Performance Tuning: Latency

Use these procedures to tune your AccelePort Linux device driver for reduced latency. For general information about latency, see About Latency.

Two methods are provided for reducing latency with the Linux driver. The first is to set a command-line flag which causes the Line Discipline to use low-latency mode. The second method is to incrementally adjust the interrupt polling period downward from its default value of 10 milliseconds until the desired latency is achieved. Both methods may be used concurrently; setting the latency flag should be tried first.

Procedure 1: Setting the Latency Flag

Use a text editor to open the device driver startup script,
/etc/rc.d/init.d/dgdm.
Find the command that starts the driver. It will be of the form:
insmod dgdm [options]
Add the option, latency=1, to the command line. Assuming that the existing command line contained no other option specifications, the command line should now look like this:
insmod dgdm latency=1
Restart Linux to effect the change.
If the latency is still too great, continue to Procedure 2: Setting the Interrupt Polling Period (leave the latency=1 flag in the command line).

Procedure 2: Setting the Interrupt Polling Period

Use a text editor to open the device driver startup script,
/etc/rc.d/init.d/dgdm.
Find the command that starts the driver. It will be of the form:
insmod dgdm [options]
Add the option, intr_period=9, to the command line (intr_period is an integer value that specifies the polling period in milliseconds; the default value is 10). Assuming that you have already added the flag, latency=1, and that the original command line contained no other option specifications, the command line should now look like this:
insmod dgdm latency=1 intr_period=9
Restart Linux to effect the change.
If the latency is still too great, continue adjusting the interrupt polling period downward (1 millisecond is the shortest supported polling period) until an acceptable latency is achieved.