On Tue, Jul 02, 2024 at 08:57:57AM +0200, David Hildenbrand wrote:
> > 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 suspect this is noise, though.
I think it's real, though small. Each prefetchw() is an instruction,
and if we can avoid issuing an instruction, we should.
> Something like:
>
> for (;;) {
> ...
> if (++loop >= nr_pages)
> break;
> p++;
> }
>
>
> Might generate slightly better code, because we know that we execute the
> loop body at least once. We use that in set_ptes(), for example.
I don't think it's worth doing. Keep the loop simple and obvious.
set_ptes() is different because we actually expect to execute the loop
exactly once (ie most folios are small). So two compares per call to
set_ptes() instead of one makes a difference. Here, we're expecting
to execute this loop, what, a million times? Doing a million-and-one
compares instead of a million makes no observable difference.
I would like to see v2 of this patch dropped, please Andrew.
