82599ES 10-Gigabit NIC cannot at 10G speed for a long time

[Sun Yalou]

Hi there,I'm developing a custom virtual switch using OVS-DPDK(v19.11.10). 
In order to achieve 10Gbit/s, I modified the OVS pmd mode from 
run-to-completion to pipeline. I used 1 thread as rx thread, 4 threads as 
worker thread and 1 thread as tx thread. it's a load balance archiecture.
I found the forwarding rate can reach 10Gbit/s in first 2 minite, and then 
drop to 5Gbit/s . My traffic generator keeps send packets at 10G rate. I 
debugged and found that packet loss occurred in `rte_eth_tx_burst`, then it 
invoke `ixgbe_xmit_pkts_vec` -> `ixgbe_xmit_fixed_burst_vec` , in 
`ixgbe_xmit_fixed_burst_vec`it should be `txq->nb_tx_free` < `nb_pkts`, it 
means there is not enough TX descriptor to be allocated.
I have tried increasing socket-mem when starting program and reducing the 
size of batch sent, neither worked. I wonder if i am missing something 
important? This question has been bugging me for weeks.
Does anyone know this issue? Thanks, BR, Yalou
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