Performance Troubleshooting

This section is intended to provide system administrators or advanced users with detailed information on how to troubleshoot performance related problems with the PCI 100BT product.Below, a few key terms are defined to help in understanding the troubleshooting information.

Key Terms:

Transmit Threshold: The transmit threshold value determines how many bytes must be in the PCI 100BT transmit FIFO before transmission of the bits onto the ethernet cable will begin. Transmit Underrun:A transmit underrun error occurs when the PCI 100BT transmitter encounters an empty transmit FIFO during the transmission of bits onto the ethernet cable. Memory Subsystem Latency:

The memory subsystem latency is defined to be the amount of time it takes to move data from system memory to an I/O device. This time includes the arbitration delay for the I/O device and for each bus bridge between the system memory controller and the I/O device.Arbitration Delay: The time it takes an I/O device, or bus bridge to acquire the I/O bus for data transfer. Transmit FIFO:

The transmit FIFO is a buffer on the PCI 100BT card used to hold data transferred from system memory to the PCI 100BT card.

The PCI 100BT product is currently optimized to achieve the best single card performance. In order to achieve this performance the PCI 100BT product has set the Transmit Threshold to an aggressive value. The Transmit Threshold is set so that transmission will begin after 512 bytes are in the transmit FIFO.While the current Transmit Threshold value allows the PCI 100BT product to achieve it's best performance, it also increases the probability of Transmit Underrun errors. A large number of Transmit Underrun errors (more than 1 out of every 1000 packets) can cause a noticeable drop in networking performance.Transmit Underrun errors may occur when there is sufficient bus contention from competing I/O devices. These errors can be monitored in two ways:

nettl -log 0xe -e PCI100bt -C 1

The nettl log file (by default is called /var/adm/nettl.LOG00) should then be monitored for the following message:

HPPCI 10/100Base-T driver encountered a Transmit Underflow

If a significant number of these messages occur, and the timestamps for each of the messages are within 30 seconds of each other, then the specified networking interface will suffer a noticeable performance drop. Corrective action must be taken to resolve this problem.

Corrective Action

The PCI 100BT product supports 3 levels of Transmit Threshold. These 3 levels are modified via the -S option of the lanadmin command as follows:

lanadmin -S TransmitThreshold nmid

where: a TransmitThreshold of 512 is most aggressive a TransmitThreshold of 1024 is somewhat aggressive a TransmitThreshold of 1500 is conservative In all of the cases above after setting the Transmit Threshold mode as specified, the lanadmin command will echo the current speed of the interface as follows; this output may be ignored (output shown for 100 Mbits/s operation):
old speed= 100000000 new speed= 100000000

After issuing the lanadmin -S you must wait at least 5 seconds before attempting to use the specified network interface. If the desired Transmit Threshold setting needs to be effective in all subsequent reboots, you must create an SD script and include it in the /sbin/init.d directory so that it gets executed on each reboot.