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

Kirill A. Shutemov
2020-03-09T09:35:39Z

On Mon, Mar 09, 2020 at 10:06:30AM +0100, Michal Hocko wrote:
> On Mon 09-03-20 03:08:20, Kirill A. Shutemov wrote:
> > On Fri, Mar 06, 2020 at 05:03:53PM -0800, Cannon Matthews wrote:
> > > Reimplement clear_gigantic_page() to clear gigabytes pages using the
> > > non-temporal streaming store instructions that bypass the cache
> > > (movnti), since an entire 1GiB region will not fit in the cache anyway.
> > >
> > > Doing an mlock() on a 512GiB 1G-hugetlb region previously would take on
> > > average 134 seconds, about 260ms/GiB which is quite slow. Using `movnti`
> > > and optimizing the control flow over the constituent small pages, this
> > > can be improved roughly by a factor of 3-4x, with the 512GiB mlock()
> > > taking only 34 seconds on average, or 67ms/GiB.
> > >
> > > The assembly code for the __clear_page_nt routine is more or less
> > > taken directly from the output of gcc with -O3 for this function with
> > > some tweaks to support arbitrary sizes and moving memory barriers:
> > >
> > > void clear_page_nt_64i (void *page)
> > > {
> > > for (int i = 0; i < GiB /sizeof(long long int); ++i)
> > > {
> > > _mm_stream_si64 (((long long int*)page) + i, 0);
> > > }
> > > sfence();
> > > }
> > >
> > > Tested:
> > > Time to `mlock()` a 512GiB region on broadwell CPU
> > > AVG time (s) % imp. ms/page
> > > clear_page_erms 133.584 - 261
> > > clear_page_nt 34.154 74.43% 67
> >
> > Some macrobenchmark would be great too.
> >
> > > An earlier version of this code was sent as an RFC patch ~July 2018
> > > https://patchwork.kernel.org/patch/10543193/ but never merged.
> >
> > Andi and I tried to use MOVNTI for large/gigantic page clearing back in
> > 2012[1]. Maybe it can be useful.
> >
> > That patchset is somewhat more complex trying to keep the memory around
> > the fault address hot in cache. In theory it should help to reduce latency
> > on the first access to the memory.
> >
> > I was not able to get convincing numbers back then for the hardware of the
> > time. Maybe it's better now.
> >
> > https://lore.kernel.org/r/1345470757-12005-1-git-send-email-kirill.shutemov@linux.intel.com
>
> Thanks for the reminder. I've had only a very vague recollection. Your
> series had a much wider scope indeed. Since then we have gained
> process_huge_page which tries to optimize normal huge pages.
>
> 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?

I cannot think of any, no.

--
Kirill A. Shutemov