Re: [PATCH] mm: Compute mTHP order efficiently

[Lance Yang]

On Mon, Sep 16, 2024 at 9:25 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Fri, Sep 13, 2024 at 02:49:02PM +0530, Dev Jain wrote:
> > We use pte_range_none() to determine whether contiguous PTEs are empty
> > for an mTHP allocation. Instead of iterating the while loop for every
> > order, use some information, which is the first set PTE found, from the
> > previous iteration, to eliminate some cases. The key to understanding
> > the correctness of the patch is that the ranges we want to examine
> > form a strictly decreasing sequence of nested intervals.
>
> This is a lot more complicated.  Do you have any numbers that indicate
> that it's faster?  Yes, it's fewer memory references, but you've gone
> from a simple linear scan that's easy to prefetch to an exponential scan
> that might confuse the prefetchers.

+1

I'm not sure if multiple mthp sizes will be enabled for common cases ;)
If not, this could be a bit more complicated, IMO.

@Barry, could you share whether OPPO typically uses multiple mthp sizes
in their scenarios?

Thanks,
Lance