INtime® 3.1 Software - tenAsys

Chapter 2: Understanding INtime software architecture
Round-robin scheduling
INtime software also provides round-robin scheduling, where equal-priority threads
take turns running. Each thread gets a time slice. If a thread is still running when its
time slice expires, that thread moves to the end of a circular queue for that priority
level where it waits until all threads ahead of it use up their time slices, as shown in
the next figure. You can adjust the length of the time slice and set the priority level
threshold where round-robin scheduling occurs.
Figure 2-5. Round-robin scheduling
Thread A
Thread B
Thread C
Threads A, B, and C are of equal priority below the round-robin priority threshold.
Thread A, the running thread, stops running when its time slice runs out. Thread A's state is saved
and it moves to the end of the queue.
Thread B, a ready thread, then becomes the running thread.
Thread A runs again when all threads in the queue either finish running or are preempted when
their time slice expires.
Higher-priority threads still preempt any running thread in the round-robin queue,
regardless of the amount of time left in its time slice.
Note
Round-robin scheduling cannot guarantee a predictable worst-case response to events because
the number of threads in the queue varies.
Handling interrupts
System hardware invokes an interrupt handler in response to an asynchronous
interrupt from an external source, based on its entry number in the IDT (Interrupt
Descriptor Table). The handler takes control immediately and saves the register
contents of the running thread so it can be restarted later. There are two ways you can
service an interrupt:
• Using a handler alone
• Using a handler/thread combination
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