Re: [PATCH v2 2/2] mm: speed up mremap by 500x on large regions

[Joel Fernandes]

On Fri, Oct 12, 2018 at 07:25:08PM -0700, Daniel Colascione wrote:
[...] 
> > But anyway, I think this runtime detection thing is not needed. THP is
> > actually expected to be as fast as this anyway, so if that's available then
> > we should already be as fast.
> 
> Ah, I think the commit message is confusing. (Or else I'm misreading
> the patch now.) It's not quite that we're disabling the feature when
> THP is enabled anywhere, but rather that we use the move_huge_pmd path
> for huge PMDs and use the new code only for non-huge PMDs. (Right?) If
> that's the case, the commit message shouldn't say "Incase THP is
> enabled, the optimization is skipped". Even if THP is enabled on a
> system generally, we might use the new PMD-moving code for mapping
> types that don't support THP-ization, right?

That is true. Ok, I guess I can update the commit message to be more accurate
about that.

> > This is for non-THP where THP cannot be enabled
> > and there is still room for some improvement. Most/all architectures will be
> > just fine with this. This flag is more of a safety-net type of thing where in
> > the future if there is this one or two weird architectures that don't play
> > well, then they can turn it off at the architecture level by not selecting
> > the flag. See my latest patches for the per-architecture compile-time
> > controls. Ideally we'd like to blanket turn it on on all, but this is just
> > playing it extra safe as Kirill and me were discussing on other threads.
> 
> Sure. I'm just pointing out that the 500x performance different turns
> the operation into a qualitatively different feature, so if we expect
> to actually ship a mainstream architecture without support for this
> thing, we should make it explicit. If we're not, we shouldn't.

We can make it explicit by enabling it in such a mainstream architecture is
my point. Also if the optimization is not doing what its supposed to, then
userspace will also just know by measuring the time.

thanks,

 - Joel