Re: [PATCH v2] RFC: clear 1G pages with streaming stores on x86

[Matthew Wilcox]

On Wed, Jul 25, 2018 at 10:30:40AM -0700, Cannon Matthews wrote:
> On Tue, Jul 24, 2018 at 10:02 PM Elliott, Robert (Persistent Memory)
> > > +     BUG_ON(pages_per_huge_page % PAGES_BETWEEN_RESCHED != 0);
> > > +     BUG_ON(!dest);
> >
> > Are those really possible conditions?  Is there a safer fallback
> > than crashing the whole kernel?
> 
> Perhaps not, I hope not anyhow,  this was something of a first pass
> with paranoid
> invariant checking, and initially I wrote this outside of the x86
> specific directory.
> 
> I suppose that would depend on:
> 
> Is page_to_virt() always available and guaranteed to return something valid?
> Will `page_per_huge_page` ever be anything other than 262144, and if so
> anything besides 512 or 1?

page_to_virt() can only return NULL for HIGHMEM, which we already know
isn't going to be supported.  pages_per_huge_page might vary in the
future, but is always going to be a power of two.  You can turn that into
a build-time assert, or just leave it for the person who tries to change
gigantic pages from being anything other than 1GB.

> It seems like on x86 these conditions will always be true, but I don't know
> enough to say for 100% certain.

They're true based on the current manuals.  If Intel want to change them,
it's fair that they should have to change this code too.

> Before I started this I experimented with all of those variants, and
> interestingly found that I could equally saturate the memory bandwidth with
> 64,128, or 256bit wide instructions on a broadwell CPU ( I did not have a
> skylake/AVX-512 machine available to run the tests on, would be a curious
> thing to see it it holds for that as well).
> 
> >From userspace I did a mmap(MAP_POPULATE), then measured the time
>  to zero a 100GiB region:
> 
> mmap(MAP_POPULATE):     27.740127291
> memset [libc, AVX]:     19.318307069
> rep stosb:              19.301119348
> movntq:                 5.874515236
> movnti:                 5.786089655
> movtndq:                5.837171599
> vmovntdq:               5.798766718
> 
> It was interesting also that both the libc memset using AVX
> instructions
> (confirmed with gdb, though maybe it's more dynamic/tricksy than I know) was
> almost identical to the `rep stosb` implementation.
> 
> I had some conversations with some platforms engineers who thought this made
> sense, but that it is likely to be highly CPU dependent, and some CPUs might be
> able to do larger bursts of transfers in parallel and get better
> performance from
> the wider instructions, but this got way over my head into hardware SDRAM
> controller design. More benchmarking would tell however.
> 
> Another thing to consider about AVX instructions is that they affect core
> frequency and power/thermals, though I can't really speak to specifics but I
> understand that using 512/256 bit instructions and zmm registers can use more
> power and limit the  frequency of other cores or something along those
> lines.
> Anyone with expertise feel free to correct me on this though. I assume this is
> also highly CPU dependent.

There's a difference between using AVX{256,512} load/store and arithmetic
instructions in terms of power draw; at least that's my recollection
from reading threads on realworldtech.  But I think it's not worth
going further than you have.  You've got a really nice speedup and it's
guaranteed to be faster on basically every microarch.  If somebody wants
to do something super-specialised for their microarch, they can submit
a patch on top of yours.