### Performance Tuning: Latency

Use this procedure to tune your AccelePort SCO OpenServer device driver for reduced latency. For a definition and general discussion of latency, see About Latency.

The SCO OpenServer device driver has interrupts enabled by default, so the only procedure necessary for reducing latency is to adjust the interrupt polling period-- 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.

The interrupt polling period is declared in the file, /etc/conf/pack.d/dxb/space.c, in the declaration, dxb_intr_period[]. This declaration is a comma-separated list which defines the interrupt polling period, in milliseconds, for each of the eight possible adapters that can be supported by the driver. If you have only one adapter installed, the first item in the list is the only one you need to adjust. If you have multiple adapters, all AccelePort RAS adapters are listed first, in the order in which they are placed on the PCI bus, followed by all AccelePort Xp adapters, in order of bus placement, followed by all DataFire RAS adapters, also in bus placement order.

By default, the polling period is 10 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).

#### Procedure

1. Use a text editor to open the file, /etc/conf/pack.d/dxb/space.c.
2. Find the following declaration:

   unsigned int dxb_intr_period[] = {10, 10, 10, 10, 10, 10, 10, 10, };

3. Set the desired polling period by changing the number that corresponds to the adapter you wish to adjust.
4. Save the file and exit from the editor.
5. Execute the following command:

   `/etc/conf/cf.d/link_unix`

6. Restart SCO OpenServer to effect the change.