poor network performance when using virtio together with NAT

[freejack]

Using VirtualBox 5.0.12 r104815 with Ubuntu 14.04 64 bit as host system I set up a guest (also Ubuntu 14.04 64 bit) without guest additions. The guest is used as headless server. I am experiencing a very poor network performance when using a virtio adapter together with NAT. I performed several tests using other combinations of bridged networking and Intel PRO/1000 MT Server NIC. Only NAT combined with virtio performs poor:

• * virtio, bridged: 75 MB/s
  * virtio, NAT: 1.5 MB/s
  * Intel PRO/1000 MT Server, bridged: 10 MB/s
  * Intel PRO/1000 MT Server, NAT: 8 MB/s

The bandwidth figures are measured downloading a one GB test file from the guest to the host via wget on the host and apache httpd on the guest. I would have expected virtio+NAT performing at least equal or slightly better than an emulated hardware NIC+NAT. As bridged virtio performes perfectly fine I am currently not supecting the virtio driver in the guest to be the cause of the issue.

[Martin]

Your guest VM is too "big" to run performant:

Host RAM: 11999MB total, 10740MB available
RamSize <integer> = 0x0000000200000000 (8 589 934 592, 8 GB)
You shold leave some more free RAM available for the host

CPUM: Physical host cores: 6
NumCPUs <integer> = 0x0000000000000006 (6)
You should not assign all physical CPU cores to a guest VM, the host needs a core for itself to run your VM effectively.

Are the performance numbers different when you configure your guest to 4 vCPU and 4-6GB RAM?

[freejack]

Thanks for the hint. But using 4 of 6 cores and 4 of 12 GB RAM for the guest did not improved the virtio+NAT bandwidth. I assume there might be an issue which is specific to this combination -- all other network settings work far better.