Performance - HP -UX 11i Installation And Update Manual

Setting the
transmit threshold

Performance

The 100 Base-T product is initially set with a conservative value for the
transmit threshold (that is, the number of bytes that must be in the
transmit FIFO buffer before transmission can begin). The transmit
threshold is set to a "store and forward" value. This means that the
entire Ethernet frame must be in the transmit FIFO before transmission
of data onto the wire will begin.
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.
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.
Appendix C
Configuring Built-In PCI Networking
Performance
141