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[Michael Kazakov <1423631@xxxxxxxxxxxxxxxxxx>]

I found the cause of the problem.
The tap interface of a instance has small  tx  length 500 packets. At a high network load (15-25 Kpps) and high CPU load on a guest some times queue overflows and the tap interface start drop network packages. I think that queue overflow occurs in moments of micro friezes (<1000ms) of a virtual machine.  I solved this problem by increasing the tx_queue up to 10000.

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Title:
  tap interface drops many packets on highload systems

Status in linux package in Ubuntu:
  Confirmed
Status in qemu package in Ubuntu:
  Incomplete
Status in qemu-kvm package in Ubuntu:
  Incomplete

Bug description:
  I use qemu-kvm in openstack. On highload hypervisor tap interface of net-highload guest drops many TX packets.
  Network options of qemu-system-x86_64  "... -netdev tap,fd=30,id=hostnet0,vhost=on,vhostfd=31 -device virtio-net-pci,netdev=hostnet0,id=net0,mac=fa:16:3e:86:67:7b,bus=pci.0,addr=0x3..."

  Network domin config:

      <interface type='bridge'>
        <mac address='fa:16:3e:86:67:7b'/>
        <source bridge='qbre4009073-0b'/>
        <target dev='tape4009073-0b'/>
        <model type='virtio'/>
        <alias name='net0'/>
        <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
      </interface>

  tape4009073-0b Link encap:Ethernet  HWaddr fe:16:3e:86:67:7b
            inet6 addr: fe80::fc16:3eff:fe86:677b/64 Scope:Link
            UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
            RX packets:1587622634 errors:0 dropped:0 overruns:0 frame:0
            TX packets:1484106438 errors:0 dropped:460259 overruns:0 carrier:0
            collisions:0 txqueuelen:500
            RX bytes:877878711500 (877.8 GB)  TX bytes:3071846828531 (3.0 TB)
  ---
  AlsaDevices:
   total 0
   crw-rw---- 1 root audio 116,  1 Feb 18 12:06 seq
   crw-rw---- 1 root audio 116, 33 Feb 18 12:06 timer
  AplayDevices: Error: [Errno 2] No such file or directory
  ApportVersion: 2.14.1-0ubuntu3.7
  Architecture: amd64
  ArecordDevices: Error: [Errno 2] No such file or directory
  AudioDevicesInUse: Error: command ['fuser', '-v', '/dev/snd/seq', '/dev/snd/timer'] failed with exit code 1:
  CRDA: Error: [Errno 2] No such file or directory
  DistroRelease: Ubuntu 14.04
  InstallationDate: Installed on 2014-07-01 (234 days ago)
  InstallationMedia: Ubuntu-Server 14.04 LTS "Trusty Tahr" - Release amd64 (20140416.2)
  MachineType: Dell Inc. PowerEdge M620
  Package: qemu-kvm
  PciMultimedia:

  ProcEnviron:
   TERM=xterm
   PATH=(custom, no user)
   LANG=en_US.UTF-8
   LC_MESSAGES=en_US.UTF-8
   SHELL=/bin/bash
  ProcFB: 0 VESA VGA
  ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.16.0-30-generic root=UUID=5965239d-820a-49a1-9d8e-23a4c1f87ce6 ro
  ProcVersionSignature: Ubuntu 3.16.0-30.40~14.04.1-generic 3.16.7-ckt3
  RelatedPackageVersions:
   linux-restricted-modules-3.16.0-30-generic N/A
   linux-backports-modules-3.16.0-30-generic  N/A
   linux-firmware                             1.127.11
  RfKill: Error: [Errno 2] No such file or directory
  Tags:  trusty
  Uname: Linux 3.16.0-30-generic x86_64
  UpgradeStatus: No upgrade log present (probably fresh install)
  UserGroups:

  _MarkForUpload: True
  dmi.bios.date: 01/21/2014
  dmi.bios.vendor: Dell Inc.
  dmi.bios.version: 2.2.7
  dmi.board.name: 0T36VK
  dmi.board.vendor: Dell Inc.
  dmi.board.version: A01
  dmi.chassis.type: 25
  dmi.chassis.vendor: Dell Inc.
  dmi.chassis.version: PowerEdge M1000e
  dmi.modalias: dmi:bvnDellInc.:bvr2.2.7:bd01/21/2014:svnDellInc.:pnPowerEdgeM620:pvr:rvnDellInc.:rn0T36VK:rvrA01:cvnDellInc.:ct25:cvrPowerEdgeM1000e:
  dmi.product.name: PowerEdge M620
  dmi.sys.vendor: Dell Inc.

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