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

Contents Overview The Scheduler Class Fixed Priority Scheduler The RealtimeThread Class RealtimeThread properties SchedulingParameters PriorityParameters ReleaseParameters Periodic RealtimeThreads overview Book: Wellings 10.2.1, 10.3, 10.4.1, 10.4.2 (part), 10.4.4, 10.5, 12.1 (part), 12.3 (part)

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

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

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

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

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); …

… 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);

… // 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(); }

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

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

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)

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

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

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

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

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

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

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

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

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

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

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

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)

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)

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

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