virtual machines network goes 100Mbit when interface is Gbit

Hello all,

I'm testing the last kvm version, 62. I was doing an iperf benchmark to test the network performance in a Feisty virtual machine and I got values surround 100Mbits when my card is actually 1Gbit. The testing environment is conformed by 2 machines, both with Gbit interfaces. The vm in kvm is running in server mode, the other (not virtualized one) is the client. The vm interface is going to a bridge called br0. Ethtool says the interface is running 100Mbits but it doesn't let me change the speed or any other parameter in the vm.

I can't either change tap0 parameters in kvm host. ¿Is anyone having the same issue?¿am I forgetting something?

Thanks for your time in advanced,
Miguel

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What emulated nic are you using? I think only the e1000 emulated nic supports 1 GB.

Quoted 17 lines · Miguel Araujo

Hello all,

I'm testing the last kvm version, 62. I was doing an iperf benchmark to test the network performance in a Feisty virtual machine and I got values surround 100Mbits when my card is actually 1Gbit. The testing environment is conformed by 2 machines, both with Gbit interfaces. The vm in kvm is running in server mode, the other (not virtualized one) is the client. The vm interface is going to a bridge called br0. Ethtool says the interface is running 100Mbits but it doesn't let me change the speed or any other parameter in the vm.

I can't either change tap0 parameters in kvm host. ¿Is anyone having the same issue?¿am I forgetting something?

Thanks for your time in advanced,
Miguel

Quoted 10 lines

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Hello,
I'm not sure if I understand you. Do you mean that the physical
network cards should be e1000 or that the virtual machines must somehow emulate e1000 network cards?

As I understand Miguel's post, he is finding an issue with the
bridge's speed (two virtual machines running on the same physical machine are limited to 100 Mbps between themselves).

Best regards,
Víctor

Quoted 36 lines · Haydn Solomon

What emulated nic are you using? I think only the e1000 emulated nic supports 1 GB.

Quoted 9 lines · Miguel Araujo

Hello all,

I'm testing the last kvm version, 62. I was doing an iperf benchmark to test the network performance in a Feisty virtual machine and I got values surround 100Mbits when my card is actually 1Gbit. The testing environment is conformed by 2 machines, both with Gbit interfaces. The vm in kvm is running in server mode, the other (not virtualized one) is the client. The vm interface is going to a bridge called br0. Ethtool says the interface is running 100Mbits but it doesn't let me change the speed or any other parameter in the vm.

I can't either change tap0 parameters in kvm host. ¿Is anyone having the same issue?¿am I forgetting something?

Thanks for your time in advanced,
Miguel

Quoted 11 lines

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Thanks for your answer Haydn, but I did not understand the issue too. Victor has explained my problem clearly, is kvm uncapable of running the bridge at 1GBit if it is not a e1000 network card?

Quoted 3 lines

Thanks for your answer Haydn, but I did not understand the issue too. Victor has explained my problem clearly, is kvm uncapable of running the bridge at 1GBit if it is not a e1000 network card?

Hey Miguel,

what Haydn meant was to force qemu to *emulate* a e1000 NIC instead of the default RTL8139. That emulation is done by qemu in userspace and has absolutely nothing to do with your physical NIC por the bridge. Chosing a NIC is done with the "model=" parameter.
kvm -net nic,model=e1000

Even better performance-wise would be to use the new virtio interface which paravirtualizes your physical NIC instead of emulating one. For that, use:
kvm -net nic,model=virtio

Of course you'll need to install the corresponding drivers in your guest. The e1000 should be supported in every guest and for the virtio interface there are windows guest drivers (see the kvm-announce mailing list for that) as well as linux guest drivers included in kernel 2.6.25 upwards.

By the way: This is good stuff f you're interested in hard disk performance, too: http://kvm.qumranet.com/kvmwiki/Virtio

Felix

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See also

http://www.linux-kvm.com/content/tip-how-setup-windows-guest-paravirtual-network-drivers

Quoted 1 line · On Thu, Apr 3, 2008 at 10:54 AM, Felix Leimbach <[email protected]…

Thanks for your answer Haydn, but

I did not understand the issue too.

> Victor has explained my problem clearly, is kvm uncapable of running

> the bridge at 1GBit if it is not a e1000 network card?

Hey Miguel,

what Haydn meant was to force qemu to *emulate* a e1000 NIC instead of the default RTL8139. That emulation is done by qemu in userspace and has absolutely nothing to do with your physical NIC por the bridge.
Chosing a NIC is done with the "model=" parameter.
kvm -net nic,model=e1000

Even better performance-wise would be to use the new virtio interface which paravirtualizes your physical NIC instead of emulating one.
For that, use:
kvm -net nic,model=virtio

Of course you'll need to install the corresponding drivers in your guest. The e1000 should be supported in every guest and for the virtio interface there are windows guest drivers (see the kvm-announce mailing list for that) as well as linux guest drivers included in kernel 2.6.25 upwards.

By the way: This is good stuff f you're interested in hard disk performance, too: http://kvm.qumranet.com/kvmwiki/Virtio

Felix

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