[Bug 769503] [NEW] KVM network I/O degraded using virtio and freezes guest sometimes

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Public bug reported:

Binary package hint: kvm

I'm just testing version 11.04 with KVM virtualization.
Server hardware: Intel 5500HCV motherboard, Intel XEON 5506 CPU, 3Ware 9690SE hw raid controller with 512 MB cache and BBU, 16GB RAM, WD RAID Edition 3 disks, raid level is raid1 with 1 spare HDD.

Distribution: 11.04 beta2
Host kernel: 2.6.38-8-server
QEMU-KVM: 0.14
Libvirt: 0.8.8
I created the virtual machine with virt-install, enabled all the accelerated drivers (virtio disk and net device), the virtio disk uses dedicated LVM volume, raw format and I switched to cache=none, as well, the NIC uses bridge (br0)
I use HUGETLBS and dedicated more than enough pages via sysctl -w vm.nr_hugepages="4400"
Guest operating system: 10.04, 64 bit version, 8GB ram, 2 VCPUs, the kernel is 2.6.35-virtual from LTS-backport. (But that doesn't matter, same things happen if I use the stock kernel, either server or virtual version)

Checked the libvirtd log and everything started up, as expected.

When I start to transfer some big files over LAN for some times it works as expected. Just transfered some images with size 7-800MB. The transfer speed is around 65-70MB/s. Starting the transfer from the host macinhe I got 95-96MB/s. If I want to transfer much bigger images - 100-200GB - from the LAN to  the virtual guest machine than the networking speed drops to 10-20-30 MB/s and CPU usage become 100% in guest, and load arises massively. (5-6 than goes up to 10-12) Sometimes the guest machine become unresponsive as well.
If I start to transfer same big files on the host machine it works as expected - 90-100MB/s, load hardly arise.

** Affects: kvm (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/769503

Title:
  KVM network I/O degraded using virtio and freezes guest sometimes