Re: [dpdk-dev] [PATCH] ethdev: rte_eth_rx_burst()nb_pktsrequirements

[Bruce Richardson <bruce.richardson@intel.com>]

On Wed, Oct 14, 2020 at 10:53:24AM +0200, Thomas Monjalon wrote:
> 14/10/2020 10:26, Morten Brørup:
> > From: Ferruh Yigit
> > > On 9/14/2020 1:42 PM, Morten Brørup wrote:
> > > > From: Bruce Richardson
> > > >> On Mon, Sep 14, 2020 at 01:05:11PM +0200, Morten Brørup wrote:
> > > >>> Updated description of rte_eth_rx_burst() to reflect what drivers,
> > > >>> when using vector instructions, expect from nb_pkts.
> > > >>>
> > > >>> Also discussed on the mailing list here:
> > > >>>
> > > >> http://inbox.dpdk.org/dev/98CBD80474FA8B44BF855DF32C47DC35C61257@smarts
> > > >> erver.smartshare.dk/
> > > >>>
> > > >>> --- a/lib/librte_ethdev/rte_ethdev.h
> > > >>> +++ b/lib/librte_ethdev/rte_ethdev.h
> > > >>> @@ -4469,6 +4469,10 @@ int
> > > >> rte_eth_dev_hairpin_capability_get(uint16_t port_id,
> > > >>>    * burst-oriented optimizations in both synchronous and asynchronous
> > > >>>    * packet processing environments with no overhead in both cases.
> > > >>>    *
> > > >>> + * @note
> > > >>> + *   Some drivers using vector instructions require that *nb_pkts*
> > > >> is
> > > >>> + *   divisible by 4 or 8, depending on the driver implementation.
> > > >>> + *
> > > >>
> > > >> Not technically true, in that the drivers will round the value down to
> > > >> the
> > > >> nearest multiple of 4 or 8. So how about rewording as:
> > > >>
> > > >> "Some drivers using vector instructions may round the *nb_pkts* driver
> > > >> to
> > > >> a multiple of 4 or 8 depending upon the driver implementation."
> > > >>
> > > >
> > > > You are correct about the driver behavior.
> > > >
> > > > However, if you pass nb_pkts=9, the driver will return 8 packets,
> > > > and thus it does not conform to the API behavior of returning nb_pkts
> > > > if they are there.
> > > >
> > > > This is why the description in this patch differs from the description we
> > > reached in the RFC discussion.
> > > >
> > > 
> > > Hi Morten, Bruce,
> > > 
> > > +1 to document the this behavior.
> > > 
> > > But in the patch the wording is more strict:
> > > "... require that *nb_pkts* is divisible by 4 or 8 ..."
> > > "... The value must be divisible by 8 in order to work with any driver."
> > > 
> > > I am not sure the requirement is that strict. Application still provide any
> > > value for 'nb_pkts', so the value doesn't "have to" be divisible 8/4.
> > > 
> > > But for vector PMD case it will return number of packets round down to 8/4.
> > > Perhaps can add for vector PMD it must be at least 4/8?
> > > 
> > > Bruce's explanation sound more accurate to me, what do you think?
> > > 
> > 
> > I aim to keep the explanation in the documentation relatively simple. Keep the parameter description short, and add the details about vector driver behavior as a note to the function.
> > 
> > The reason for all this is the existing documentation describing how to use the rte_eth_rx_burst() function at high level:
> > 
> > The rte_eth_rx_burst() function returns the number of packets actually retrieved [...]. A return value equal to nb_pkts indicates [...] that other received packets remain in the input queue. Applications implementing a "retrieve as much received packets as possible" policy can check this specific case and keep invoking the rte_eth_rx_burst() function until a value less than nb_pkts is returned.
> > 
> > As an alternative to my proposed solution, we could add that vector drivers round down to 4 or 8, and the application's comparison of the nb_pkts and return value must consider this. But that would make the above description strangely complex, rather than just requiring that nb_pkts for vector drivers must be divisible by 4 or 8.
> > 
> > And as a minor detail, keeping my proposed restriction would also eliminate the vector drivers' need to round down.
> > 
> > I don't see a need to be able to call rte_eth_rx_burst() with a value not divisible by 4 or 8 for a vector driver, so my proposed restriction is a tradeoff favoring simplicity over unnecessary flexibility.
> 
> It makes sense to me.
>

That sounds reasonable for what we have now. We just need to standardize on
either 4 or 8 as the required factor of the input size. I would suggest
having it as 4, and look to put in fallback paths for the few drivers which
don't support less than 8. I think that 8 is too large a min burst size to
support, for any apps that want small bursts for lower latency.

/Bruce