How to resolve UDP packets being dropped given netstat reporting SndbufErrors

I have a server instance on an OpenStack that at a pretty high load starts loosing UDP packets. I captured all outgoing packets using tcpdump and some of them are missing, even though application logs imply that they should have been sent. Usual packet size is around 60-120 bytes.

Running netstat -s gives:

[root@myServer] ~> netstat -s | grep Udp: -A 5
Udp:
    3855490640 packets received
    133199 packets to unknown port received.
    89 packet receive errors
    4116940753 packets sent
    SndbufErrors: 1396176

When the server is under load, SndbufErrors keeps increasing. I tried to figure out what might be causing it but with no luck even though it feels like this should be covered somewhere.

Q: What might be the reasons for this and how could I resolve this?

Investigation I've done:

1. Running ifconfig -a doesn't show any errors:

     UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
     RX packets:6361554048 errors:0 dropped:0 overruns:0 frame:0
     TX packets:6902945025 errors:0 dropped:0 overruns:0 carrier:0
     collisions:0 txqueuelen:1000
   

   I tried increasing txqueuelen to 10'000 (by running ifconfig eth1 txqueuelen 10000), but it didn't make a difference.

2. Running several sysctl commands I get:

   net.core.rmem_max = 124928
   net.core.wmem_max = 4194304
   net.core.rmem_default = 124928
   net.core.wmem_default = 124928
   

   I tried increasing net.core.rmem_max and net.core.wmem_max to much bigger number 16'777'216, but still keep getting the same errors.

3. Running sar -n UDP 1 1 gives (approximated values, but no errors):

   05:47:31 PM    idgm/s    odgm/s  noport/s idgmerr/s
   05:48:46 PM  23000.00  24000.00      0.00      0.00
   

4. Running ethtool on the Openstack VM instance mostly results in Operation not supported. Running ethtool on the Openstack host server choosing its interface that is used to communicate with outside world, I get:

   [root@myServer] ~> ethtool em1
       Speed: 1000Mb/s
       ... 
   
   [root@myServer] ~> ethtool -g em1
       Ring parameters for em1:
       Pre-set maximums:
       RX:             4096
       RX Mini:        0
       RX Jumbo:       0
       TX:             4096
       Current hardware settings:
       RX:             256
       RX Mini:        0
       RX Jumbo:       0
       TX:             256
   

   I am not convinced that it can be related though as errors I see are inside the VM and not the Openstack host server. Update: I increased the RX and TX values, but to no success.