> From: Honnappa Nagarahalli [mailto:honnappa.nagaraha...@arm.com] > Sent: Monday, 8 November 2021 05.33 > > <snip> > > > > >>>>>>>>> Current mempool per core cache implementation is based on > > > >>>>> pointer > > > >>>>>>>>> For most architectures, each pointer consumes 64b Replace > it > > > >>>>> with > > > >>>>>>>>> index-based implementation, where in each buffer is > > > >>>>>>>>> addressed > > > >>>>> by > > > >>>>>>>>> (pool address + index) > > > >>>> > > > >>>> I like Dharmik's suggestion very much. CPU cache is a critical > > > >>>> and limited resource. > > > >>>> > > > >>>> DPDK has a tendency of using pointers where indexes could be > used > > > >>>> instead. I suppose pointers provide the additional flexibility > of > > > >>>> mixing entries from different memory pools, e.g. multiple mbuf > > > >> pools. > > > >>>> > > > >> > > > >> Agreed, thank you! > > > >> > > > >>>>>>>> > > > >>>>>>>> I don't think it is going to work: > > > >>>>>>>> On 64-bit systems difference between pool address and it's > > > elem > > > >>>>>>>> address could be bigger than 4GB. > > > >>>>>>> Are you talking about a case where the memory pool size is > > > >>>>>>> more > > > >>>>> than 4GB? > > > >>>>>> > > > >>>>>> That is one possible scenario. > > > >>>> > > > >>>> That could be solved by making the index an element index > instead > > > of > > > >> a > > > >>>> pointer offset: address = (pool address + index * element > size). > > > >>> > > > >>> Or instead of scaling the index with the element size, which is > > > only > > > >> known at runtime, the index could be more efficiently scaled by > a > > > >> compile time constant such as RTE_MEMPOOL_ALIGN (= > > > >> RTE_CACHE_LINE_SIZE). With a cache line size of 64 byte, that > would > > > >> allow indexing into mempools up to 256 GB in size. > > > >>> > > > >> > > > >> Looking at this snippet [1] from > rte_mempool_op_populate_helper(), > > > >> there is an ‘offset’ added to avoid objects to cross page > > > boundaries. > > > >> If my understanding is correct, using the index of element > instead > > > of a > > > >> pointer offset will pose a challenge for some of the corner > cases. > > > >> > > > >> [1] > > > >> for (i = 0; i < max_objs; i++) { > > > >> /* avoid objects to cross page boundaries */ > > > >> if (check_obj_bounds(va + off, pg_sz, > total_elt_sz) > > > >> < > > > >> 0) { > > > >> off += RTE_PTR_ALIGN_CEIL(va + off, > pg_sz) - > > > >> (va + off); > > > >> if (flags & > RTE_MEMPOOL_POPULATE_F_ALIGN_OBJ) > > > >> off += total_elt_sz - > > > >> (((uintptr_t)(va + off - > 1) % > > > >> total_elt_sz) + > 1); > > > >> } > > > >> > > > > > > > > OK. Alternatively to scaling the index with a cache line size, > you > > > can scale it with sizeof(uintptr_t) to be able to address 32 or 16 > GB > > > mempools on respectively 64 bit and 32 bit architectures. Both x86 > and > > > ARM CPUs have instructions to access memory with an added offset > > > multiplied by 4 or 8. So that should be high performance. > > > > > > Yes, agreed this can be done. > > > Cache line size can also be used when ‘MEMPOOL_F_NO_CACHE_ALIGN’ is > > > not enabled. > > > On a side note, I wanted to better understand the need for having > the > > > 'MEMPOOL_F_NO_CACHE_ALIGN' option. > > > > The description of this field is misleading, and should be corrected. > > The correct description would be: Don't need to align objs on cache > lines. > > > > It is useful for mempools containing very small objects, to conserve > memory. > I think we can assume that mbuf pools are created with the > 'MEMPOOL_F_NO_CACHE_ALIGN' flag set. With this we can use offset > calculated with cache line size as the unit.
You mean MEMPOOL_F_NO_CACHE_ALIGN flag not set. ;-) I agree. And since the flag is a hint only, it can be ignored if the mempool library is scaling the index with the cache line size. However, a mempool may contain other objects than mbufs, and those objects may be small, so ignoring the MEMPOOL_F_NO_CACHE_ALIGN flag may cost a lot of memory for such mempools. > > > > > > > > > > > > > >>>> > > > >>>>>> Another possibility - user populates mempool himself with > some > > > >>>>> external > > > >>>>>> memory by calling rte_mempool_populate_iova() directly. > > > >>>>> Is the concern that IOVA might not be contiguous for all the > > > memory > > > >>>>> used by the mempool? > > > >>>>> > > > >>>>>> I suppose such situation can even occur even with normal > > > >>>>>> rte_mempool_create(), though it should be a really rare one. > > > >>>>> All in all, this feature needs to be configurable during > compile > > > >>>> time. > > > >>> > > > >
