Cylic Executive (CE) scheduling - real-time

Question

I am writing pseudocode for a CE scheduling algo. By the looks of it, task E is never going to be complete. Can anyone see where I'm going wrong? Am I choosing the correct interrupt time of 25 msec for this cyclic executive schedule?
Task Period p msec Exec Time msec
A 25 10
B 25 5
C 50 5
D 50 5
E 100 2
while(true)
wait_for_int (waits 25ms)
taskA()
taskB()
taskC()
taskD()
wait_for_int (waits 25ms)
taskA()
taskB()
wait_for_int (waits 25ms)
taskA()
taskB()
taskC()
taskD()
wait_for_int (waits 25ms)
taskA()
taskB()
endloop;

Answer 1

You are going wrong by thinking that all five tasks need to run in the same 25 millisecond period. That's not the case. All five tasks need to run every 100 milliseconds, and some tasks need to run more than once in that 100 millisecond period, but never do all five tasks need to run in the same 25 millisecond period.
For example, tasks C and D run every 50 milliseconds. But they don't have to run in the same 25 millisecond phase. They can run out of phase by 25 milliseconds. If you divide the 100 millisecond period into 25 millisecond phases then at most you need to run only four tasks in any given phase.
(If you break the 100 milliseconds into smaller phases, such as 5 milliseconds, then you might be able to design it such that no two tasks ever need to run in the same phase.)
Read this article, Multi-rate Main Loop Tasking, for a detailed explanation of what you're trying to do along with a great example.

Answer 2

You need to interleave C and D so that E can be executed in any 25ms period:
Period 0ms 25ms 50ms 100ms
-----------------------
A A A A
B B B B
C D C D
- - - E
-----------------------
Exec Time 20ms 20ms 20ms 22ms