Re: [PATCH] mm: limit THP alignment – performance gain observed in AI inference workloads

[Dev Jain <dev.jain@arm.com>]

On 01/07/25 11:23 am, Lorenzo Stoakes wrote:
> On Tue, Jul 01, 2025 at 11:15:25AM +0530, Dev Jain wrote:
>> Sorry I am not following, don't know in detail about the VMA merge stuff.
>> Are you saying the after the patch, the VMAs will eventually get merged?
>> Is it possible in the kernel to get a merge in the "future"; as I understand
>> it only happens at mmap() time?
>>
>> Suppose before the patch, you have two consecutive VMAs between (PMD, 2*PMD) size.
>> If they are able to get merged after the patch, why won't they be merged before the patch,
>> since the VMA characteristics are the same?
>>
>>
> Rik's patch aligned each to 2 MiB boundary. So you'd get gaps:
>
>
>    0            2MB                      4MB           6MB                      8MB          10MB
>    |-------------.------|                 |-------------.------|                 |-------------.------|
>    |             .      |		 |             .      |                 |             .      |
>    |             .      |		 |             .      |                 |             .      |
>    |-------------.------|		 |-------------.------|                 |-------------.------|
>      huge mapped  4k m'd

The effort to draw this is appreciated!

I understood the alignment, what I am asking is this:

In __get_unmapped_area(), we will return a THP-aligned addr from
thp_get_unmapped_area_vmflags(). Now for the diagram you have
drawn, suppose that before the patch, we first mmap() the
8MB-start chunk. Then we mmap the 4MB start chunk.
We go to __mmap_region(), and we see that the 8MB-start chunk
has mergeable characteristics, so we merge. So the gap goes away?

>
> If you don't force alignment then subsequent mappings will be adjacent to one
> another and those non-huge page parts can be merged.
>
> Vlasta's fix up means we only try to get the THP up-front if the length is
> already aligned at which point you won't end up with these gaps.