Low UDP packets-per-second performance on RHEL/CentOS 6 compared to RHEL/CentOS 7

[alexagr]

I'm running three virtual machines in Azure cloud on the same virtual instances:

• VM size: DS3_v2 (4 vCPUs, 14 GB of RAM)
• Accelerated networking is disabled - therefore hv_netvsc network driver is in use

Three machines have different versions of RHEL/CentOS installed:

• VM#1
  • RHEL 6.8
• VM#2
  • CentOS 6.9
  • LIS 4.2.5.1
• VM#3
  • RHEL 7.4

I need to run VoIP application that receives and transmits large amount of small UDP packets - and therefore depends on packets-per-second (PPS) performance. I've noted that VM#1 and VM#2 perform significantly worse than VM#3. The problem is related to PPS performance:

• VM#1 and VM#2 are capable of handling 70 KPPS
• VM#3 is capable of handling 250 KPPS

The problem can be easily reproduced using NUTTCP test tool:

• take two VMs of the same type
• install 'nuttcp' and 'sysstat' packages on both VMs
• run the following commands on the 1st VM:
  • nuttcp -S
  • sar -n DEV 1
• run the following commands on the 2nd CM:
  • nuttcp -u -l64 -R999999999999999p -T30 -N 128 10.3.2.7 &
  • nuttcp -u -l64 -R999999999999999p -T30 -N 128 10.3.2.7 &
  • nuttcp -u -l64 -R999999999999999p -T30 -N 128 10.3.2.7 &
    (10.3.2.7 is IP of the 1st VM; I'm running three 'client' instances - to make sure that I reach max capacity supported by the VM)
• check “rxpck/s” number produced by “sar” on the 1st VM

Is there any way to improve PPS performance on RHEL/CentOS 6 VMs?