> From: Bruce Richardson [mailto:bruce.richard...@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. So I am asking for a public #define __rte_memcpy_aligned I can use to meet the alignment requirements for optimal rte_memcpy().
