Applying FPPT to AEH in the RTSJ

1. Applying FPPT to AEH in the RTSJ ▶ AEH in the RTSJ ▶ AEH Implementations ▶ Critical Sequences in the Dynamic 1:N mapping ▶ Schedulability Analysis for FPPT ▶ Preemption Threshold Assignment ▶ Finding rmax ▶ Applying FPPT to AEH in the RTSJ ▶ Conclusions

2. AEH in the RTSJ • The driving design goal of AEH is to have a lightweight concurrency mechanism • Handlers should not incur the same overhead as real-time threads do • The lightweightness requirement can only be achieved by minimising the number of server threads

3. AEH Implementations • 1:1 Mapping Model - Static: bound asynchronous event handlers and OVM - Dynamic: jRate • 1:N Mapping Model - Static: Jamaics - Dynamic: Java RTS 2.0

4. Critical Sequences in the Dynamic 1:N Mapping • In a fixed priority system where every handler may self-suspend (Blocking Handlers), a critical sequence of handler releases for a set of handlers occurs when • a handler is released after or at the release time of a previous handler, and • before or during the self-suspension of the previous handler, that is, ri is in the range [rj , rj + ssj ] exclusively, for every handler in the set. • Where ri denote the release and the execution time of a handler i and the greater the value i, the higher the priority. ssi denotes the finishing time of the self-suspension of handler i.

5. Critical Sequences in the Dynamic 1:N Mapping • In a fixed priority system where handlers do not self-suspend (Non-Blocking Handlers), a critical sequence of the handler releases occurs when • handlers are released in a priority-ascending manner in turn and • one is released in the middle of a previously released handler being executed, that is, ri is in the range [ri-1, ri-1 + ssi-1] exclusively, for every handler in a set.

6. Critical Sequences in the Dynamic 1:N mapping Based on the critical sequences • For Blocking Handlers - Used by the Blocking AEH model: - The required number of server threads is dependent on the number of released handlers • For Non-Blocking Handlers - Used by the non-blocking AEH model: - The required number of server threads is dependent on the number of priority levels

7. Fixed-Priority Preemptive Scheduling with Preemption Threshold • Is primarily introduced to feasibly schedule a task set unschedulable either by FPP or FPNP (3-6% improvement) • Assigns two priority levels to each task, a regular priority and a preemption threshold • The regular priority of a task is used when the task is queued, but when the task gets the CPU its active priority is raised to its preemption threshold • Effectively reduces the number of active priority levels, by allowing a task not to be preempted by a higher regular priority task up to its preemption threshold • Note that, FPPT’s schedulability improvement comes at the expense of the increased worst-case response time of higher priority tasks

8. Schedulability Analysis for FPPT

9. Preemption Threshold Assignment 1 2 3 rmax rmin 4 FPP FPPT FPNP 6 5

10. Finding rmax 1 2 rmin rmax

11. A Simple Example • An unschedulable task set with 8 tasks such that t8 = (1, 10), t7 = (1, 15), t6 = (4, 40), t5 = (10, 60), t4 = (20, 80), t3 = (15, 100), t2 = (10, 200), t1 = (16, 240) as w1 = 293 However, it is schedulable with rmin and rmax. ▪ rmin = {8,7,6,5,4,4,4,2} schedulable with 6 servers ▪ rmax = {8,8,8,7,6,6,7,6} schedulable with 3 servers

12. Applying FPPT to AEH • The non-blocking AEH model has been extended to support FPPT • The ImportanceParameters class is used to denote the preemption threshold of a handler • A server thread in the model now raises its priority to the preemption threshold of the current handler when it gets the CPU • This hides the use of importance value from the base priority-scheduler in order for existing mechanisms to work correctly with the scheduler

13. Conclusions • In order to achieve the lightweight requirement of AEH in the RTSJ, the critical sequences of the dynamic 1:N mapping are derived • Based on the critical sequences for non-blocking handlers, it is shown that the number of server threads required is dependent on the number of active priority levels in the system • FPPT is discussed to reduce the number of effective priority levels for non-blocking handlers • FPPT is applied to the non-blocking AEH implementation to further reduce the number of server threads • This allows AEH to further reduce the number of server threads required to be smaller than that of the priority levels even in the worst case • This enables the lightweight requirement can be better achieved for AEH in the RTSJ

14. Thank youQ & A