Re: [dpdk-dev] [PATCH v8 1/3] eal/x86: run-time dispatch over memcpy

["Li, Xiaoyun" <xiaoyun.li@intel.com>]

> -----Original Message-----
> From: Thomas Monjalon [mailto:thomas@monjalon.net]
> Sent: Thursday, November 2, 2017 18:45
> To: Wang, Zhihong <zhihong.wang@intel.com>; Li, Xiaoyun
> <xiaoyun.li@intel.com>
> Cc: dev@dpdk.org; Richardson, Bruce <bruce.richardson@intel.com>;
> Ananyev, Konstantin <konstantin.ananyev@intel.com>; Lu, Wenzhuo
> <wenzhuo.lu@intel.com>; Zhang, Helin <helin.zhang@intel.com>;
> ophirmu@mellanox.com
> Subject: Re: [dpdk-dev] [PATCH v8 1/3] eal/x86: run-time dispatch over
> memcpy
> 
> 02/11/2017 11:22, Wang, Zhihong:
> > > I don't know what is creating this drop exactly.
> > > When doing different tests on different environments, we do not see
> > > this drop.
> > > If nobody else can see such issue, I guess we can ignore it.
> >
> > Hi Thomas, Xiaoyun,
> >
> > With this patch (commit 84cc318424d49372dd2a5fbf3cf84426bf95acce) I
> > see more than 20% performance drop in vhost loopback test with testpmd
> > macswap for 256 bytes packets, which means it impacts actual
> > vSwitching performance.
> >
> > Suggest we fix it or revert it for this release.
> 
> I think we need more numbers to take a decision.
> What is the benefit of this patch? In which use-cases?

 The benefit is that if compile it on a lower platform (such as only supports SSE),
when it run on higher platforms (such as AVX2 or AVX512). It would still can get ISA benefit (AVX2).
User case seems to be that some customers want it in cloud environment and don't want to compile on all platforms.

> What are the drawbacks? In which use-cases?

The drawback is perf drop. So far, see lot of drop in mellanox case and vhost case.

Should I send the revert patch or you revert it directly?

> 
> Please, it is a call to test performance with and without this patch in more
> environments (CPU, packet size, applications).