Solarflare Solarflare XtremeScale series, User Manual, Page: 293 / 400

Solarflare Server Adapter User Guide
Solarflare Adapters on VMware
Issue 20 © Solarflare Communications 2017 286
LRO should not be enabled if you are using the host to forward packets from one
interface to another; for example if the host is performing IP routing or acting as a
layer2 bridge.
LRO is supported, and enabled by default, on VMware versions later than ESX 3.5.
TCP Protocol Tuning
TCP Performance can also be improved by tuning kernel TCP settings. Settings
include adjusting send and receive buffer sizes, connection backlog, congestion
control, etc.
Typically it is sufficient to tune just the max buffer value. It defines the largest size
the buffer can grow to. Suggested alternate values are max=500000 (1/2 Mbyte).
Factors such as link latency, packet loss and CPU cache size all influence the affect of
the max buffer size values. The minimum and default values can be left at their
defaults minimum=4096 and default=87380.
For advice on tuning the guest TCP stack consult the documentation for the guest
operating system.
Receive Side Scaling (RSS)
Solarflare adapters support Receive Side Scaling (RSS). RSS enables packet receive ‐
processing to scale with the number of available CPU cores. RSS requires a platform
that supports MSI ‐ X interrupts. RSS is enabled by default.
When RSS is enabled the controller uses multiple receive queues into which to
deliver incoming packets. The receive queue selected for an incoming packet is
chosen in such a way as to ensure that packets within a TCP stream are all sent to
the same receive queue – this ensures that packet ‐ ordering within each stream is
maintained. Each receive queue has its own dedicated MSI ‐ X interrupt which ideally
should be tied to a dedicated CPU core. This allows the receive side TCP processing
to be distributed amongst the available CPU cores, providing a considerable
performance advantage over a conventional adapter architecture in which all
received packets for a given interface are processed by just one CPU core.
RSS will be enabled whenever NetQueue is not and Solarflare recommend using
NetQueue on VMware ESX hosts.
Other Considerations
PCI Express Lane Configurations
The PCI Express (PCIe) interface used to connect the adapter to the server can
function at different widths. This is independent of the physical slot size used to
connect the adapter. The possible widths are multiples x1, x2, x4, x8 and x16 lanes
of (2.5Gbps for PCIe Gen. 1, 5.0 Gbps for PCIe Gen. 2) in each direction. Solarflare
Adapters are designed for x8 lane operation.