### Performance Tuning: Latency

Use one of these procedures to tune your AccelePort HP-UX device driver for reduced latency. For a definition and general discussion of latency, see About Latency.

The polling period is the delay between a character's appearance in the adapter's buffers and the transmission of that character from the adapter to the operating system.

By default, the polling period is 20 milliseconds. To reduce the latency of character reception, try smaller integer values until you are satisfied with the results. Note that reducing the polling period correspondingly increases system overhead, and may impact overall system performance. The minimum polling period is 1 millisecond. Setting a value of 0 disables interrupts (this may improve overall performance in systems with a large number of ports, where low latency is not a major concern).

The polling period may be changed globally for all AccelePort adapters, or individually for each adapter.

#### Procedure: Changing the Global Polling Rate

1. Enter the command `dgdxb_config` at an HP-UX root prompt.

   The Digi DGDXB Adapter Viewer window appears

2. Choose `View/Configure Global Polling Rate`.
3. Enter a new polling period in milliseconds in the `Change Global Polling Rate To` field.

   The dgdxb_config program will ask you to confirm the change.

4. Answer `y` to save the change.

#### Procedure: Tuning Per-adapter Interrupts

1. Enter the command `dgdxb_config` at an HP-UX root prompt.

   The Digi DGDXB Adapter Viewer window appears

2. Choose `View/Configure Board`.
3. Choose the Adapter for which you will change the polling period.

   The `Adapter Information` and `Tunable Adapter Parameters` tables will appear.

4. Under `Tunable Adapter Parameters`, select `Interrupt Rate(ms)`.
5. Enter a new polling period in milliseconds.
6. Enter Q to exit and save the change.

   The dgdxb_config program will ask you to confirm the change.

7. Answer `y` to save the change.

   A prompt will appear asking you if you wish to immediately reload the adapter.

8. Read the adapter reloading information and answer the prompt appropriately.