
Issue 11
© Solarflare Communications 2014
225
Solarflare Server Adapter
User Guide
Large Receive Offload (LRO)
TCP Large Receive Offload (LRO) is a feature whereby the adapter coalesces multiple packets
received on a TCP connection into a single larger packet before passing this onto the network stack
for receive processing. This reduces CPU utilization and improves peak throughput when the CPU is
fully utilized. The effectiveness of LRO is bounded by the interrupt moderation delay and in itself,
enabling LRO does not negatively impact latency. LRO is a Solarflare proprietary mechanism similar
to the Windows Receive Side Coalescing feature. LRO is disabled by default and should not be
enabled if the host is forwarding received packets from one interface to another.
LRO is set by changing the Large Receive Offload settings in the Network Adapter’s Advanced
Properties Page. TCP /IP checksum offloads must be enabled for LRO to work. The Solarflare network
adapter driver does not enable LRO by default..
Receive Side Scaling (RSS)
Receive Side Scaling (RSS) was first supported as part of the scalable networking pack for Windows
Server 2003 and has been improved with each subsequent operating system release. RSS is enabled
by default and will be used on network adapters that support it. Solarflare recommend that RSS is
enabled for best networking performance.
For further information about using RSS on Windows platforms see the Microsoft white paper
"Scalable Networking: Eliminating the Receive Processing Bottleneck—Introducing RSS"
This is available from:
http://download.microsoft.com/download/5/D/6/5D6EAF2B-7DDF-476B-93DC-7CF0072878E6/
NDIS_RSS.doc
On Windows Server 2008 R2 and Windows 7, specific RSS parameters can be tuned on a per-adapter
basis. For details see the Microsoft white paper
"Networking Deployment Guide: Deploying High-
Speed Networking Features"
available from:
http://download.microsoft.com/download/8/E/D/8EDE21BC-0E3B-4E14-AAEA-9E2B03917A09/
HSN_Deployment_Guide.doc
Solarflare network adapters optimize RSS settings by default on Windows operating systems and
offer a number of RSS interrupt balancing modes via the network adapter's advanced property page
in Device Manager and Solarflare's adapter management tools.
RSS NUMA Node
The adapter driver chooses a subset of the available CPU cores to handle transmit and receive
processing. The RSS NUMA Node setting can be used to constrain the set of CPU cores considered
for processing to those on the given NUMA Node.
NOTE:
LRO should
NOT
be enabled when using the host to forward packets from one
interface to another. For example, if the host is performing IP routing.