HP 3000 Manuals HP 3000 Manuals
Chapter 6 Software Interrupts NOWAIT I/O requires an application to "poll" to see if the requested I/O has completed. Each time the check is made, there is some overhead, whether the I/O has completed or not. The application is faced with a difficult trade-off. The more often the application polls, the greater the overhead, and the poorer its overall performance becomes. If it polls less frequently, this increases the delay between when the I/O can complete and when the application completes it, thus reducing performance. One solution to this dilemma is to use software interrupts. When software interrupts are enabled, MPE XL signals the application when to complete the I/O. There is no need for repeated polling; the application completes the I/O only when signaled, so the I/O always completes on the first try. Software interrupts are a special case of NOWAIT I/O. The difference is that MPE XL interrupts the process when the I/O can be completed; the process does not need to poll to determine whether the I/O can be completed. Most of the discussion about NOWAIT I/O also applies to software interrupts. Like NOWAIT I/O, a call to IOWAIT or IODONTWAIT is needed to complete an I/O request.