Last month we published a Tech Note summarizing networking throughput results using ESX Server 3.0.1 and XenEnterprise 3.2.0. Multiple NICs were used in order to achieve the maximum throughput possible in a single uniprocessor VM. While these results are very useful for evaluating the virtualization overhead of networking, a more common configuration is to spread the networking load across multiple VMs. We present results for multi-VM networking in a new paper just published. Only a single 1 Gbps NIC is used per VM, but with up to four VMs running simultaneously. This simulates a consolidation scenario of several machines each with substantial, but not extreme, networking I/O. Unlike the multi-NIC paper, there is no exact native analog, but we ran the same total load in a SMP native Windows machine for comparison. The results are similar to the earlier ones: ESX stays close to native performance, achieving up to 3400 Mbps for the 4-VM case. XenEnterprise peaks at 3 VMs and falls off to 62-69% of the ESX throughput with 4 VMs. According to the XenEnterprise documentation only three physical NICs are supported in the host, even though the UI let us configure and run four physical NICs without error or warning. This is not surprising given the performance. We then tried a couple of experiments (like making dom0 use more than 1 CPU) to fix the bottleneck, but only succeeded in further reducing the throughput. The virtualization layer in ESX is always SMP, and together with a battle-tested scheduler and support for 32 e1000 NICs, scales to many heavily-loaded VMs. Let us know if you're able to reach the limits of ESX networking!