Re: Slow-tier Page Promotion discussion recap and open questions

Raghavendra K T
2024-12-20T11:29:02Z

On 12/18/2024 8:51 PM, Nadav Amit wrote:
>
>
>> On 18 Dec 2024, at 6:19, David Rientjes <rientjes@google.com> wrote:
>>
>> Hi everybody,
>>
>> We had a very interactive discussion last week led by RaghavendraKT on
>> slow-tier page promotion intended for memory tiering platforms, thank
>> you! Thanks as well to everybody who attended and provided great
>> questions, suggestions, and feedback.
>>
>> The RFC patch series "mm: slowtier page promotion based on PTE A bit"[1]
>> is a proposal to allow for asynchronous page promotion based on memory
>> accesses as an alternative to NUMA Balancing based promotions. There was
>> widespread interest in this topic and the discussion surfaced multiple
>> use cases and requirements, very focused on CXL use cases.
>>
>
> Just sharing my 2 cents.
>
> IIUC, the suggested approach has two benefits:
>
> 1. Fewer/no page-faults (as A-bit is used to detect usage)
> 2. Batching
>
> While (2) seems like a win that might be added un top of AutoNUMA, (1)
> is more delicate. As indicated in the patch-set, the "exact destination”
> is lost. At the same time, the last time I checked, the A-bit setting
> wasn’t free and cost something like 550 cycles (others saw similar
> results [1]).
>
> So considering empty page-fault is ~1050 cycles (2014 number Linus
> measured [2]), there is a question how big of a win it is...
>
>
> [1] https://lore.kernel.org/all/20160620000606.GB3194@blaptop/
> [2] Google+ post RIP

Thanks for the feedback. (as I noted in other post), can A bit scanning
that detects hot VMA information be fed to NUMAB=1 scanning?

Regards
- Raghu