Exception Handling Policies

Exception Handling Policies

The FRS control process can establish one of four policies for handling overrun and underrun exceptions. When it detects an exception, the Frame Scheduler can:

Send a signal to the FRS controller
Inject an additional minor frame
Extend the frame by a specified number of microseconds
Steal a specified number of microseconds from the following frame

The default action is to send a signal (the specific signals are listed under "Setting Frame Scheduler Signals"). The scheduler continues to run. The FRS control process can then take action, for example, terminating the Frame Scheduler.

Injecting a Repeat Frame

The policy of injecting an additional minor frame can be used with any time base. The Frame Scheduler inserts another complete minor frame, essentially repeating the minor frame in which the exception occurred. In the case of an overrun, the activity processes that did not finish have another frame's worth of time to complete. In the case of an underrun, there is that much more time for the waiting process to wake up. Because exactly one frame is inserted, all other processes remain synchronized to the time base.

Extending the Current Frame

The policies of extending the frame, either with more time or by stealing time from the next frame, are allowed only when the time base is an on-chip or high-resolution timer (see "Selecting a Time Base").

When adding time, the current frame is made longer by a fixed amount of time. Since the minor frame becomes a variable length, it is possible for the Frame Scheduler to drop out of synch with an external device.

When stealing time from the following frame, the Frame Scheduler returns to the original time base at the end of the following minor frame--provided that the processes queued to that following frame can finish their work in a reduced amount of time. If they do not, the Frame Scheduler will steal time from the next frame still.

Dealing With Multiple Exceptions

You decide how many consecutive exceptions are allowed within a single minor frame. After injecting, stretching, or stealing time that many times, the Frame Scheduler stops trying to recover, and sends a signal instead.

The count of exceptions is reset when a minor frame completes with no remaining exceptions.