Le 14 janv. 2016 22:39, "Wang, Zhihong" <zhihong.wang at intel.com> a ?crit : > > > > > -----Original Message----- > > From: Stephen Hemminger [mailto:stephen at networkplumber.org] > > Sent: Friday, January 15, 2016 12:49 AM > > To: Wang, Zhihong <zhihong.wang at intel.com> > > Cc: dev at dpdk.org; Ananyev, Konstantin <konstantin.ananyev at intel.com>; > > Richardson, Bruce <bruce.richardson at intel.com>; Xie, Huawei > > <huawei.xie at intel.com> > > Subject: Re: [PATCH 0/4] Optimize memcpy for AVX512 platforms > > > > On Thu, 14 Jan 2016 01:13:18 -0500 > > Zhihong Wang <zhihong.wang at intel.com> wrote: > > > > > This patch set optimizes DPDK memcpy for AVX512 platforms, to make full > > > utilization of hardware resources and deliver high performance. > > > > > > In current DPDK, memcpy holds a large proportion of execution time in > > > libs like Vhost, especially for large packets, and this patch can bring > > > considerable benefits. > > > > > > The implementation is based on the current DPDK memcpy framework, some > > > background introduction can be found in these threads: > > > http://dpdk.org/ml/archives/dev/2014-November/008158.html > > > http://dpdk.org/ml/archives/dev/2015-January/011800.html > > > > > > Code changes are: > > > > > > 1. Read CPUID to check if AVX512 is supported by CPU > > > > > > 2. Predefine AVX512 macro if AVX512 is enabled by compiler > > > > > > 3. Implement AVX512 memcpy and choose the right implementation based > > on > > > predefined macros > > > > > > 4. Decide alignment unit for memcpy perf test based on predefined macros > > > > > > Zhihong Wang (4): > > > lib/librte_eal: Identify AVX512 CPU flag > > > mk: Predefine AVX512 macro for compiler > > > lib/librte_eal: Optimize memcpy for AVX512 platforms > > > app/test: Adjust alignment unit for memcpy perf test > > > > > > app/test/test_memcpy_perf.c | 6 + > > > .../common/include/arch/x86/rte_cpuflags.h | 2 + > > > .../common/include/arch/x86/rte_memcpy.h | 247 > > ++++++++++++++++++++- > > > mk/rte.cpuflags.mk | 4 + > > > 4 files changed, 255 insertions(+), 4 deletions(-) > > > > > > > This really looks like code that could benefit from Gcc > > function multiversioning. The current cpuflags model is useless/flawed > > in real product deployment > > > I've tried gcc function multi versioning, with a simple add() function > which returns a + b, and a loop calling it for millions of times. Turned > out this mechanism adds 17% extra time to execute, overall it's a lot > of extra overhead. > > Quote the gcc wiki: "GCC takes care of doing the dispatching to call > the right version at runtime". So it loses inlining and adds extra > dispatching overhead. > > Also this mechanism works only for C++, right? > > I think using predefined macros at compile time is more efficient and > suits DPDK more. >
I agree with you: performance first. So having a mix of runtime and compile time would work. For those who are ok with some performance drops, they can go with runtime. > Could you please give an example when the current CPU flags model > stop working? So I can fix it. >