Applying fppt to aeh in the rtsj
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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 r max ▶ Applying FPPT to AEH in the RTSJ

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Applying fppt to aeh in the rtsj

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


Aeh in the rtsj
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


Aeh implementations
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


Critical sequences in the dynamic 1 n mapping
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.


Critical sequences in the dynamic 1 n mapping1
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.


Critical sequences in the dynamic 1 n mapping2
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


Fixed priority preemptive scheduling with preemption threshold
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



Preemption threshold assignment
Preemption Threshold Assignment Threshold

1

2

3

rmax

rmin

4

FPP

FPPT

FPNP

6

5


Finding r max
Finding Thresholdrmax

1

2

rmin

rmax


A simple example
A Simple Example Threshold

  • 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


Applying fppt to aeh
Applying FPPT to AEH Threshold

  • 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


Conclusions
Conclusions Threshold

  • 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


Thank you q a
Thank you ThresholdQ & A