Bulk create of VMs with default security group fails due to high load of openvswitch

Issue

• With the deployment 'nova + neutron + openvswitch', when we bulk create about 500 VM with a default security group, the CPU usage of neutron-server and openvswitch agent is very high, especially the CPU usage of openvswitch agent will be 100%, this will cause creating VMs failed.

• When creating a stack that deploys 60 instances with 8 nics, most of the instances are created with no ip addresses. Steps to reproduced via OpenStack :

1. create 8 networks 
2. create single instance with 8 nics --> instance is up and running and all ip addresses are exist
3. create 60 instances with 8 nics using heat file - stack
   all instances are running but most of them without ip addresses 

result : 
 tag was not created in  "ovs-vsctl show" after creating an Instance