The Cardiac Pacemaker – SystemJ versus Safety
The cardiac pacemaker – SystemJ versus Safety Critical Java
Heejong Park, Avinash Malik, Muhammad Nadeem, and Zoran Salcic.
University of Auckland, NZ
Why is it interesting?
Study difference between classical (RTOS) model of computation vs. the synchronous model of computation
Improved state representation and formal functional and non-functional (timing) verification of programs designed in reactive languages
Different design decisions for implementing the same safety critical application.
Approach taken for comparison
Implementing the cardiac pacemaker control logic in SystemJ and comparing with the SCJ implementation.
Features compared:
◦ Scheduling policies
◦ Real-time and response-time analysis
◦ Memory model (if you are interested, after the presentation).
The cardiac pace-maker control logic – DDDR operating mode
• The dips below the X-axis are the pacing signals
• Scenario D: A normal working heart.
• Scenario A: Atrial and ventricle pace are produced
• Scenario B: Only atrial pace produced
• Scenario C: Only ventricle pace produced
The general SystemJ model of computation
Globally Asynchronous
Locally Synchronous
(GALS) MoC.
Signals are used for communication within each synchronous island
(clock-domain).
Channels and modified-
CSP style rendezvous between reactions in different clock-domains.
SystemJ syntax
What is each reaction really?
S/O1
L1
!S/O2 input signal S;
L1: pause ; present (S) emit O1; else emit O2;
L2: pause ;
L2
Synchronous composition
L1 L2
L1L
2
A/B C/D A&&C/
{B,D}
L1’ L2’
L1’
L2’
The cardiac pacemaker in SystemJ
Only synchronous parallel composition
All communication via signals
Input and Output to the heart model also via signals
No need for asynchrony, because only one mode runs at any given time
SCJ vs. SystemJ – functional correctness
States are explicitly demarcated at pause
Smaller state space compared to SCJ
Every FSM transition is atomic
◦ Easier to verify, since synchrony avoids interleaving altogether .
◦ Further reduction in state space, because change in signals
(and update of data) is not visible until completion of transition.
We verified for the pacemaker liveness properties
(via SPIN model-checker)
◦ If Ventricle/Atrial sense is not detected Ventricular/Atrial pace will always be generated.
SCJ vs. SystemJ – tasks and scheduling model
Task priority
◦ SCJ needs unique priority for each task
◦ All SystemJ reactions have equal priority (or no priority)
Task ordering
◦ Priorities and schedule together determine task-ordering in SCJ
◦ Reactions in SystemJ can be run in any order – more optimization chances, outputs are always deterministic.
Response time
◦ SCJ (RTOS) definition – time from release to completion of a task.
◦ Time from one or more inputs to generation of one or more outputs via one or more tasks (reactions/CDs) interacting together.
Event handling
◦ SCJ supports Periodic and Aperiodic event handlers, no sporadic events (?) and what happens with multiple incoming events?
◦ SystemJ can be considered to have only sporadic event handling with minimum statically guaranteed inter-arrival time. Multiple events can always be captured.
SystemJ – timing model guaranteeing real-time properties
A
A
𝑊
𝐴𝐵
I/{}
𝑊𝐶𝑅𝑇
B
B
𝑊
𝐵𝐶
T/{O}
𝑊𝐶𝑅𝑇
C
C
WCRT = max(𝑊
𝐴𝐵
, 𝑊
𝐵𝐶
)
𝐼 → 𝑂 (response time from I to O) =
WCRT * 2
Verifying real-time property: []𝐼 →⋄
𝑀
𝑂
SCJ vs. SystemJ
Timers
◦ SCJ handles timing via one shot timer handlers, triggered via external timers
◦ SystemJ converts wait statements to logical time – bounded self-transitions.
The resultant system is still real-time analyzable.
The wait is exact.
𝑊𝐶𝑅𝑇 d<10
A
𝑊𝐶𝑅𝑇 d==10
B
Experimental results
We run the pacemaker on 3 different platforms:
◦ Standard JOP – all SystemJ compiled to simple
Java
◦ TP-JOP, separated control and dataprocessing.
◦ JOP+, JOP with support for reactivity and control processing.
Experimental results
Logic Element usage
Average Tick times (us)
Generated memory footprint (KB)
Conclusions
SystemJ is easier to verify (functional and nonfunctional):
◦ One is just programming an automaton
◦ Reduced state space representation, every change in data/signal is not a new state, only pause makes a new state
Time (real and logical) is a first class language construct.
SystemJ allows handling multiple events, since it is clock-driven.
No preemption of transitions, preemptions only allowed once transition is finished.
Correct by construction achieved via SPIN and SMT.
Verified, pacemaker control-logic implemented in
SystemJ.
