Knowledge Base

Esxtop might show receive packets dropped at the virtual switch if the virtual machine’s network driver runs out of receive (RX) buffer memory the packets will be treated in a FIFO (First in first out basis) which the network can be degraded. Even though esxtop shows the packets as dropped at the virtual switch, they are actually dropped between the virtual switch and the guest operating system driver.

 

The number of dropped packets can be reduced by increasing the Rx buffers for the virtual network driver. For more information, see vCenter Server 5.1 performance charts report dropped network packets (2052917).

 

Warning: The changes described in this article may impact guest networking at the time of the change. You may need to reboot the guest to recover. It is recommended that you schedule downtime for the applications running within the guest to which you are making changes prior to performing the steps.

For the E1000 virtual network driver, Rx buffers can be modified from the guest operating system in exactly the same way as on the physical machine. The default value is 256, and the maximum value is 4096. Determine an appropriate setting by experimenting with different buffer sizes.

 

To determine the appropriate setting:

For more information on a related issue, see High rate of dropped packets for guests using E1000 or E1000E virtual network adapter on the same ESXi 5.x host (2056468).

Ethernet x.numRecvBuffers=128

Where x refers to your virtual NIC.

 

Where x refers to your virtual NIC and value refers to the new value for the Rx buffer size.

The default Rx ring size is 256 and the maximum is 4096. The default setting can be modified from within the guest operating system.

Example for a Linux guest operating system:

ethtool -G ethX rx value

Where X refers to the Ethernet interface ID in the guest operating system, and value refers to the new value for the Rx ring size.

Additionally, a Linux virtual machine enabled with Large Receive Offload (LRO) functionality on a VMXNET3 device might experience packet drops on the receiver side when the Rx ring #2 runs out of memory. This occurs when the virtual machine is handling packets generated by LRO.

As of ESXi 5.1 Update 3, the Rx ring #2 can be configured through the rx-jumbo parameter in ethtool. The maximum ring size for this parameter is 2048.

Where X refers to the Ethernet interface ID in the guest operating system, and value refers to the new value for the Rx ring #2.

In ESXi/ESX 3.x.x, you cannot configure the ring size of the VMXNET3 network interface card in Windows guest operating systems. In ESXi/ESX 4.0 Update 2 and later, you can configure the following parameters from the Device Manager (a Control Panel dialog box) in Windows guest operating systems: Rx Ring #1 Size, Rx Ring #2 Size, Tx Ring Size, Small Rx Buffers, and Large Rx Buffers.

The default value of the size of the first Rx ring, Rx Ring #1 Size, is 512. You can modify the number of Rx buffers separately using the Small Rx Buffers parameter. The default value is 1024.
For some processes (for example, traffic that arrives in burst), you might need to increase the size of the ring, while for others (for example, applications that are slow in processing receive traffic) you might increase the number of the receive buffers.

When jumbo frames are enabled, you might use a second ring, Rx Ring #2 Size. The default value of RX Ring #2 Size is 32. The number of large buffers that are used in both RX Ring #1 and #2 Sizes when jumbo frames are enabled is controlled by Large Rx Buffers. The default value of Large Rx Buffers is 768.