T-Kernel Specification (1.B0.02)

10 CHAPTER 2. CONCEPTS UNDERLYING THE T-KERNEL SPECIFICATION
tasks is determined as follows based on the priority of each task. If there are multiple tasks that can be
run, the one with the highest precedence goes to RUN state and the others go to READY state.
In determining precedence among tasks, of those tasks having different priority levels, that with the
highest priority has the highest precedence. Among tasks having the same priority, the one first going
to a run state (RUN state or READY state) has the highest precedence. It is possible, however, to use
a system call to change the precedence among tasks having the same priority.
When the task with the highest precedence changes from one task to another, a dispatch occurs immediately
and the task in RUN state is switched over. If no dispatch occurs, however, the switching of the
task in RUN state is held off until dispatch occurs.
Additional Note
According to the scheduling rules adopted in the T-Kernel specification, so long as there is a highprecedence
task in a run state, a task with lower precedence will simply not run. That is, unless the
highest-precedence goes to WAIT state or for other reason cannot run, other tasks are not run. This is a
fundamental difference from TSS (Time Sharing System) scheduling in which multiple tasks are treated
equally. It is possible, however, to issue a system call changing the precedence among tasks having the
same priority. An application can use such a system call to realize round-robin scheduling, which is a
typical kind of TSS scheduling.
precedence
priority
Hight 〈Priority 1〉
✻
〈Priority 2〉
✲ Task A ✲
✲ Task B ✲ Task C ✲ Task D ✲
❄
Low 〈Priority 3〉
✲
Task E
✲
Figure 2.2: Precedence in Initial State
Figure 2.2 and following illustrate how the task that first goes to a run state (RUN state or READY
state) gains precedence among tasks having the same priority. Figure 2.2 shows the precedence among
tasks after Task A of priority 1, Task E of priority 3, and Tasks B, C and D of priority 2 are started in
that order. The task with the highest precedence, Task A, goes to RUN state.
When Task A exits, Task B with the next-highest precedence goes to RUN state (Figure 2.3). When
Task A is again started, Task B is preempted and reverts to READY state; but since Task B went to
a run state earlier than Task C and Task D, it still has the highest precedence among tasks with the
same priority. In other words, the task precedence reverts to that in Figure 2.2.
Next, consider what happens when Task B goes to WAIT state in the conditions in Figure 2.3. Since
task precedence is defined among tasks that can be run, the precedence among tasks becomes as shown
in Figure 2.4. Thereafter when the Task B wait state is released, Task B went to run state after Task
C and Task D, and thus assumes the lowest precedence among tasks of the same priority (Figure 2.5).
Summarizing the above, immediately after a task that went from READY state to RUN state reverts
to READY state, it has the highest precedence among tasks of the same priority; but after a task goes
from RUN state to WAIT state and then the wait is released, its precedence is the lowest among tasks
of the same priority.
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T-Kernel 1.B0.02