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From: Alexander Belyakov <abelyako@gmail.com>
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Subject: [dpdk-dev] Surprisingly high TCP ACK packets drop counter
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Hello,

we have simple test application on top of DPDK which sole purpose is to
forward as much packets as possible. Generally we easily achieve 14.5Mpps
with two 82599EB (one as input and one as output). The only suprising
exception is forwarding pure TCP ACK flood when performace always drops to
approximately 7Mpps.

For simplicity consider two different types of traffic:
1) TCP SYN flood is forwarded at 14.5Mpps rate,
2) pure TCP ACK flood is forwarded only at 7Mpps rate.

Both SYN and ACK packets have exactly the same length.

It is worth to mention, this forwarding application looks at Ethernet and
IP headers, but never deals with L4 headers.

We tracked down issue to RX circuit. To be specific, there are 4 RX queues
initialized on input port and rte_eth_stats_get() shows uniform packet
distribution (q_ipackets) among them, while q_errors remain zero for all
queues. The only drop counter quickly increasing in the case of pure ACK
flood is ierrors, while rx_nombuf remains zero.

We tried different kinds of traffic generators, but always got the same
result: 7Mpps (instead of expected 14Mpps) for TCP packets with ACK flag
bit set while all other flag bits dropped. Source IPs and ports are
selected randomly.

Please let us know if anyone is aware of such strange behavior and where
should we look at to narrow down the problem.

Thanks in advance,
Alexander Belyakov