On Thu, Dec 08, 2016 at 09:53:12AM +0000, Yang, Zhiyong wrote: > Hi， Konstantin: > > > -----Original Message----- > > From: Ananyev, Konstantin > > Sent: Thursday, December 8, 2016 5:26 PM > > To: Yang, Zhiyong <zhiyong.y...@intel.com>; Thomas Monjalon > > <thomas.monja...@6wind.com> > > Cc: dev@dpdk.org; yuanhan....@linux.intel.com; Richardson, Bruce > > <bruce.richard...@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 > > > > > > 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 > > For most cases, memset is used such as memset(address, 0, sizeof(struct > xxx)); > The use case here is small by accident, I only give an example here. > but rte_memset is introduced to need consider generic case. > sizeof(struct xxx) is not limited to very small size, such as less than SSE > register size. > I just want to say that the size for the most use case is not very large, So > cycles consumed > Is not large. It is not suited to choose function at run-time since overhead > is considered. > For small copies with sizes specified at compile time, do compilers not fully inline the memset call with a fixed-size equivalent. I believe some compilers used to do so with memcpy - which is why we had a macro for it in DPDK, so that compile-time constant copies would use regular memcpy. If that is also the case for memset, then we should perhaps specify that rte_memset is only for relatively large copies, e.g. >64 bytes. In that case, run-time detection may be worthwhile.
/Bruce
