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Poor TCP performance may occur in Linux virtual machines with LRO enabled (1027511)

Details

Large Receive Offload (LRO) functionality is enabled by default on VMXNET2 (Enhanced) and VMXNET3 devices. In some scenarios, Linux TCP/IP stack performs poorly when handling LRO-generated packets. As a result, having Large Receive Offload enabled for these devices in a virtual machine running a Linux guest operating system can cause poor TCP performance.

Solution

This issue has been resolved in ESXi/ESX 4.1 Update 2. For more/related information, see VMware ESX 4.1 Update 2 Release Notes.

This issue does not generally affect Linux virtual machines on ESXi 5.0. Also ensure your Linux virtual machines have VMware Tools version 5 installed.

VMware recommends that you disable Large Receive Offload functionality for virtual interfaces in Linux virtual machines on ESXi/ESX 4.0 if you notice poor TCP performance.

On virtual machines using VMXNET3 virtual network devices and running a guest OS with Linux kernel 2.6.24 or later, use the ethtool command to disable Large Receive Offload.

To disable Large Receive Offload using the ethtool command:

  1. Use the ethtool command to disable LRO for each VMXNET virtual network interface:

    # ethtool -K ethX lro off

    For example:

    # ethtool -K eth0 lro off

    Note: The configuration change made with the ethtool command does not persist across a reboot. To persist across a reboot, add the ethtool command to a startup script such as rc.local.

For virtual machines running an earlier version of the Linux kernel or using VMXNET2 devices, disable Large Receive Offload by setting the module load time parameter for the vmxnet2 or vmxnet3 Linux driver to include disable_lro=1.

To set the module load time parameter:

Note: These steps may not work for RedHat Enterprise Linux/CentOS 5.x virtual machines when updating the /etc/modprobe.conf file.

  1. Using a text editor, open the /etc/modules or /etc/modules.conf file used by the Linux distribution.
  2. Locate the alias entry for the VMXNET or VMXNET3 driver:

    alias eth0 vmxnet

    or

    alias eth0 vmxnet3

  3. Append an options entry to disable LRO on a new line under the alias.

    For example:

    alias eth0 vmxnet
    options vmxnet disable_lro=1

    or

    alias eth0 vmxnet3
    options vmxnet3 disable_lro=1

  4. Save the configuration file.
  5. Reboot the Linux virtual machine for the changes to take effect.

To disable LRO on these versions on Linux:

  1. Using a text editor, open the /etc/rc.local file and add these two lines to the file:

    rmmod vmxnet
    modprobe vmxnet disable_lro=1

  2. Save the configuration file.
  3. Run both commands from the command line or reboot the virtual machine for the changes to take effect.

Additional Information

Using promiscuous mode in the Linux virtual machine leads to resolving the performance issue too because activating this mode leads to disabling LRO.

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