Re: [dpdk-dev] [PATCH 1/4] eal/common: introduce rte_memset on IA platform

[Bruce Richardson <bruce.richardson@intel.com>]

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.yang@intel.com>; Thomas Monjalon
> > <thomas.monjalon@6wind.com>
> > Cc: dev@dpdk.org; yuanhan.liu@linux.intel.com; Richardson, Bruce
> > <bruce.richardson@intel.com>; De Lara Guarch, Pablo
> > <pablo.de.lara.guarch@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.monjalon@6wind.com]
> > > > Sent: Friday, December 2, 2016 6:25 PM
> > > > To: Yang, Zhiyong <zhiyong.yang@intel.com>
> > > > Cc: dev@dpdk.org; yuanhan.liu@linux.intel.com; Richardson, Bruce
> > > > <bruce.richardson@intel.com>; Ananyev, Konstantin
> > > > <konstantin.ananyev@intel.com>; De Lara Guarch, Pablo
> > > > <pablo.de.lara.guarch@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