_Performance Testing issues_

Branch: dpdk-next-virtio

13d7733f099af8c7d0c11cc84d4b476177bd5ebe --> performance testing fail

Test environment and result as below:

Ubuntu 20.04
Kernel: 4.15.0-generic
Compiler: gcc 7.4
NIC: Mellanox ConnectX-5 100000 Mbps
Target: x86_64-native-linuxapp-gcc
Fail/Total: 2/5

Detail performance results: 
+------------+---------+----------+-------------+------------------------------+
| frame_size | txd/rxd | num_cpus | num_threads |  throughput difference from  |
|            |         |          |             |           expected           |
+============+=========+==========+=============+==============================+
| 64         | 256     | 1        | 1           | -7.9%                        |
+------------+---------+----------+-------------+------------------------------+
| 128        | 256     | 1        | 1           | -4.3%                        |
+------------+---------+----------+-------------+------------------------------+
| 1024       | 256     | 1        | 1           | -0.3%                        |
+------------+---------+----------+-------------+------------------------------+
| 256        | 256     | 1        | 1           | -1.4%                        |
+------------+---------+----------+-------------+------------------------------+
| 1518       | 256     | 1        | 1           | -0.1%                        |
+------------+---------+----------+-------------+------------------------------+

To view detailed results, visit:
https://lab.dpdk.org/results/dashboard/tarballs/17544/

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