Page 97 - DCAP608_REAL TIME SYSTEMS

Real Time Systems

Notes into non-preemptive procedures). Now the lower bound by constraint 1 has been reduced. Now
we can divide each job in (20,5) into a chain of three slices with execution time 1,3,1. So (20,5) now
in fact is (20,1), (20,3), (20,1). Now we have 5 tasks and we can choose frame size 4.

Figure 9.4: Example of Job Slices

Source: ansari.szabist-isb.edu.pk/RTS/RTS0508.ppt
We have not chosen to decompose (20,5) into (20,3) and (20,2) as it would not be possible to fit
these tasks together with T1 and T2 in five frames of size 4. T1 must be scheduled in each frame.
T2 must be scheduled in 4 out of 5 frames. This leaves one frame with 3 units of time for T3. The
other frames only have one unit of time left for T3.

We can schedule two subtasks each with 1 unit of execution time in these frames, but there is not
time in any frame for a subtask with execution time 2.

Task Draw a diagram for a cyclic schedule with frame size 4.

9.4 Cyclic Executives

The clock driven scheduler must be modified to accommodate the restriction that scheduling
decisions are made only at frame boundaries. The term cyclic executives refer to a scheduler that
deterministically interleaves and sequentializes the execution of periodic tasks on a CUP
according to a given cyclic schedule. It also makes scheduling decision only at the beginning of
each frame and deterministically interleaves the execution of periodic tasks.
However it allows aperiodic and sporadic jobs to use the time not used by periodic tasks. The
cyclic executives take over the processor and executes at each of the clock interrupts, which occur
at the beginning of frames. When it executes, the cyclic executive copies the table entry for the
current frame into the current block. It then wakes up a job, called period task server (Not
needed where periodic tasks never overrun, executive just executes the job slices) , and lets the
server execute the job slices in the current block. Upon the completion of the periodic task
server, the cyclic executive wakes ups the aperiodic jobs in the aperiodic job queue in turn and
allows them to use the remaining time in the frame.
The supposition here is that whenever the server or a job completes, the cyclic executives wake
up and executes. The system may also have an aperiodic task server, which when awaked
executes aperiodic jobs in the aperiodic job queue. In addition to scheduling, the cyclic executive
also checks for overruns at the beginning of each frame. If the last job executed in the previous
frame is not complete at that time, the cyclic executive preempts the execution of the job if the
last job is an aperiodic job. The job remains in the aperiodic job queue and will be resumed
whenever there is time again for aperiodic jobs.
If the cyclic executive finds the periodic task server still executing at the time of a clock interrupt,
a frame overrun occurs. After checking for overruns, the cyclic executive makes sure that all the
job slices scheduled in the current block are ready for execution and then wakes up the periodic
task server to execute them. If there is still time after all the slices in the current block are

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