> Subject: Re: Network throughput limits for local VM <-> VM
> communication
>
> Fischer, Anna wrote:
> > Not sure I understand. As far as I can see the packets are replicated
> on the tun/tap interface before they actually enter the bridge. So this
> is not about the bridge learning MAC addresses and flooding frames to
> unknown destinations. So I think this is different.
> >
>
> Okay.
>
> You said:
>
> > However, without VLANs, the tun
> > interface will pass packets to all tap interfaces. It has to, as it
> > doesn't know to which one the packet has to go to.
>
> Well, it shouldn't. The tun interface should pass the packets to just
> one tap interface.
>
> Can you post the qemu command line you're using? There's a gotcha
> there
> that can result in what you're seeing.
Sorry for the late reply on this issue. The command line I am using looks
roughly like this:
/usr/bin/qemu-system-x86_64 -m 1024 -smp 2 -name FC10-2 -uuid
b811b278-fae2-a3cc-d51d-8f5b078b2477 -boot c -drive
file=,if=ide,media=cdrom,index=2 -drive
file=/var/lib/libvirt/images/FC10-2.img,if=virtio,index=0,boot=on -net
nic,macaddr=54:52:00:11:ae:79,model=e1000 -net tap net
nic,macaddr=54:52:00:11:ae:78,model=e1000 -net tap -serial pty -parallel none
-usb -vnc 127.0.0.1:2 -k en-gb -soundhw es1370
This is my "routing VM" that has two network interfaces and routes packets
between two subnets. It has one interface plugged into bridge virbr0 and the
other interface is plugged into virbr1:
brctl show
bridge name bridge id STP enabled interfaces
virbr0 8000.8ac1d18c63ec no vnet0
vnet1
virbr1 8000.2ebfcbb9ed70 no vnet2
vnet3
If I use the e1000 virtual NIC model, I see performance drop significantly
compared to using virtio_net. However, with virtio_net I have the network
stalling after a few seconds of high-throughput traffic (as I mentioned in my
previous post). Just to reiterate my scenario: I run three guests on the same
physical machine, one guest is my routing VM that is routing IP network traffic
between the other two guests.
I am also wondering about the fact that I do not seem to get CPU utilization
maxed out in this case while throughput does not go any higher. I do not
understand what is stopping KVM from using more CPU for guest I/O processing?
There is nothing else running on my machine. I have analyzed the amount of CPU
that each KVM thread is using, and I can see that the thread that is running
the VCPU of the routing VM which is processing interrupts of the e1000 virtual
network card is using the highest amount of CPU. Is there any way that I can
optimize my network set-up? Maybe some specific configuration of the e1000
driver within the guest? Are there any known issues with this?
I also see very difference CPU utilization and network throughput figures when
pinning threads to CPU cores using taskset. At one point I managed to double
the throughput, but I could not reproduce that setup for some reason. What are
the major issues that I would need to pay attention to when pinning threads to
cores in order to optimize my specific set-up so that I can achieve better
network I/O performance?
Thanks for your help.
Anna
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