1. G53SRP: Asynchronous Events in RTSJ
Async EventsReal Time Scheduling
Real Time Scheduling
20/11/2018
20/11/2018
G53SRP: Asynchronous Events in RTSJ
Chris Greenhalgh
School of Computer Science 1
G53SRP
G53SRPG53SRP 1

2. Contents Overview AsyncEvent class AsyncEventHandler class
Aperiodic events and AperiodicParameters
Sporadic events and SporadicParameters
Timers and timer-based events
POSIX Signals
Summary
Book: Wellings chapter 11, , 2

3. Class Overview <<interface>> java.lang. Runnable
<<interface>> javax.realtime. Schedulable
extends
<<abstract>> javax.realtime. Scheduler
has
javax.realtime. PriorityScheduler
(Default)
Class Overview
javax.realtime. AsyncEvent
implements
javax.realtime. AsyncEventHandler
has
javax.realtime. Timer
extends
javax.realtime. BoundAsyncEventHandler
extends
javax.realtime. OneShotTimer
extends
javax.realtime. PeriodicTimer

4. Overview Asynchronous events are data-less occurrences
Fired by program, triggered by environment (interrupts, signals)
AsyncEvent instances represent the event
AsyncEventHandler instances can be registered with an AsyncEvent instance
Implements Schedulable c.f. RealtimeThread
The handler is released when the event fires

5. Async EventsReal Time Scheduling
20/11/2018
AsyncEvent
An AsyncEvent may be triggered by
Another part of the application calling fire()
A low-level system event (such as an interrupt) to which “happening” it is bound using bindTo
Not supported in demonstration RTSJ RI
The passage of time
For OneShotTimer or PeriodicTimer subclasses
POSIX (UNIX) signals can be handled but do any AsyncEvent object is not visible
See POSIXSignalHandler
G53SRPG53SRP

6. AsyncEvent class (i) Set of current handlers to release
package javax.realtime;
public class AsyncEvent {
public AsyncEvent();
// set of handlers
public void addHandler(AsyncEventHandler h);
public void removeHandler(AsyncEventHandler h);
public void setHandler(AsyncEventHandler h);
public boolean handledBy(AsyncEventHandler h);
public ReleaseParameters createReleaseParameters(); …
Set of current handlers to release
(each handler in list at most once)
Default ReleaseParameters for event
(if needed by handler)
- probably AperiodicParameters

7. AsyncEvent class (ii) Set of implementation- Specific “happenings”
that will fire this event
e.g. interrupt, signal …
public void bindTo(String happening);
// throws UnknownHappeningException
public void unBindTo(String happening);
public void fire();
// throws MITViolationException, // ArrivalTimeQueueOverflowException }
Explicit firing of
event by program
See later
(Sporadic)

8. AsyncEventHandler Specifies the code to run to handle an event
Associated with an AsyncEvent using addHandler or setHandler
Any “arguments” for the handler must be passed via (e.g.) a shared variable or data value queue
But note that a version of fire with an argument is being added to later version(s) of RTSJ

9. AsyncEventHandler class (i)
package javax.realtime;
public class AsyncEventHandler implements Schedulable {
public AsyncEventHandler();
// various other constructor options (with defaults)…
public AsyncEventHandler(
SchedulingParameters scheduling,
ReleaseParameters release,
MemoryParameters memory,
MemoryArea area,
ProcessingGroupParameters group,
boolean nonheap, Runnable logic); …

10. AsyncEventHandler parameters
See RealtimeThread
Part of common Schedulable interface / abstraction
SchedulingParameters, ReleaseParameters, MemoryParameters, MemoryArea, ProcessingGroupParameters
noheap
throws IllegalParameterException if event handler or parameters are on heap (GC issue)
logic
optional Runnable for handler, run by default handleAsyncEvent method [not run]

11. AsyncEventHandler class (ii)…
// various parameter getters and setters
// from Schedulable (see RealtimeThread)
// various Feasibility-testing methods
// from Schedulable (see later notes)

12. AsyncEventHandler class (iii)…
public final void run();
// event handling
public void handleAsyncEvent();
// event/release count
protected int getAndClearPendingFireCount();
protected int getAndDecrementPendingFireCount();
protected int getAndIncrementPendingFireCount();
protected int getPendingFireCount(); }

13. AsyncEventHandler notes (i)
Each AsyncEventHandler has its own “Arrival Time Queue”
= Release time queue
Each time the associated event is fired, that time is added to the arrival time queue
Subject to arrival time queue and minimum inter-arrival time behaviour (see later)
E.g. [T1][T2][]
= Releases at T1 and T2

14. AsyncEventHandler notes (ii)
The system will schedule a call to handleAsyncEvent() for each time in the queue
Each with its own deadline & cost
Specify code to run by
Overriding handleAsyncEvent()
Or passing a Runnable to constructor (called by default handleAsyncEvent())
(run() is defined by EventHandler super-class and cannot be overriden)
See HelloAsyncEventHandler.java
and HelloAsyncEventHandler2.java

15. AsyncEventHandler notes (iii)
The number of times in the queue is visible in the API as “fire count”
I.e. how many times the event has been fired and not yet handled, including any currently running release
Event handler code can check and manipulate the queue, e.g. to remove additional release(s)
See getPendingFireCount() etc.
See SlowAsyncEventHandler.java

16. “Periodic”, “aperiodic” and “sporadic” events
Irrespective of its actual cause or nature, a realtime event can be classified as
Periodic – occurring at some fixed interval
Aperiodic – occurring with no temporal pattern
Sporadic – occurring from time to time but not “too” often
See later notes on Minimum Inter-arrival Time
This must be known to assess the system’s schedulability
Using the AsyncEventHandler’s ReleaseParameters…

17. ReleaseParameters classes
<<abstract>> javax.realtime. ReleaseParameters
extends
javax.realtime. PeriodicParameters
javax.realtime. AperiodicParameters
javax.realtime. SporadicParameters
extends

18. Aperiodic events An aperiodic event can occur at any time
Successive releases may occur arbitrary closely together
=> CPU requirement potentially infinite
=> can never guarantee to meet deadlines with aperiodic events
Releases occurring before the previous release completes are queued
The release time is recorded
Various queue behaviours are possible…

19. AperiodicParameters class (i)
Package javax.realtime;
public class AperiodicParameters
extends ReleaseParameters {
// parameters as per ReleaseParameters
public AperiodicParameters();
public AperiodicPamameters(
RelativeCost cost,
RelativeTime deadline,
AsyncEventHandler overrunHandler,
AsyncEventHandler missHandler); …

20. AperiodicParameters class (ii)…
// arrival queue
public int getInitialArrivalTimeQueueLength();
public void setInitialArrivalTimeQueueLength(int initial);
public String getArrivalTimeQueueOverflowBehavior();
public void setArrivalTimeQueueOverflowBehavior(
String behvaviour);
// behavior constants
public static final String arrivalTimeQueueOverflowExcept;
public static final String arrivalTimeQueueOverflowIgnore;
public static final String arrivalTimeQueueOverflowReplace;
public static final String arrivalTimeQueueOverflowSave; }

21. Arrival Time Queue Length
EventHandler arrival time queue has initial length specified by AperiodicParameters
Note: queue includes current release
So queue length 0 with queue behaviour EXCEPT, IGNORE or REPLACE will never fire
New firings are added to empty slots in the queue
If queue is full then depends on arrival time queue behaviour…

22. Arrival Time Queue Behaviors
E.g. queue length 3, first release (T) running…
[T][T+4][T+6]
Event fired again:
EXCEPT – throw ArrivalTimeQueueOverflowException from fire()
like ignore if caused by external happening
IGNORE – ignore firing; queue unchanged
REPLACE – replace the last time in the queue
E.g. [T][T+4][T+10] (affects last release deadline)
SAVE – extend the queue (default)
E.g. [T][T+4][T+6][T+10]
See AperiodicAsyncEventHandler.java

23. Sporadic events
Sporadic events are similar to aperiodic events but also have a minimum inter-arrival time (MIT)
Equivalently, a maximum rate
MIT used for analysis and scheduling
Worst-case equivalent is a periodic event with MIT as period
Checked at run-time
Has an arrival time queue
From AperiodicParameters super-class, including queue length and associated queue-full behaviour
Plus additional constraints…

24. SporadicParameters class (i)
Package javax.realtime;
public class SporadicParameters
extends AperiodicParameters {
// parameters as per ReleaseParameters, plus…
public AperiodicParameters(
RelativeTime minInterarrival);
public AperiodicPamameters(
RelativeTime minInterarrival,
RelativeCost cost,
RelativeTime deadline,
AsyncEventHandler overrunHandler,
AsyncEventHandler missHandler); …

25. SporadicParameters class (ii)…
public RelativeTime getMinimumInterarrival();
public void setMinimumInterarrival(
RelativeTime minInterarrival);
// MIT violation…
public String getMitViolationBehavior();
public void setMitViolationBehavior(String behavior);
// behavior options
public static final String mitViolationExcept;
public static final String mitViolationIgnore;
public static final String mitViolationReplace;
public static final String mitViolationSave; }

26. Minimal Interarrival Behaviors
E.g. queue length 3, MIT=3…
[T][T+3][]
Event fired again at T+4 – MIT violation!:
EXCEPT – throw MITViolationException from fire()
like ignore if caused by external happening
IGNORE – ignore firing; queue unchanged
REPLACE – replace the last time in the queue
E.g. [T][T+4][] (no effect if last already released)
SAVE – add to queue (extend if required) (default)
E.g. [T][T+3][T+6]
Note: time from last release increased to MIT for starting handler but NOT deadline checking
See SporadicAsyncEventHandler.java

27. Timer overview Comparable to java.util.Timer/ TimerTask, but…
Realtime Timer extends AsyncEvent
OneShotTimer and PeriodicTimer extend Timer for one-shot or periodic use  So
Threading is handled by realtime VM or BoundAsyncEventHandler (see later)
Realtime equivalent of TimerTask is a regular realtime AsyncEventHandler

28. Timer class (i) Package javax.realtime; public abstract class Timer
extends AsyncEvent {
protected Timer(HighResolutionTime time,
Clock clock,
AsyncEventHandler handler);
// parameters
protected ReleaseParameters createReleaseParameters();
public Clock getClock();
// state…
public void destroy(); …

29. Fires when time reached?
Timer class (ii) …
public void start();
public void start(boolean disabled);
public void stop();
public void enable();
public void disable();
public void reschedule(HighReslutionTime time);
public AbsoluteTime getFireTime();
public boolean isRunning();
public void fire(); }
Timer active,
i.e. “clock” running?
Fires when time reached?
(default to true)
(next) fire time
active & enabled?

30. OneShotTimer class (i)
Package javax.realtime;
public class OneShotTimer extends Timer {
// default clock is system realtime clock
protected OneShotTimer(HighResolutionTime fireTime,
Clock clock,
AsyncEventHandler handler); }

31. OneShotTimer notes Fires once fireTime may be
RelativeTime – absolute fireTime determined in start()
AbsoluteTime – specifies fireTime directly
If in past then fires immediately
After firing becomes inactive (and disabled)
Can be fired again by reschule(time) then start()
See HelloOneShotTimer.java, HelloOneShotTimer2.java

32. PeriodicTimer class (i)
Package javax.realtime;
public class PeriodicTimer extends Timer {
// default clock is system realtime clock
protected PeriodicTimer(HighResolutionTime start,
RelativeTime interval,
Clock clock,
AsyncEventHandler handler);
// from Timer…
protected ReleaseParameters createReleaseParameters();
public AbsoluteTime getFireTime();
// properties
public RelativeTime getInterval();
public void setInterval(RelativeTime interval); }

33. PeriodicTimer notes (i)
Fires repeatedly at interval until stopped
fireTime as per OneShotTimer
Note: if disabled then intervals still pass but fire is not called (fireTime shows next possible fire)
E.g. due to fire at 10, 20, 30, …; disabled at 19, re-enabled at 21 => fire at 10, (not 20) 30
See HelloPeriodicTimer.java, HelloPeriodicTimer2.java
DisablePeriodicTimer.java

34. PeriodicTimer notes (ii)
Initialised with RelativeTime fireTime:
When start() is called the (next) absolute fireTime is calculated from initial relative fireTime and the current time
Initialised with AbsoluteTime fireTime:
When time start() is called if fireTime has passed one* fire is done immediately
But ignored if disabled
Can stop and re-start a timer
but generally wouldn’t
* Updates to RTSJ should allow a range of “catch-up” options

35. BoundAsyncEventHandler
Extends AsyncEventHandler
Same constructor arguments
Permanently associated with a RealtimeThread
Avoids any additional delay in assigning a fired AsyncEventHandler to a system RealtimeThread
(normally the realtime VM provides/creates realtime threads to run AsyncEventHandlers as needed)

36. BoundAsyncEventHandler class
package javax.realtime;
public class BoundAsyncEventHandler
extends AsyncEventHandler {
// as per AsyncEventHandler…
public BoundAsyncEventHandler();
public BoundAsyncEventHandler(
SchedulingParameters scheduling,
ReleaseParameters release,
MemoryParameters memory,
MemoryArea area,
ProcessingGroupParameters group,
boolean nonheap, Runnable logic); }

37. POSIXSignalHandler Required on POSIX platforms (only)
Standard way to register handlers for POSIX signals
C.f. AsyncEvent bound to corresponding happening (platform-dependent)
But note that there is not a (visible) AsyncEvent for any POSIX signals
See HelloPOSIXSignalHandler.java
HelloBoundAsyncEvent.java

38. POSIXSignalHandler class
package javax.realtime;
public class POSIXSignalHandler {
// signals…
public static final int SIGABRT;
public static final int SIGINT; …
public static void addHandler(int signal,
AsyncEventHandler handler);
public static void removeHandler(int signal,
public static void setHandler(int signal, }

39. Summary (i) AsyncEvent instances represent asynchronous events
Fired by program or bound to external signals or interrupts
AsyncEventHandler handles firing of event
Implements Schedulable
Same realtime parameters as RealtimeThread
Calls handleAsyncEvent or Runnable logic
Aperiodic events have no known arrival pattern
Described by AperiodicParameters ReleaseParameters
Can be (partially) managed via Arrival Time Queue

40. Summary (ii) Sporadic events have a minimum interarrival time
Described by SporadicParameters
MIT violations detected and responded to, e.g. ignore, save
Timed events supported by concrete Timer subclasses
OneShotTimer – fires once
PeriodicTimer – fires repeatedly with specified period
States include in/active, dis/en-abled, relative/absolute
POSIX Signals exposed to handlers via POSIXSignalHandler