Re: No packets received if burst is too small in rte_eth_rx_burst

[Thomas Monjalon <thomas.monjalon@6wind.com>]

2016-12-19 10:25, Bruce Richardson:
> On Sat, Dec 17, 2016 at 11:43:25AM +0100, tom.barbette@ulg.ac.be wrote:
> > Hi,
> > 
> > Your comments made me saw the line "PMD: i40e_set_rx_function(): Vector rx enabled, please make sure RX burst size no less than 4 (port=0)."
> > 
> > The problem was probably that I was under this limit... Is there a way to get that limit through a function or something? 
> > 
> > With 16.04 I received sometimes 5 or 7 packets with a burst_size of 4 which respects this limit. I see that "[dpdk-dev] net/i40e: fix out-of-bounds writes during vector Rx" fixed that, as the limit was in fact 32 no matter the message.
> > 
> > At the end, what should be the minimal rx burst size? How to find it at runtime for any NIC? I imagine that vector rx will create a problem if I give a burst size of 1 even with a recent DPDK version, right?
> > 
> 
> Sadly, there doesn't appear to be any way to discover this, and the i40e
> driver requires at least a burst size of 4 even with the latest DPDK.
> From i40e_rxtx_vec_sse.c:
> 
> 243         /* nb_pkts has to be floor-aligned to RTE_I40E_DESCS_PER_LOOP */
> 244         nb_pkts = RTE_ALIGN_FLOOR(nb_pkts, RTE_I40E_DESCS_PER_LOOP);
> 245
> 
> I think in this case the gap is not so much having a discovery mechanism
> to determine min burst size, but rather a driver gap so as to allow some
> form of slower-path fallback when we get below min-size bursts for the
> vector driver.

Yes this is a severe bug.

Please Tom, could you keep asking/monitoring this bug until it is fixed?
Thanks for reporting.