[EXT] Re: [PATCH v5 2/2] app/testpmd: add command to process Rx metadata negotiation

[Thomas Monjalon]

01/02/2023 11:58, Andrew Rybchenko:
> On 2/1/23 13:48, Jerin Jacob wrote:
> > On Wed, Feb 1, 2023 at 2:59 PM Andrew Rybchenko
> > <andrew.rybchenko at oktetlabs.ru> wrote:
> >> Frankly speaking I don't understand why default value is so
> >> important if we have a way to change it. Reasons should be
> >> really strong to change existing defaults.
> > 
> > The only reason is, typically testpmd will be used performance
> > benchmarking as an industry standard. It is difficult to tell/educate
> > the QA or customers
> > that, "BTW if you need to get better performance add more flag to
> > testpmd command line".

I disagree.
When you do performance benchmark, you tune settings accordingly.

> > To make that worst, only some PMD needs to give the additional
> > parameter to get better number.
> > And also, testpmd usage will be treated as application modeling.
> > 
> > Since this feature only used on sfc and cnxk driver, What is the
> > situation with sfc driver?
> > Keeping it as negotiated and not use the feature, will impact the per
> > core performance of sfc or
> > is it just PCI bandwidth thing which really dont show any difference in testpmd?
> 
> Yes, sfc could run faster if no Rx metadata are negotiated. So,
> it is better to negotiate nothing by default. But it is always
> painful to change defaults. You need to explain that now you
> need to negotiate Rx metadata to use mark, flag and tunnel offloads. 
> Yes, it will be required on sfc and cnxk only.
> As an sfc maintainer I don't mind to change testpmd defaults.

If we change testpmd defaults to "do nothing",
then we should disable MBUF_FAST_FREE as well.