rte_memcpy alignment

[Ananyev, Konstantin]

> 
> > From: Ananyev, Konstantin [mailto:konstantin.ananyev at intel.com]
> > Sent: Friday, 14 January 2022 11.54
> >
> > > From: Morten Brørup <mb at smartsharesystems.com>
> > > Sent: Friday, January 14, 2022 9:54 AM
> > >
> > > > From: Bruce Richardson [mailto:bruce.richardson at intel.com]
> > > > Sent: Friday, 14 January 2022 10.11
> > > >
> > > > On Fri, Jan 14, 2022 at 09:56:50AM +0100, Morten Brørup wrote:
> > > > > Dear ARM/POWER/x86 maintainers,
> > > > >
> > > > > The architecture specific rte_memcpy() provides optimized
> > variants to
> > > > copy aligned data. However, the alignment requirements depend on
> > the
> > > > hardware architecture, and there is no common definition for the
> > > > alignment.
> > > > >
> > > > > DPDK provides __rte_cache_aligned for cache optimization
> > purposes,
> > > > with architecture specific values. Would you consider providing an
> > > > __rte_memcpy_aligned for rte_memcpy() optimization purposes?
> > > > >
> > > > > Or should I just use __rte_cache_aligned, although it is
> > overkill?
> > > > >
> > > > >
> > > > > Specifically, I am working on a mempool optimization where the
> > objs
> > > > field in the rte_mempool_cache structure may benefit by being
> > aligned
> > > > for optimized rte_memcpy().
> > > > >
> > > > For me the difficulty with such a memcpy proposal - apart from
> > probably
> > > > adding to the amount of memcpy code we have to maintain - is the
> > > > specific meaning
> > > > of what "aligned" in the memcpy case. Unlike for a struct
> > definition,
> > > > the
> > > > possible meaning of aligned in memcpy could be:
> > > > * the source address is aligned
> > > > * the destination address is aligned
> > > > * both source and destination is aligned
> > > > * both source and destination are aligned and the copy length is a
> > > > multiple
> > > >   of the alignment length
> > > > * the data is aligned to a cacheline boundary
> > > > * the data is aligned to the largest load-store size for system
> > > > * the data is aligned to the boundary suitable for the copy size,
> > e.g.
> > > >   memcpy of 8 bytes is 8-byte aligned etc.
> > > >
> > > > Can you clarify a bit more on your own thinking here? Personally, I
> > am
> > > > a
> > > > little dubious of the benefit of general memcpy optimization, but I
> > do
> > > > believe that for specific usecases there is value is having their
> > own
> > > > copy
> > > > operations which include constraints for that specific usecase. For
> > > > example, in the AVX-512 ice/i40e PMD code, we fold the memcpy from
> > the
> > > > mempool cache into the descriptor rearm function because we know we
> > can
> > > > always do 64-byte loads and stores, and also because we know that
> > for
> > > > each
> > > > load in the copy, we can reuse the data just after storing it
> > (giving
> > > > good
> > > > perf boost). Perhaps something similar could work for you in your
> > > > mempool
> > > > optimization.
> > > >
> > > > /Bruce
> > >
> > > I'm going to copy array of pointers, specifically the 'objs' array in
> > the rte_mempool_cache structure.
> > >
> > > The 'objs' array starts at byte 24, which is only 8 byte aligned. So
> > it always fails the ALIGNMENT_MASK test in the x86 specific
> > > rte_memcpy(), and thus cannot ever use the optimized
> > rte_memcpy_aligned() function to copy the array, but will use the
> > > rte_memcpy_generic() function.
> > >
> > > If the 'objs' array was optimally aligned, and the other array that
> > is being copied to/from is also optimally aligned, rte_memcpy() would
> > use
> > > the optimized rte_memcpy_aligned() function.
> > >
> > > Please also note that the value of ALIGNMENT_MASK depends on which
> > vector instruction set DPDK is being compiled with.
> > >
> > > The other CPU architectures have similar stuff in their rte_memcpy()
> > implementations, and their alignment requirements are also different.
> > >
> > > Please also note that rte_memcpy() becomes even more optimized when
> > the size of the memcpy() operation is known at compile time.
> >
> > If the size is known at compile time, rte_memcpy() probably an overkill
> > - modern compilers usually generate fast enough code for such cases.
> >
> > >
> > > So I am asking for a public #define __rte_memcpy_aligned I can use to
> > meet the alignment requirements for optimal rte_memcpy().
> >
> > Even on x86 ALIGNMENT_MASK could have different values (15/31/63)
> > depending on ISA.
> > So probably 64 as 'generic' one is the safest bet.
> 
> I will use cache line alignment for now.
> 
> > Though I wonder do we really need such micro-optimizations here?
> 
> I'm not sure, but since it's available, I will use it. :-)
> 
> And the mempool get/put functions are very frequently used, so I think we should squeeze out every bit of performance we can.

Well it wouldn't come for free, right?
You would probably need to do some extra checking and add handling for non-aligned cases.
Anyway, will probably just wait for the patch before going into further discussions :)

> 
> > Would it be such huge difference if you call rte_memcpy_aligned()
> > instead of rte_memcpy()?
> 
> rte_memcpy_aligned() is x86 only.
>