Packet Accumulation in RX and TX in bnx2x

Packet Accumulation in RX and TX in bnx2x

[Usman Tanveer]

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Hi,

I have a *BroadCom NetXtreme II BCM57810 10 Gigabit *with a *bnx2x* driver.
I was trying to measure the performance of the network card. I'm running
DPDK-L2FWD application to receive and send packets with MAX_PKT_BURST = 1.
but I noticed that packets are being accumulated somewhere in bnx2x_rxtx.
There is some sort of batching/buffering happening in rxtx. So, the latency
of the packets is increasing as they have to wait.
The first packet of the batch is received with the minimum latency and
incoming packets are received with the accumulated latency of the previous
packets. After a specific number of packets (6-7 packets), the same pattern
repeats i.e. a packet arrives with the minimum latency and incoming packets
with accumulated latency until the specific number of packets arrives. I've
copied the latency measured for some contiguous packets.

I tried to explore bnx2x_recv_pkts() and bnx2x_xmit_pkts() in bnx2x_rxtx.c,
but didn't get any clue why the packets were being accumulated.

Sequence Latency (microseconds)
4825105 9.37207
4825106 10.72168
4825107 15.06543
4825108 19.394043
4825109 22.665039
4825110 26.979004
4825111 8.74707
4825112 11.145996
4825113 14.439941
4825114 18.701172
4825115 23.082031
4825116 27.402832
4825117 9.164062
4825118 11.623047
4825119 15.944824
4825120 19.066406
4825121 23.589355
4825122 27.909668
4825123 9.701172
4825124 11.13916
4825125 15.489746
4825126 19.780762
4825127 23.111816
4825128 27.403809

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