Optimizations of Large Receive Offload in Xen

Xen provides us with logically independent computing environments (domains) and I/O devices can be multiplexed so that each domain considers as if it has own instances of I/O devices. These benefits come with the performance overhead and network interface is one of most typical cases. Previously, we ported the large receive offload (LRO) into the physical and virtual network interfaces of Xen and evaluated its effectiveness. In this paper, two optimizations are attempted to further improve the network performance of Xen. First, copying packets at the bridge within the driver domain is eliminated. The aggregated packets are flushed to the upper layer in the network stack when the kernel polls the network device driver Our second optimization is to increase the number of aggregated packets by waiting for every other polling before flushing the packets. Compared to the original LRO, the first optimization reduces the packet handling overhead in the driver domain front 13.4 to 13.0 (clock cycles per transferred byte). However it also increases the overhead in the guest domain from 7.1 to 7.7 and the overall improvement in throughput is negligible. The second optimization reduces the overhead in driver and guest domains from 13.4 to 3.3 and from 7.1 to 5.9, respectively The receive throughput is improved from 577Mbps to 748Mbps.