TX drops on instance tap interfaces and their cause in Red Hat OpenStack Platform

Issue

During a test of a VNF using host-only networking, high packet loss can be observed in the TX queue of the instance's tap interface. The test setup sends packets from one VM on a node to another VM on the same node. The packet loss appears in bursts.

The following shows a high number of dropped packets in the tap's TX queue.

ip -s -s link ls dev tapc18eb09e-01
69: tapc18eb09e-01: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master qbrc18eb09e-01 state UNKNOWN mode DEFAULT qlen 1000
    link/ether fe:16:3e:a5:17:c0 brd ff:ff:ff:ff:ff:ff
    RX: bytes  packets  errors  dropped overrun mcast   
    5500034259301 132047795 0       0       0       0       
    RX errors: length   crc     frame   fifo    missed
               0        0       0       0       0       
    TX: bytes  packets  errors  dropped carrier collsns 
    5481296464 81741449 0       11155280 0       0       
    TX errors: aborted  fifo   window heartbeat transns
               0        0       0       0       0

Note: the queue length is shown above as qlen 1000.