm/957f2242-56d4-4bf0-8aeb-9d60fbea8c8c@amd.com/
> 
> Instead what I am doing here is to take out demotion from the scenario but still
> retain the access pattern of the benchmark by pushing out the data to lower tier
> when the benchmark reaches steady allocation state.
> 

Balbir