I am testing network throughput between two guests residing on the same 
physical machine. I use a bridge to pass packets between those guests and the 
virtio NIC model. I am wondering why the throughput only goes up to about 
970Mbps. Should we not be able to achieve much higher throughput if the packets 
do not actually go out on the physical wire? What are the limitations for 
throughput performance under KVM/virtio? I can see that by default the 
interfaces (the tap devices) have TX queue length set to 500, and I wonder if 
increasing this would make any difference? Also, are there other things I would 
need to consider to achieve higher throughput numbers for local guest <-> guest 
communication? The CPU is not maxed out at all, and shows as being idle for 
most of the time while the throughput does not increase any more.

I run KVM under standard Fedora Core 10 with a Linux kernel 2.6.27.

Thanks,
Anna

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