1. G53SRP: Real Time Threads in RTSJ (part I)
Real Time Scheduling
23/07/2018
G53SRP: Real Time Threads in RTSJ (part I)
Chris Greenhalgh
School of Computer Science
G53SRP

2. Contents Overview The Scheduler Class Fixed Priority Scheduler
The RealtimeThread Class
RealtimeThread properties
SchedulingParameters
PriorityParameters
ReleaseParameters
Periodic RealtimeThreads overview
Book: Wellings , 10.3, , (part), , 10.5, 12.1 (part), 12.3 (part)

3. Class Overview … <<interface>> java.lang. Runnable
<<interface>> javax.realtime. Schedulable
implements
extends
implements
java.lang. Thread
<<abstract>> javax.realtime. Scheduler
has
javax.realtime. RealtimeThread
extends
javax.realtime. PriorityScheduler
extends
(Default) …
javax.realtime. NoHeapRealtimeThread
extends

4. Checking and setting the Scheduler
package javax.realtime; public abstract class Scheduler { public static Scheduler getDefaultScheduler(); public static void setDefaultScheduler(Scheduler s); public String getPolicyName(); // other Scheduler methods … }
See HelloScheduler.java

5. The Priority-based Scheduler
package javax.realtime; public class PriorityScheduler extends Scheduler { public static PriorityScheduler instance(); // Scheduler-related info: priorities // Note: all RT Priorities > standard (1-10) public int getMinPriority(); public int getMaxPriority(); // at least min+27 public int getNormPriority(); // other PriorityScheduler methods … }
See HelloScheduler.java

6. The Priority-based Scheduler
Only standard scheduler specified in RTSJ
Used as initial default scheduler
Schedules realtime threads according to priority
Single fixed priority assigned to each thread
Fully pre-emptive
i.e. a higher priority thread that becomes runnable immediately pre-empts a lower priority thread
Does not time-slice threads of equal priority
Running thread may yield to allow another equal-priority thread to run

7. The RealtimeThread class
package javax.realtime; public class RealtimeThread extends java.lang.Thread implements Schedulable { public RealtimeThread( SchedulingParameters scheduling, ReleaseParameters release, MemoryParameters memory, MemoryArea area, ProcessingGroupParameters group, java.lang.Runnable logic); // partial cons. – defaults are all null public RealtimeThread(); public RealtimeThread(SchedulingParameters s); public RealtimeThread(SchedulingParameters s, ReleaseParameters release); …

8. … public static RealtimeThread currentRealtimeThread(); // from Thread public void run(); // runs logic by default public void start(); public void interrupt(); // changed: see ATC public void join(); public void join(long millis); public void join(long millis, int nanos); static public void sleep(long millis); static public void sleep(long millis, int nanos); // high-res versions static void sleep(Clock clock, HighResolutionTime t); static void sleep(HighResolutionTime time);

9. … // scheduler public Scheduler getScheduler(); public void setScheduler(Scheduler scheduler); // from Thread public void setPriority(int priority); public int getPriority(); // getters & setters for all parameters // (release, scheduler, memory, proc.group)… // periodic threads (see later) static boolean waitForNextPeriod(); static boolean waitForNextPeriodInterruptible(); public void schedulePeriodic(); public void deschedulePeriodic(); }

10. E.g. HelloRealtimeThread.java
import javax.realtime.RealtimeThread; public class HelloRealtimeThread extends RealtimeThread { public void run() { RealtimeThread rt = RealtimeThread.currentRealtimeThread(); System.out.println("Hello from "+rt); } public static void main(String args[]) { RealtimeThread rt = new HelloRealtimeThread(); rt.start(); try { rt.join(); } catch (InterruptedException ie) { }
Code run with realtime support

11. RealtimeThread (Schedulable) properties
SchedulingParameters (subclass)
Scheduler-specific thread information, e.g. priority
ReleaseParameters (subclass)
Temporal characteristics of the thread, e.g. release time, period, deadline, cost
MemoryParameters
memory usage of thread, e.g. max size, max rate of allocation

12. RealtimeThread (Schedulable) properties (cont.)
MemoryArea
type/area of memory in which new objects will be allocated by thread
ProcessingGroupParameters
optional group limits (cost per period) for a set of schedulable objects
java.lang.Runnable
the logic to be run by the default run method (or over-ride run method)

13. SchedulingParameters classes
<<abstract>> javax.realtime. SchedulingParameters
Empty class
javax.realtime. PriorityParameters
extends
javax.realtime. ImportanceParameters
extends

14. PriorityParameters class
package javax.realtime; public abstract class PriorityParameters extends SchedulingParameters { public PriorityParameters(int priority); public int getPriority(); public void setPriority(int priority); } 14

15. PriorityParameters notes
SchedulingParameters depend on Scheduler used
PriorityParameters used with (e.g.) PriorityScheduler
Equivalent to Thread get/setPriority

16. Priority notes Realtime Priorities > normal thread priorities
Normal priorities
1-10
Time-sliced – priority affects time budget
RT priorities
11-? (at least 28)
Strict order – only highest priority runs
See NRTPriority.java
See RTPriority.java

17. ImportanceParameters
Extends PriorityParameters
Adds an integer importance property
May be used by scheduler in deadline over-run situations
E.g. to identify safety-critical tasks for preferential scheduling
Optional – depends on scheduler implementation

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

19. ReleaseParameters class
package javax.realtime; public abstract class ReleaseParameters { protected ReleaseParameters( RelativeTime cost, RelativeTime deadline, AsyncEventHandler overrunHandler, AsyncEventHandler missHandler); // getters & setters … } 19

20. ReleaseParameters notes
For every type of realtime thread or other Schedulable
cost
Time units per release
May (or may not) be enforced as maximum
May (or may not) be used for feasibility test
deadline
Latest completion time after release time

21. PeriodicParameters class
package javax.realtime; public class PeriodicParameters extends ReleaseParameters { public PeriodicParameters( HighResolutionTime start, RelativeTime period, RelativeTime cost, RelativeTime deadline, AsyncEventHandler overrunHandler, AsyncEventHandler missHandler); // getters & setters … }
Specific to Periodic tasks 21

22. PeriodicParameters notes
start may be AbsoluteTime or RelativeTime
Default deadline = period

23. Periodic RealtimeThread
Follows standard code pattern:
public void run() {
boolean ok = true;
while(ok) {
// periodic task code …
ok = RealtimeThread.
waitForNextPeriod(); }
// contingency…
See HelloPeriodicThread.java

24. Example Periodic RealtimeThread (no problems)
Process Release Time
Process
Process Completion Time
Deadline met
Start
Period…
Call to waitForNextPeriod
RT1
Actual
task execution
waitForNextPeriod blocking
1000
2000
3000
4000
5000
Time (ms)

25. Summary Scheduler exposed through Scheduler abstract class
Fixed Priority Scheduler provided as default –PriorityScheduler
RealtimeThread class extends java.lang.Thread
implements Schedulable as well as Runnable (see later)

26. Summary (2) RealtimeThread properties
SchedulingParameters – for Scheduler
PriorityParameters – for PriorityScheduler, specifies thread priority (like Thread.get/setPriority)
ReleaseParameters
Defines thread deadline and cost
Subclassed as PeriodicParameters, AperiodicParameters & SporadicParameters

27. Summary (3) PeriodicParameters
Defines thread start time/delay and period
Supported by RealtimeThread.waitForNextPeriod