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

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Bruce Richardson [mailto:bruce.richardson@intel.com]
> Sent: Wednesday, October 14, 2020 1:39 PM
> 
> On Wed, Oct 14, 2020 at 01:14:13PM +0200, Morten Brørup wrote:
> > > From: Bruce Richardson [mailto:bruce.richardson@intel.com]
> > > Sent: Wednesday, October 14, 2020 11:30 AM
> > >
> > > 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.
> > >
> >
> > 8 works with all drivers today, so I prefer 8. Also, it seems more future
> proof for even higher bandwidths. If you need low latency, don't use vector
> drivers. Also, DPDK generally seems to prefer throughput over latency, so 8
> seems like the better choice to me.
> >
> > Bruce, if you have a very strong preference for standardizing on 4 and
> you can convince the affected driver developers to modify their drivers to
> support 4, we can go ahead with that.
> >
> Let's go with 8 for now, and we can reduce in future if we can.

Great. If y'all agree now, can I get some Acks, then?

-Morten