Using IRQbalance to Improve Network Throughput in XenServer
If you are running XenServer 5.6 FP1 or later, there is a little trick you can use to improve network throughput on the host.
By default, XenServer uses the netback process to process network traffic, and each host is limited to four instances of netback, with one instance running on each of dom0’s vCPUs. When a VM starts, each of its VIFs (Virtual InterFaces) is assigned to a netback instance in a round-robin fashion.