Re: [dpdk-dev] No packets received if burst is too small in rte_eth_rx_burst

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

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.

/Bruce

> Thanks,
> Tom
> 
> Tom Barbette 
> PhD Student @ Université de Liège 
> 
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> 
> ----- Mail original -----
> De: "Bruce Richardson" <bruce.richardson@intel.com>
> À: "tom barbette" <tom.barbette@ulg.ac.be>
> Cc: dev@dpdk.org
> Envoyé: Mercredi 14 Décembre 2016 17:52:21
> Objet: Re: [dpdk-dev] No packets received if burst is too small in rte_eth_rx_burst
> 
> On Wed, Dec 14, 2016 at 04:13:53PM +0100, tom.barbette@ulg.ac.be wrote:
> > Hi list,
> > 
> > Between 2.2.0 and 16.04 (up to at least 16.07.2 if not current), with the XL710 controller I do not get any packet when calling rte_eth_rx_burst if nb_pkts is too small. I would say smaller than 32. The input rate is not big, if that helps. But It should definitely get at least one packet per second.
> > 
> > Any ideas? Is that a bug or expected behaviour? Could be caused by other ABI changes?
> > 
> Does this issue still occur even if you disable the vector driver in
> your build-time configuration?
> 
> /Bruce