[dpdk-dev] [PATCH 0/4] Optimize memcpy for AVX512 platforms

Vincent JARDIN vincent.jardin at 6wind.com
Fri Jan 15 23:03:28 CET 2016


Le 14 janv. 2016 22:39, "Wang, Zhihong" <zhihong.wang at intel.com> a écrit :
>
>
>
> > -----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.
>


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