G53SRP: Real Time Threads in RTSJ (part I)

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G53SRP Real Time Threads in RTSJ (part I)

• Chris Greenhalgh
• School of Computer Science

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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)

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Class Overview
ltltinterfacegtgtjava.lang.Runnable
implements
java.lang.Thread

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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
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The Priority-based Scheduler

• package javax.realtime
• public class PriorityScheduler extends Scheduler
  
• public static PriorityScheduler instance()
• // Scheduler-related info priorities
• // Note all RT Priorities gt standard (1-10)
• public int getMinPriority()
• public int getMaxPriority() // at least min27
• public int getNormPriority()
• // other PriorityScheduler methods

See HelloScheduler.java
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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

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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)

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• public static RealtimeThread currentRealtimeThre
  ad()
• // 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)

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• // 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()

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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)

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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

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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)

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SchedulingParameters classes
ltltabstractgtgtjavax.realtime.SchedulingParameters
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PriorityParameters class

• package javax.realtime
• public abstract class PriorityParameters
• extends SchedulingParameters
• public PriorityParameters(int priority)
• public int getPriority()
• public void setPriority(int priority)

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PriorityParameters notes

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

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Priority notes

• Realtime Priorities gt 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
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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

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ReleaseParameters classes
ltltabstractgtgtjavax.realtime.ReleaseParameters
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ReleaseParameters class

• package javax.realtime
• public abstract class ReleaseParameters
• protected ReleaseParameters(
• RelativeTime cost,
• RelativeTime deadline,
• AsyncEventHandler overrunHandler,
• AsyncEventHandler missHandler)
• // getters setters

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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

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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

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PeriodicParameters notes

• start may be AbsoluteTime or RelativeTime
• Default deadline period

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Periodic RealtimeThread

• Follows standard code pattern
• public void run()
• boolean ok true
• while(ok)
• // periodic task code
• ok RealtimeThread.
• waitForNextPeriod()
• // contingency

See HelloPeriodicThread.java
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Example Periodic RealtimeThread (no problems)
Process Release Time
Process
Process Completion Time Deadline met
Call to waitForNextPeriod
RT1
Actual task execution
Time (ms)
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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)

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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

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Summary (3)

• PeriodicParameters
• Defines thread start time/delay and period
• Supported by RealtimeThread.waitForNextPeriod