Hi Zhiyong, > > HI, Thomas: > Sorry for late reply. I have been being always considering your > suggestion. > > > -----Original Message----- > > From: Thomas Monjalon [mailto:thomas.monja...@6wind.com] > > Sent: Friday, December 2, 2016 6:25 PM > > To: Yang, Zhiyong <zhiyong.y...@intel.com> > > Cc: dev@dpdk.org; yuanhan....@linux.intel.com; Richardson, Bruce > > <bruce.richard...@intel.com>; Ananyev, Konstantin > > <konstantin.anan...@intel.com>; De Lara Guarch, Pablo > > <pablo.de.lara.gua...@intel.com> > > Subject: Re: [dpdk-dev] [PATCH 1/4] eal/common: introduce rte_memset on > > IA platform > > > > 2016-12-05 16:26, Zhiyong Yang: > > > +#ifndef _RTE_MEMSET_X86_64_H_ > > > > Is this implementation specific to 64-bit? > > > > Yes. > > > > + > > > +#define rte_memset memset > > > + > > > +#else > > > + > > > +static void * > > > +rte_memset(void *dst, int a, size_t n); > > > + > > > +#endif > > > > If I understand well, rte_memset (as rte_memcpy) is using the most recent > > instructions available (and enabled) when compiling. > > It is not adapting the instructions to the run-time CPU. > > There is no need to downgrade at run-time the instruction set as it is > > obviously not a supported case, but it would be nice to be able to upgrade a > > "default compilation" at run-time as it is done in rte_acl. > > I explain this case more clearly for reference: > > > > We can have AVX512 supported in the compiler but disable it when compiling > > (CONFIG_RTE_MACHINE=snb) in order to build a binary running almost > > everywhere. > > When running this binary on a CPU having AVX512 support, it will not benefit > > of the AVX512 improvement. > > Though, we can compile an AVX512 version of some functions and use them > > only if the running CPU is capable. > > This kind of miracle can be achieved in two ways: > > > > 1/ For generic C code compiled with a recent GCC, a function can be built > > for > > several CPUs thanks to the attribute target_clones. > > > > 2/ For manually optimized functions using CPU-specific intrinsics or asm, > > it is > > possible to build them with non-default flags thanks to the attribute > > target. > > > > 3/ For manually optimized files using CPU-specific intrinsics or asm, we use > > specifics flags in the makefile. > > > > The function clone in case 1/ is dynamically chosen at run-time through > > ifunc > > resolver. > > The specific functions in cases 2/ and 3/ must chosen at run-time by > > initializing a function pointer thanks to rte_cpu_get_flag_enabled(). > > > > Note that rte_hash and software crypto PMDs have a run-time check with > > rte_cpu_get_flag_enabled() but do not override CFLAGS in the Makefile. > > Next step for these libraries? > > > > Back to rte_memset, I think you should try the solution 2/. > > I have read the ACL code, if I understand well , for complex algo > implementation, > it is good idea, but Choosing functions at run time will bring some overhead. > For frequently called function > Which consumes small cycles, the overhead maybe is more than the gains > optimizations brings > For example, for most applications in dpdk, memset only set N = 10 or > 12bytes. It consumes fewer cycles.
But then what the point to have an rte_memset() using vector instructions at all? >From what you are saying the most common case is even less then SSE register >size. Konstantin > > Thanks > Zhiyong
