Examples

Precision-Timed Actuation (discussion Dec 2018)

Given a time unit c,

• H3 reacts sporadically >= 100c (e.g., 10, 120, 230, ...)
• H4 reacts periodically with period 50c (e.g., 0, 50, 100, ...)
• Delay adds 100c to the timestamp of each incoming event
• Actuate shall start executing H5 before r.t. clock exceeds time stamp of incoming events

+--------+
|        |          +--------+     +-------+     +---------+
|   H3   +----------> H1     |     |       |     |         |
|        |          |        +-----> Delay +-----> Actuate |
+--------+    +-----> H2     |     |  100  |     |   (H5)  |
              |     +--------+     +-------+     +---------+
              |
+--------+    |
|        |    |
|   H4   +----+
|        |
+--------+

We can construct a dependency graph:

H3 ---> H1 ---> H2 ---> H5
            |
H4 ---------+

A feasible schedule requires that:

• WCET(H3) + WCET(H1) + WCET(H2) <= 100c
• WCET(H4) + WCET(H1) + WCET(H2) <= 100c

Preemption Example

          T = 1s          C = 300ms
       +---------+      +----------+
       |         |      |   Corr   |
       |   GPS   +------> r1       +----+
       |         |      |          |    |
       +---------+      +----------+    |     +----------------+         D = 100ms
                                        |     |                |        +--------+
         T = 100ms                      +-----> r2             |        |        |
       +---------+                            |       Ctrl     +-------->  Act.  |
       |         |           +----------------> r3             |        |        |
       |  IMU    +-----------+                |                |        +--------+
       |         |                            +----------------+
       +---------+

This example needs the following:

• r3 needs to preempt r1.
• The event from GPS needs a delay of 300ms between Corr and Ctrl, so Ctrl never sees an older event.

If we want to avoid preemption, as this hurts WCET analysis:

• Split reactor Corr. into three (or more) reactors and add a delay of 100 ms after each one.

For both solutions, the scheduler needs a "safe to process" analysis for reaction r3 to execute while r1 is still executing for an older time-stamped event.

Preemption can be avoided when there are enough cores (or hardware threads in PRET) available to execute r1 and r3 concurrently.