On 06/17/2009 10:36 AM, Fischer, Anna wrote:
/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
Okay, like I suspected, qemu has a trap here and you walked into it.
The -net option plugs the device you specify into a virtual hub. The
command line you provided plugs the two virtual NICs and the two tap
devices into one virtual hub, so any packet received from any of the
four clients will be propagated to the other three.
To get this to work right, specify the vlan= parameter which says which
virtual hub a component is plugged into. Note this has nothing to do
with 802.blah vlans.
So your command line should look like
qemu ... -net nic,...,vlan=0 -net tap,...,vlan=0 -net nic,...,vlan=1
-net tap,...,vlan=1
This will give you two virtual hubs, each bridging a virtual nic to a
tap device.
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
Please redo the tests with qemu vlans but without 802.blah vlans, so we
see what happens without packet duplication.
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?
There are known issues with lack of flow control while sending packets
out of a guest. If the guest runs tcp that tends to correct for it, but
if you run a lower level protocol that doesn't have its own flow
control, the guest may spend a lot of cpu generating packets that are
eventually dropped. We are working on fixing 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?
It's black magic, unfortunately. But please retry with the fixed
configuration and we'll continue from there.
--
Do not meddle in the internals of kernels, for they are subtle and quick to
panic.
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