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SubjectRe: [PATCH v3 09/21] x86/asm: add clear_pages_movnt()
On Fri, Jun 10, 2022 at 3:11 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> On Mon, Jun 6, 2022 at 11:39 PM Ankur Arora <ankur.a.arora@oracle.com> wrote:
> >
> > Add clear_pages_movnt(), which uses MOVNTI as the underlying primitive.
> > With this, page-clearing can skip the memory hierarchy, thus providing
> > a non cache-polluting implementation of clear_pages().
> >
> > MOVNTI, from the Intel SDM, Volume 2B, 4-101:
> > "The non-temporal hint is implemented by using a write combining (WC)
> > memory type protocol when writing the data to memory. Using this
> > protocol, the processor does not write the data into the cache
> > hierarchy, nor does it fetch the corresponding cache line from memory
> > into the cache hierarchy."
> >
> > The AMD Arch Manual has something similar to say as well.
> >
> > One use-case is to zero large extents without bringing in never-to-be-
> > accessed cachelines. Also, often clear_pages_movnt() based clearing is
> > faster once extent sizes are O(LLC-size).
> >
> > As the excerpt notes, MOVNTI is weakly ordered with respect to other
> > instructions operating on the memory hierarchy. This needs to be
> > handled by the caller by executing an SFENCE when done.
> >
> > The implementation is straight-forward: unroll the inner loop to keep
> > the code similar to memset_movnti(), so that we can gauge
> > clear_pages_movnt() performance via perf bench mem memset.
> >
> > # Intel Icelakex
> > # Performance comparison of 'perf bench mem memset -l 1' for x86-64-stosb
> > # (X86_FEATURE_ERMS) and x86-64-movnt:
> >
> > System: Oracle X9-2 (2 nodes * 32 cores * 2 threads)
> > Processor: Intel Xeon(R) Platinum 8358 CPU @ 2.60GHz (Icelakex, 6:106:6)
> > Memory: 512 GB evenly split between nodes
> > LLC-size: 48MB for each node (32-cores * 2-threads)
> > no_turbo: 1, Microcode: 0xd0001e0, scaling-governor: performance
> >
> > x86-64-stosb (5 runs) x86-64-movnt (5 runs) Delta(%)
> > ---------------------- --------------------- --------
> > size BW ( stdev) BW ( stdev)
> >
> > 2MB 14.37 GB/s ( +- 1.55) 12.59 GB/s ( +- 1.20) -12.38%
> > 16MB 16.93 GB/s ( +- 2.61) 15.91 GB/s ( +- 2.74) -6.02%
> > 128MB 12.12 GB/s ( +- 1.06) 22.33 GB/s ( +- 1.84) +84.24%
> > 1024MB 12.12 GB/s ( +- 0.02) 23.92 GB/s ( +- 0.14) +97.35%
> > 4096MB 12.08 GB/s ( +- 0.02) 23.98 GB/s ( +- 0.18) +98.50%
>
> For these sizes it may be worth it to save/rstor an xmm register to do
> the memset:
>
> Just on my Tigerlake laptop:
> model name : 11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz
>
> movntdq xmm (5 runs) movnti GPR (5 runs)
> Delta(%)
> ----------------------- -----------------------
> size BW GB/s ( +- stdev) BW GB/s ( +-
> stdev) %
> 2 MB 35.71 GB/s ( +- 1.02) 34.62 GB/s ( +-
> 0.77) -3.15%
> 16 MB 36.43 GB/s ( +- 0.35) 31.3 GB/s ( +-
> 0.1) -16.39%
> 128 MB 35.6 GB/s ( +- 0.83) 30.82 GB/s ( +-
> 0.08) -15.5%
> 1024 MB 36.85 GB/s ( +- 0.26) 30.71 GB/s ( +-
> 0.2) -20.0%


Also (again just from Tigerlake laptop) I found the trend favor
`rep stosb` more (as opposed to non-cacheable writes) when
there are multiple threads competing for BW:

https://docs.google.com/spreadsheets/d/1f6N9EVqHg71cDIR-RALLR76F_ovW5gzwIWr26yLCmS0/edit?usp=sharing
> >
> > Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
> > ---
> > arch/x86/include/asm/page_64.h | 1 +
> > arch/x86/lib/clear_page_64.S | 21 +++++++++++++++++++++
> > 2 files changed, 22 insertions(+)
> >
> > diff --git a/arch/x86/include/asm/page_64.h b/arch/x86/include/asm/page_64.h
> > index a88a3508888a..3affc4ecb8da 100644
> > --- a/arch/x86/include/asm/page_64.h
> > +++ b/arch/x86/include/asm/page_64.h
> > @@ -55,6 +55,7 @@ extern unsigned long __phys_addr_symbol(unsigned long);
> > void clear_pages_orig(void *page, unsigned long npages);
> > void clear_pages_rep(void *page, unsigned long npages);
> > void clear_pages_erms(void *page, unsigned long npages);
> > +void clear_pages_movnt(void *page, unsigned long npages);
> >
> > #define __HAVE_ARCH_CLEAR_USER_PAGES
> > static inline void clear_pages(void *page, unsigned int npages)
> > diff --git a/arch/x86/lib/clear_page_64.S b/arch/x86/lib/clear_page_64.S
> > index 2cc3b681734a..83d14f1c9f57 100644
> > --- a/arch/x86/lib/clear_page_64.S
> > +++ b/arch/x86/lib/clear_page_64.S
> > @@ -58,3 +58,24 @@ SYM_FUNC_START(clear_pages_erms)
> > RET
> > SYM_FUNC_END(clear_pages_erms)
> > EXPORT_SYMBOL_GPL(clear_pages_erms)
> > +
> > +SYM_FUNC_START(clear_pages_movnt)
> > + xorl %eax,%eax
> > + movq %rsi,%rcx
> > + shlq $PAGE_SHIFT, %rcx
> > +
> > + .p2align 4
> > +.Lstart:
> > + movnti %rax, 0x00(%rdi)
> > + movnti %rax, 0x08(%rdi)
> > + movnti %rax, 0x10(%rdi)
> > + movnti %rax, 0x18(%rdi)
> > + movnti %rax, 0x20(%rdi)
> > + movnti %rax, 0x28(%rdi)
> > + movnti %rax, 0x30(%rdi)
> > + movnti %rax, 0x38(%rdi)
> > + addq $0x40, %rdi
> > + subl $0x40, %ecx
> > + ja .Lstart
> > + RET
> > +SYM_FUNC_END(clear_pages_movnt)
> > --
> > 2.31.1
> >

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