Compare UDP throughput between the regular userspace virtio-net under build qemu-kvm-0.12.1.2-2.113.el6_0.1.x86_64 and build qemu-kvm-0.12.1.2-2.109.el6.x86_64 by using the following steps:
1. start guest by:
/usr/libexec/qemu-kvm -enable-kvm -m 2048 -smp 2 -name rhel6 -uuid `uuidgen`
-rtc base=utc -boot c -drive file=/var/lib/libvirt/images/rhel6.img,if=none,cache=none,id=drive-virtio-disk0,format=raw,boot=on -device virtio-blk-pci,bus=pci.0,addr=0x3,drive=drive-virtio-disk0,id=virtio-disk0 -netdev tap,id=hostnet0 -device virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:00:e9:19:a9,bus=pci.0,addr=0x7 -usb -device usb-tablet,id=input0 -vnc :1 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x6 -monitor stdio
2. 2vcpus, pin each of them to host cpu.
   tasket -pc 0 $thread_id1
   tasket -pc 1 $thread_id2
3. on guest, pin the interrupt to the second vcpu.
   echo $i > /proc/irq/$irqcur/smp_affinity 
4. run netperf from guest to external host:
#  for b in 32 64 128 256 512 1024 2048 4096 8192 16834 32768 65507; do
netperf -t UDP_STREAM -f m -H 192.162.0.3 -P 0 -l 10 -- -m $b; done
5. got the average throughput

qemu-kvm-0.12.1.2-2.109.el6.x86_64:
Send    Send    Elasped  Throughput
Socket  Message Time
Size    Size
(bytes)	(bytes)	(secs)	(10^6 bits/sec)
124928	32	10	42.00
124928	64	10	85.19
124928	128	10	167.27
124928	256	10	341.25
124928	512	10	643.83
124928	1024	10	931.37
124928	2048	10	932.34
124928	4096	10	952.12
124928	8192	10	952.23
124928	16834	10	955.88
124928	32768	10	956.31
124928	65507	10	968.33

qemu-kvm-0.12.1.2-2.113.el6_0.1.x86_64:
Send    Send    Elasped  Throughput  Speedup
Socket  Message Time              
Size    Size
(bytes)	(bytes)	(secs)(10^6bps) (percentage)
124928	32	10	71.71	70.74%
124928	64	10	146.12	71.52%
124928	128	10	292.51	74.87%
124928	256	10	607.26	77.95%
124928	512	10	1066.01	65.57%
124928	1024	10	899.22	-3.45%
124928	2048	10	936.58	0.45%
124928	4096	10	953.62	0.16%
124928	8192	10	955.28	0.32%
124928	16834	10	958.77	0.30%
124928	32768	10	962.41	0.64%
124928	65507	10	970.40	0.21%

Summary:
  The bottom half transmitter improves the tx throughput in UDP_STREAM.

    Technical note added. If any revisions are required, please edit the "Technical Notes" field
    accordingly. All revisions will be proofread by the Engineering Content Services team.
    
    New Contents:
Prior to this update, virtio-net used a packet transmission algorithm that was using a timer to delay a transmission in an attempt to batch multiple packets together. However, this typically resulted in a higher latency. With this update, the default algorithm has been changed to use an asynchronous bottom half transmitter, improving the performance.

An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on therefore solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.

http://rhn.redhat.com/errata/RHBA-2010-0855.html