Device Service Time

Device Service Time

The time spent servicing an interrupt should be negligible. The interrupt handler should do very little processing, only wake up a sleeping user process and possibly start another device operation. Time-consuming operations such as allocating buffers or locking down buffer pages should be done in the request entry points for read(), write(), or ioctl(). When this is the case, device service time is minimal.

Device drivers supplied by SGI indeed spend negligible time in interrupt service. Device drivers from third parties are an unknown quantity. Hence the 200 microsecond guarantee is not in force when third-party device drivers are used on the same CPU at a superior priority to the time-critical interrupts.