Re: [PATCH v7 13/16] mm: memory: support clearing page ranges

[Ankur Arora <ankur.a.arora@oracle.com>]

Raghavendra K T <rkodsara@amd.com> writes:

> On 9/18/2025 3:14 AM, Andrew Morton wrote:
>> On Wed, 17 Sep 2025 08:24:15 -0700 Ankur Arora <ankur.a.arora@oracle.com> wrote:
>>
>>> Change folio_zero_user() to clear contiguous page ranges instead of
>>> clearing using the current page-at-a-time approach. Exposing the largest
>>> feasible length can be useful in enabling processors to optimize based
>>> on extent.
>> This patch is something which MM developers might care to take a closer
>> look at.
>>
>>> However, clearing in large chunks can have two problems:
>>>
>>>   - cache locality when clearing small folios (< MAX_ORDER_NR_PAGES)
>>>     (larger folios don't have any expectation of cache locality).
>>>
>>>   - preemption latency when clearing large folios.
>>>
>>> Handle the first by splitting the clearing in three parts: the
>>> faulting page and its immediate locality, its left and right
>>> regions; with the local neighbourhood cleared last.
>> Has this optimization been shown to be beneficial?
>> If so, are you able to share some measurements?
>> If not, maybe it should be removed?
>>
>
> I reverted the effect of this patch by hard coding
>
> #define PAGE_CONTIG_NR 1
>
> I see that benefit for voluntary kernel is lost without this change
>
> (for rep stosb)
>
> with PAGE_CONTIG_NR equivalent to 8MB
>
> Preempt mode: voluntary
>
> # Running 'mem/mmap' benchmark:
> # function 'demand' (Demand loaded mmap())
> # Copying 64GB bytes ...
>
>       34.533414 GB/sec
>
>
> with PAGE_CONTIG_NR equivalent to 4KB
>
> # Running 'mem/mmap' benchmark:
> # function 'demand' (Demand loaded mmap())
> # Copying 64GB bytes ...
>
>       20.766059 GB/sec
>
> For now (barring David's recommendations),
> feel free to add
>
> Reviewed-by: Raghavendra K T <raghavendra.kt@amd.com>

Thanks Raghu.

--
ankur