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SubjectRe: [PATCH] mm: clear 1G pages with streaming stores on x86
On Mon, Mar 9, 2020 at 11:37 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Mon, Mar 09, 2020 at 08:38:31AM -0700, Andi Kleen wrote:
> > > Gigantic huge pages are a bit different. They are much less dynamic from
> > > the usage POV in my experience. Micro-optimizations for the first access
> > > tends to not matter at all as it is usually pre-allocation scenario. On
> > > the other hand, speeding up the initialization sounds like a good thing
> > > in general. It will be a single time benefit but if the additional code
> > > is not hard to maintain then I would be inclined to take it even with
> > > "artificial" numbers state above. There really shouldn't be other downsides
> > > except for the code maintenance, right?
> >
> > There's a cautious tale of the old crappy RAID5 XOR assembler functions which
> > were optimized a long time ago for the Pentium1, and stayed around,
> > even though the compiler could actually do a better job.
> >
> > String instructions are constantly improving in performance (Broadwell is
> > very old at this point) Most likely over time (and maybe even today
> > on newer CPUs) you would need much more sophisticated unrolled MOVNTI variants
> > (or maybe even AVX-*) to be competitive.
>
> Presumably you have access to current and maybe even some unreleased
> CPUs ... I mean, he's posted the patches, so you can test this hypothesis.

I don't have the data at hand, but could reproduce it if strongly
desired, but I've
also tested this on skylake and cascade lake, and we've had success running
with this for a while now.

When developing this originally, I tested all of this compared with
AVX-* instructions as well
as the string ops, they all seemed to be functionally equivalent, and
all were beat out by this
MOVNTI thing for large regions of 1G pages.

There is probably room to further optimize the MOVNTI stuff with
better loop unrolling or
optimizations, if anyone has specific suggestions I'm happy to try to
incorporate them, but
this has shown to be effective as written so far, and I think I lack
that assembly expertise to
micro optimize further on my own.

But just in general, while there are probably some ways this could be
made better, it does a
good job so far for the workloads that are more specific to 1G pages.

Making it work for 2MiB in a convincing general purpose way is a
harder problem and feels
out of scope, and further optimizations can always be added later on
for some other things.

I'm working on a v2 of this patch addressing some of the nits
mentioned by Andrew, should have
that hopefully soon.

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