[dpdk-dev] ixgbe packet drops not accounted for

[Zoltan Kiss <zoltan.kiss@linaro.org>]

Hi,

I've seen an odd behaviour in my test setup, which affected my test 
results, so I set up a much simpler scenario.
I'm using netmap pktgen as a packet source, it creates a steady 14.2 
Mpps of 64 byte UDP packets over one port of a 82599ES dual port card. 
This traffic then goes to an another similar machine with the same dual 
port NIC, where it get forwarded out on the other port. The packet sink 
runs on the same machine as the generator, it's also netmap pktgen, and 
it tells me there is a big fluctuation of throughput between 13 and 14 
Mpps, the average comes out around 13.4 Mpps. After I've stripped down 
my test app to nothing but calling rx and tx functions in a loop (it 
doesn't even modifies the MAC address as DPDK l2fwd does), I've started 
to check what rte_eth_tx_burst() tells us. I've added it's return value 
to a counter, and a separate thread printed it out every second 
(sleep(1)), and I've found it reports a steady 14.05 Mpps output. I've 
checked with rte_eth_stats_get(), it gives me the same numbers, and no 
indication of any failure.
When I've connected the generator to the sink directly, it was able to 
receive all the packets, so it's not that the sink is not able to count 
them all. I've even replaced the cables with each other to see if the 
one towards the sink drops some packets, but nothing changed.
I had the impression that once rte_eth_tx_burst() managed to place the 
packets on the descriptor ring, it will go out in some finite time, or 
if the card itself drops it, it will appear in the stats at least, but 
the oerrors and q_errors values are always 0.
Does anyone has an idea where could those packets (avg 0.6 Mpps) get 
dropped?

Regards,

Zoltan Kiss