Re: [PATCH] mm: clear 1G pages with streaming stores on x86

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Tue, Mar 10, 2020 at 11:35:54PM -0400, Arvind Sankar wrote:
> On Wed, Mar 11, 2020 at 03:54:47AM +0300, Kirill A. Shutemov wrote:
> > On Tue, Mar 10, 2020 at 05:21:30PM -0700, Cannon Matthews wrote:
> > > 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.
> > 
> > Andi's point is that string instructions might be a better bet in a long
> > run. You may win something with MOVNTI on current CPUs, but it may become
> > a burden on newer microarchitectures when string instructions improves.
> > Nobody realistically would re-validate if MOVNTI microoptimazation still
> > make sense for every new microarchitecture.
> >
> 
> The rationale for MOVNTI instruction is supposed to be that it avoids
> cache pollution. Aside from the bench that shows MOVNTI to be faster for
> the move itself, shouldn't it have an additional benefit in not trashing
> the CPU caches?
> 
> As string instructions improve, why wouldn't the same improvements be
> applied to MOVNTI?

String instructions inherently more flexible. Implementation can choose
caching strategy depending on the operation size (cx) and other factors.
Like if operation is large enough and cache is full of dirty cache lines
that expensive to free up, it can choose to bypass cache. MOVNTI is more
strict on semantics and more opaque to CPU.

And more importantly string instructions, unlike MOVNTI, is something that
generated often by compiler and used in standard libraries a lot. It is
and will be focus of optimization of CPU architects.

-- 
 Kirill A. Shutemov