1. Real Time Java : Synchronization
Maung Aung Han,
Marc Loy
Jihua Zhong
Seminor in Real-time Systems
Instructor: Professor Insup Lee
1/17/2019
CIS 642

2. CIS 642: Real Time Java: Synchronization
Real time Java issues
Threads
Release characteristics & failures
Scheduling
Synchronization
Time and timers
Memory management
Asynchronous event handling
Asynchronous transfer of control
Exceptions
System-level options
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CIS 642: Real Time Java: Synchronization

3. Multitasking – Multithread
Multitasking(operating system related)
Cooperative(non-preemptive)
Preemptive
Multiple process
Each process has own variables
Multithread
Threads share the same data
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CIS 642: Real Time Java: Synchronization

4. CIS 642: Real Time Java: Synchronization
Multithread
Extremely useful in practice
browser deal with multiple hosts
Open window or view another page while downloading data
Java use a thread to do garbage collection in the background
Very complicated !!
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CIS 642: Real Time Java: Synchronization

5. Example: without Synchronization
From Core java 2
unsync.html
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CIS 642: Real Time Java: Synchronization

6. Example : possible output
Transactions:10000 Sum:
Transactions:20000 Sum:
Transactions:30000 Sum:
Transactions:40000 Sum:
Transactions:50000 Sum:
Transactions:60000 Sum:
Transactions:70000 Sum:
Transactions:80000 Sum:
Transactions:90001 Sum:
Transactions: Sum:
Transactions: Sum:
Transactions: Sum:
Transactions: Sum: 94792
Transactions: Sum: 94792
Transactions: Sum: 94792 …
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CIS 642: Real Time Java: Synchronization

7. CIS 642: Real Time Java: Synchronization
Why Synchronization?
Race condition!
Two or more threads access to same object and each call a method that modifier the state of the object. Corrupted object result!
So synchronization needed in the some method.
1/17/2019
CIS 642: Real Time Java: Synchronization

8. CIS 642: Real Time Java: Synchronization
Why Synchronization?
Unsynchronized
Synchronized
Thread 1
Thread 2
Thread 1
Thread 2
Time
transfer
transfer
transfer
transfer
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CIS 642: Real Time Java: Synchronization

9. CIS 642: Real Time Java: Synchronization
How to Synchronize?
For Regular Java Threads
Object Locks
Key word : synchronized
unlock object lock
Wait method: Called inside synchronized method and the current thread is blocked and put into wait list
Notify / NotifyAll method: remove the thread from wait list
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CIS 642: Real Time Java: Synchronization

10. Example: with Synchronization
From Core java 2
sunc.html
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CIS 642: Real Time Java: Synchronization

11. RT Issues: Synchronization
Risk of unbounded priority inversions
Priorities can uncover or existing race conditions
Need well defined thread and locking semantics
running outside block
blocked at guard
waiting (blocked) on
a condition variable
synchronized block
running inside block
priority key:
high
middle
low
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CIS 642: Real Time Java: Synchronization

12. CIS 642: Real Time Java: Synchronization
RT Threads
Realtime Thread
NoHeapRealtimeThread
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CIS 642: Real Time Java: Synchronization

13. Semantics & requirements
Priority
Loosely (not strict)
Waite Queues
Thread waiting to enter synchronization block are in execution eligibility(EE) order
Blocked thread that becomes ready to run
Explicitly set by itself or other threads
Thread performs a yield
Preempted with a thread with higher EE
FIFO for threads with same EE
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CIS 642: Real Time Java: Synchronization

14. Semantics & requirements (contd.)
Priority Inversion Avoidance
Any conforming implementation must provide an implementation of the synchronized primitive with default behavior that ensure that there is no unbounded priority inversion.This must apply to code running with both the implementation and real-time threads.
The priority inheritance protocol must be implemented by default and can be overridden
Support the priority ceiling emulation protocol
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CIS 642: Real Time Java: Synchronization

15. Semantics & requirements (contd.)
Priority Inversion Avoidance
NoHeapRealtimeThreads have an implicit execution eligibility that must be higher than that of the garbage collector , but regular java threads never does.
Note that: if the execution of NoHeapRealtimeThreads must not be delayed by the execution of garbage collector, it is impossible for a NoHeapRealtimeThread to synchronize on an object accessed by regular Java Threads. RTSJ provides three wait-free queue classes to provide protected, non-blocking, shared access to objects access by both regular Java threads and NoHeapRealtimeThreads
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CIS 642: Real Time Java: Synchronization

16. Semantics & requirements (contd.)
Determinism
Conforming implementations shall provide a fixed upper bound on the time required to enter a synchronized block for an unlocked monitor
Clearly document the behavior of the algorithm which provide the monitor control
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CIS 642: Real Time Java: Synchronization

17. Priority inheritance protocol (default)
If the thread t1 attempts to acquire a lock held by a lower-priority thread t2, the priority of t2 is raised to that of t1 as long as t2 holds the lock ( recursively if t2 is itself waiting to acquire a lock held by a even lower priority thread)
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CIS 642: Real Time Java: Synchronization

18. Priority ceiling emulation protocol (highest locker protocol)
A monitor is given a priority ceiling when it is created
when a thread enters synchronized code, its priority raised to the monitor’s ceiling priority, thus ensuring mutually exclusive access to the code since it will not be preempted by any thread that could possibly attempt to enter the same monitor
If, through programming error, a thread has a higher priority than the ceiling of the monitor it is attempting to enter, then an exception is thrown
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CIS 642: Real Time Java: Synchronization

19. Implementation: Monitor Control
PriorityCeilingEmulation
PriorityInheritance
public abstract class MonitorControl {
public MonitorControl()
public static void setMonitorControl(MonitorControl policy)
//control the default monitor behavior for object monitors used //by synchronized statement and methods in the system
public static void setMonitorControl(java.lang.Object monitor, MonitorControl policy)
//policy only affect the indicated object policy
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CIS 642: Real Time Java: Synchronization

20. Implementation: PriorityCeilingEmulation
Objects under the influence of this protocol have the effect that a thread entering the monitor has its effective priority- for priority-based dispatching-raised to the ceiling on the entry, and is restored to its previous effective priority when it exits the monitor.
public class PriorityCeilingEmulation extends MonitorControl
public PriorityCeilingEmulation(int ceiling)
// ceiling – Priority ceiling value
public int getDefaultCeiling()
//get the priority ceiling for an object
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CIS 642: Real Time Java: Synchronization

21. Implementation: PriorityInheritance
a thread entering the monitor will boost the effective priority of the thread in the monitor to its own effective priority. And will be restored to its previous effective priority when it exits the monitor.
public class PriorityInheritance extends MonitorControl
public PriorityInheritance()
// ceiling – Priority ceiling value
public static PriorityInheritance instance()
//return a pointer to the singleton PriorityInheritance
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CIS 642: Real Time Java: Synchronization

22. Implementation: WaitFreeDequeue
The wait-free queue classes facilitate communication and synchronization between instances of RealtimeThread and java.lang.Thread
public class WaitFreeDequeue
public WaitFreeDequeue(java.lang.Thread writer, java.lang.Thread reader, int maximum, MemoryArea area)
public java.lang.Object blockingRead() //when queueis empty
public boolean blockingWrite(java.lang.Object object) //full
public boolean force(java.lang.Object object)
public java.lang.Object nonblockingRead()
public java.lang.Object nonblockingWrite()
SYN
UNSYN
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CIS 642: Real Time Java: Synchronization

23. Implementation: WaitFreeReadQueue
Problem:
synchronizated access objects shared between real-time threads and regular java threads might cause the real-time threads to incur delays due to execution of the garbage collector
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CIS 642: Real Time Java: Synchronization

24. Implementation: WaitFreeReadQueue
public class WaitFreeReadQueue
public WaitFreeReadQueue(java.lang.Thread writer, java.lang.Thread reader, intmaximum, MemoryArea memory)
public WaitFreeReadQueue(java.lang.Thread writer, java.lang.Thread reader, intmaximum, MemoryArea memory, boolean notify)
public void clear()
public boolean isEmpty()
public boolean isFull()
public java.lang.Object Read()
public int size()
public void waitForData()
public synchronized boolean write(java.lang.Object object)
//this call blocks on queue full and will wait until there is space in the queue
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CIS 642: Real Time Java: Synchronization

25. Implementation: WaitFreeWriteQueue
public class WaitFreeWriteQueue
public WaitFreeWriteQueue(java.lang.Thread writer, java.lang.Thread reader, intmaximum, MemoryArea memory)
public WaitFreeWriteQueue(java.lang.Thread writer, java.lang.Thread reader, intmaximum, MemoryArea memory, boolean notify)
public void clear()
public boolean isEmpty()
public boolean isFull()
public java.lang.Object Read() // block on the queue empty
public int size()
public void waitForData()
public synchronized boolean write(java.lang.Object object)
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CIS 642: Real Time Java: Synchronization

26. CIS 642: Real Time Java: Synchronization
Summary
Synchronization is very important in real world and complicated for implementation
Efficiency for real time java are less discussed
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CIS 642: Real Time Java: Synchronization

27. CIS 642: Real Time Java: Synchronization
Reference
Core Java 2. Vol 2, Ch1
1/17/2019
CIS 642: Real Time Java: Synchronization