Re: [PATCH] mm/page_alloc: remove prefetchw() on freeing page to buddy system

[Andrew Morton <akpm@linux-foundation.org>]

On Tue,  2 Jul 2024 02:09:31 +0000 Wei Yang <richard.weiyang@gmail.com> wrote:

> The prefetchw() is introduced from an ancient patch[1].
> 
> The change log says:
> 
>     The basic idea is to free higher order pages instead of going
>     through every single one.  Also, some unnecessary atomic operations
>     are done away with and replaced with non-atomic equivalents, and
>     prefetching is done where it helps the most.  For a more in-depth
>     discusion of this patch, please see the linux-ia64 archives (topic
>     is "free bootmem feedback patch").
> 
> So there are several changes improve the bootmem freeing, in which the
> most basic idea is freeing higher order pages. And as Matthew says,
> "Itanium CPUs of this era had no prefetchers."
> 
> I did 10 round bootup tests before and after this change, the data
> doesn't prove prefetchw() help speeding up bootmem freeing. The sum of
> the 10 round bootmem freeing time after prefetchw() removal even 5.2%
> faster than before.

I don't think I've ever seen prefetch make a damn bit of difference.

> [1]: https://lore.kernel.org/linux-ia64/40F46962.4090604@sgi.com/
> 
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> Suggested-by: Matthew Wilcox <willy@infradead.org>
> CC: David Hildenbrand <david@redhat.com>
> 
> ---
> The patch is based on mm-stable with David's change.

Oh help.  David makes many changes.  Please identify patches with much
care.  Fully quoting the email title works, as does a link.