Re: [dpdk-dev] [PATCH v5 1/5] i40e: Use constant random hash keys

["Zhang, Helin" <helin.zhang@intel.com>]

> -----Original Message-----
> From: Thomas Monjalon [mailto:thomas.monjalon@6wind.com]
> Sent: Monday, November 3, 2014 4:59 PM
> To: Zhang, Helin
> Cc: dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH v5 1/5] i40e: Use constant random hash keys
> 
> 2014-11-03 08:18, Zhang, Helin:
> > From: Thomas Monjalon [mailto:thomas.monjalon@6wind.com]
> > > The title is a bit surprising:
> > > - it should be about RSS
> >
> > RSS makes use of hash function to route received packets, though hash
> > function can be used for other cases, e.g. Flow director.
> 
> Yes but this patch is only changing rss_key_default so I guess it's only related to
> RSS, right?
Yes, it is related to RSS.

> 
> > > - a constant cannot be really random ;)
> >
> > The hash keys are generated by libc random function.
> > It is preparatory to avoid calling random function for each port.
> 
> Here, you remove the call to rte_rand by a constant value.
Yes, actually the hardware just needs random keys by default, no matter what the
real data it is. So array of random data would be better, as no need of cpu cycles,
and it is safer in multi-threads environments.

> 
> > > 2014-10-21 11:14, Helin Zhang:
> > > > To be simpler, and remove the race condition, it uses prepared
> > > > constant random hash keys to replace runtime generating the hash keys.
> > >
> > > Could you explain what is the role of rss_key_default?
> >
> > Hash function needs to be configured with keys, before end users
> > configured them with specific keys, we need to provide a default keys
> > which is generated by libc random function.
> > The random keys can get the hash function to route the received
> > packets to all the queues well-proportioned.

Regards,
Helin