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[Stéphan Kochen <stephan@xxxxxxxxx>]

Thanks for the continued help fixing this!

I couldn't reproduce it using redis-benchmark on EC2, but that's okay.

Scatter/gather IO is solely a performance flag in the driver? As in, it
won't affect applications?

The only effect I noticed after disabling it is that it's apparently
required for jumbo framing:

    vif vif-0 eth0: Reducing MTU because no SG offload

And it dropped to 1500. But I can live with that.

Also, do you have a link to the upstream discussion?

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https://bugs.launchpad.net/bugs/1317811

Title:
  Dropped packets on EC2, "xen_netfront: xennet: skb rides the rocket: x
  slots"

Status in “linux” package in Ubuntu:
  In Progress

Bug description:
  Running Ubuntu 14.04 LTS on EC2, we see a lot of the following in the
  kernel log:

      xen_netfront: xennet: skb rides the rocket: 19 slots

  Each of these messages corresponds to a dropped TX packet, and
  eventually causes our application's connections to break and timeout.

  The problem appears when network load increases. We have Node.js
  processes doing pubsub with a Redis server, and these are most visibly
  affected, showing frequent connection loss. The processes talk to each
  other using the private addresses EC2 allocates to the machines.

  Notably, the default MTU on the network interface seems to have gone
  up from 1500 on 13.10, to 9000 in 14.04 LTS. Reducing the MTU back to
  1500 seems to drastically reduce dropped packets. (Can't say for
  certain if it completely eliminates the problem.)

  The machines we run are started from ami-896c96fe.

  ProblemType: Bug
  DistroRelease: Ubuntu 14.04
  Package: linux-image-3.13.0-24-generic 3.13.0-24.46
  ProcVersionSignature: User Name 3.13.0-24.46-generic 3.13.9
  Uname: Linux 3.13.0-24-generic x86_64
  AlsaDevices:
   total 0
   crw-rw---- 1 root audio 116,  1 May  9 09:01 seq
   crw-rw---- 1 root audio 116, 33 May  9 09:01 timer
  AplayDevices: Error: [Errno 2] No such file or directory: 'aplay'
  ApportVersion: 2.14.1-0ubuntu3
  Architecture: amd64
  ArecordDevices: Error: [Errno 2] No such file or directory: 'arecord'
  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: 'iw'
  Date: Fri May  9 09:11:18 2014
  Ec2AMI: ami-896c96fe
  Ec2AMIManifest: (unknown)
  Ec2AvailabilityZone: eu-west-1c
  Ec2InstanceType: c3.large
  Ec2Kernel: aki-52a34525
  Ec2Ramdisk: unavailable
  IwConfig: Error: [Errno 2] No such file or directory: 'iwconfig'
  Lspci:
   
  Lsusb: Error: command ['lsusb'] failed with exit code 1: unable to initialize libusb: -99
  PciMultimedia:
   
  ProcFB:
   
  ProcKernelCmdLine: root=LABEL=cloudimg-rootfs ro console=hvc0
  RelatedPackageVersions:
   linux-restricted-modules-3.13.0-24-generic N/A
   linux-backports-modules-3.13.0-24-generic  N/A
   linux-firmware                             N/A
  RfKill: Error: [Errno 2] No such file or directory: 'rfkill'
  SourcePackage: linux
  UpgradeStatus: No upgrade log present (probably fresh install)
  --- 
  AlsaDevices:
   total 0
   crw-rw---- 1 root audio 116,  1 May  9 09:54 seq
   crw-rw---- 1 root audio 116, 33 May  9 09:54 timer
  AplayDevices: Error: [Errno 2] No such file or directory
  ApportVersion: 2.14.1-0ubuntu3
  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
  CurrentDmesg: [   24.724129] init: plymouth-upstart-bridge main process ended, respawning
  DistroRelease: Ubuntu 14.04
  Ec2AMI: ami-896c96fe
  Ec2AMIManifest: (unknown)
  Ec2AvailabilityZone: eu-west-1c
  Ec2InstanceType: c3.large
  Ec2Kernel: aki-52a34525
  Ec2Ramdisk: unavailable
  IwConfig: Error: [Errno 2] No such file or directory
  Lspci:
   
  Lsusb: Error: command ['lsusb'] failed with exit code 1: unable to initialize libusb: -99
  Package: linux (not installed)
  PciMultimedia:
   
  ProcFB:
   
  ProcKernelCmdLine: root=LABEL=cloudimg-rootfs ro console=hvc0
  ProcVersionSignature: User Name 3.13.0-24.46-generic 3.13.9
  RelatedPackageVersions:
   linux-restricted-modules-3.13.0-24-generic N/A
   linux-backports-modules-3.13.0-24-generic  N/A
   linux-firmware                             N/A
  RfKill: Error: [Errno 2] No such file or directory
  Tags:  trusty ec2-images
  Uname: Linux 3.13.0-24-generic x86_64
  UpgradeStatus: No upgrade log present (probably fresh install)
  UserGroups: adm audio cdrom dialout dip floppy netdev plugdev sudo video
  _MarkForUpload: True

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