Re: [dpdk-dev] [RFC] ethdev: rte_eth_rx_burst() requirements for nb_pkts

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

On Thu, Aug 27, 2020 at 10:40:11AM +0200, Morten Brørup wrote:
> Jeff and Ethernet API maintainers Thomas, Ferruh and Andrew,
> 
> I'm hijacking this patch thread to propose a small API modification that prevents unnecessarily performance degradations.
> 
> > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Jeff Guo
> > Sent: Thursday, August 27, 2020 9:55 AM
> > 
> > The limitation of burst size in vector rx was removed, since it should
> > retrieve as much received packets as possible. And also the scattered
> > receive path should use a wrapper function to achieve the goal of
> > burst maximizing.
> > 
> > This patch set aims to maximize vector rx burst for for
> > ixgbe/i40e/ice/iavf PMDs.
> > 
> 
> Now I'm going to be pedantic and say that it still doesn't conform to the rte_eth_rx_burst() API, because the API does not specify any minimum requirement for nb_pkts.
> 
> In theory, that could also be fixed in the driver by calling the non-vector function from the vector functions if nb_pkts is too small for the vector implementation.
> 
> However, I think that calling rte_eth_rx_burst() with a small nb_pkts is silly and not in the spirit of DPDK, and introducing an additional comparison for a small nb_pkts in the driver vector functions would degrade their performance (only slightly, but anyway).
> 

Actually, I'd like to see a confirmed measurement showing a slowdown before
we discard such an option. :-) While I agree that using small bursts is not
keeping with the design approach of DPDK of using large bursts to amortize
costs and allow prefetching, there are cases where a user/app may want a
small burst size, e.g. 4, for latency reasons, and we need a way to support
that.

Since the path selection is dynamic, we need to either:
a) provide a way for the user to specify that they will use smaller bursts
and so that vector functions should not be used
b) have the vector functions transparently fallback to the scalar ones if
used with smaller bursts

Of these, option b) is simpler, and should be low cost since any check is
just once per burst, and - assuming an app is written using the same
request-size each time - should be entirely predictable after the first
call.

/Bruce