HP-UX 11i Version 1 Installation and Update Guide, June 2003

Configuring Built-In PCI Networking
PCI 10/100Base-TX and 100Base-FX/9000
Appendix D 207
The initial transmit threshold value is set to eliminate transmit
underruns (that is, when the transmitter encounters an empty transmit
FIFO during the transmission). However, it can also reduce transmit
performance by not being able to pre-fetch the next packet until the
current frame is completely transferred. This causes a slight delay
between frames, resulting in an overall drop in transmit performance.
The transmit threshold value is adjustable. Adjusting the threshold to
either 1024 or 512 bytes can increase performance. If the transmit
threshold is 1024 or 512, the CORE 100 Base-T interface will start
transmitting data onto the wire after 1024 or 512 bytes are in the
transmit FIFO. This allows the CORE 100 Base-T hardware to start
pre-fetching the next frame in the transmit FIFO before the current
frame is completely transmitted. This results in less time between
subsequent frames and therefore increases the transmit performance.
Setting the
transmit threshold
The CORE 100 Base-T product supports three levels of transmit
threshold. These three levels are modified via lanadmin:
lanadmin -S transmit_threshold PPA
where:
transmit_threshold of 512 is most aggressive.
transmit_threshold of 1024 is somewhat aggressive.
transmit_threshold of 1500 is conservative.
After issuing lanadmin -S, you must wait at least 8 seconds before
attempting to use the specified network interface.
If you want the transmit threshold setting to be effective in all
subsequent reboots, you must create an SD script and include the
command in the /sbin/init.d directory so it gets executed on each reboot.
CAUTION Be careful when adjusting the transmit threshold. While lowering the
threshold may increase performance, it also makes the CORE 100
Base-T interface more susceptible to Transmit Underrun errors.
A large number of transmit underrun errors (more than 1 out of every
1000 packets) may cause a noticeable drop in networking performance.
The performance drop will depend on the application being used.
Applications that send data in a continuous stream (FTP, for example)