Ethernet Support Guide (for HP-UX 11i v1, 11i v2)

Introduction to Technology
Gigabit Ethernet Overview
Chapter 1
18
Optional: If desired, you can also use the lanadmin -X command to temporarily set
these values; but if you use the lanadmin command, your settings will not be saved
across a reboot. The exact lanadmin commands are as follows (note that the -X is
capitalized in these commands):
lanadmin -X send_cko_on ppa
lanadmin -X recv_cko_on ppa
Please refer to the lanadmin man page for details of these CKO options.
Support for TCP Segmentation Offload (TSO)
TCP Segmentation Offload (TSO) is a mechanism by which the host stack offloads
certain portions of outbound TCP packet processing to the Network Interface Card (NIC)
thereby reducing host CPU utilization. This functionality can significantly reduce the
load on the server for certain applications which primarily transmit large amounts of
data from the system. Examples include web servicing, NFS, and file transfer
applications.
How It Works
The reduction in CPU utilization is achieved primarily by allowing the host to transmit
large frames (frames larger than the links Maximum Transmission Unit or MTU) to the
NIC which are subsequently carved up into smaller, MTU-sized frames by the NIC,
before transmission on the wire. Thus instead of processing many small MTU-sized
frames during transmit, the host sends fewer larger VMTU (Virtual MTU) sized frames
thereby increasing the efficiency of the data transfer in the host. The VMTU is typically
much larger than the links MTU; for example, on a typical Ethernet card, the link MTU
is 1500 bytes while a VMTU could be as large as 64Kbytes. Greater than 50% reduction
in CPU utilization has been observed on some FTP workloads.
NOTE Not all applications benefit from the TSO mechanism. Only data intensive applications
which transmit large data buffers using TCP over IPv4 are improved. Other types of
applications will not significantly benefit from the TSO mechanism. Performance
improvements vary depending upon the platform used. Systems which support
hardware partitioning will notice a decrease in per-card throughput in addition to the
significant reduction in CPU utilization.
Managing TSO
TSO is disabled by default. To enable TSO on each specific interface, use the lanadmin
command to set the “ vmtu ” tunable to a non-zero value. See the next section for details.
New lanadmin Command Option: vmtu
The lanadmin command is updated to get or set a new tunable called “ vmtu ” -- the
virtual MTU of a link that supports TSO. The following command will list the TSO
capability of the link:
# lanadmin –x vmtu <ppa>
Driver/Hardware supports TCP Segmentation Offload. Current VMTU = 32160.
The syntax of the command to set the vmtu is as follows (note: the X is capitalized):
# lanadmin -X vmtu new_vmtu_value <ppa>