Introduction
What’s Java?
Java vs C/C++
From C to Java
JavaTM & Real-Time
Advantages, Limitations, & RTSJ
Java Threads
Thread Example
Interrupts
Christos K LOUKINAS
Monitors
Timers
Christos.Kloukinas@imag.fr
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Verimag
(http://www-verimag.imag.fr/)
Grenoble, France
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.1/56
Introduction
Introduction
What’s Java?
Java vs C/C++
Java is a language with lots of merits
We will examine:
Its advantages for Real-Time programming;
From C to Java
Java Threads
Thread Example
Its disadvantages for Real-Time programming; and,
Interrupts
Monitors
Timers
The RTSJ proposal.
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.2/56
What’s Java?
Java is:
Introduction
Object-Oriented;
What’s Java?
Java vs C/C++
From C to Java
Type-Safe;
Java Threads
Thread Example
Interrupts
Portable;
Monitors
Timers
JNI
Library-Rich; and,
CNI
Disadvantages
Speed
has automatic Garbage Collection.
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.3/56
Java versus C/C++
O-O helps with structuring code and decreasing code
dependencies.
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Increased type-safety, bound-checks and lack of
pointer-arithmetic increase overall safety and allow for
aggressive compiler optimisations.
Monitors
Timers
JNI
CNI
Disadvantages
Built-in exceptions, threads & synchronisation means
that the compiler knows how to do “The Right Thing”.
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Built-in GC means no more dangling pointers & a lot
fewer memory leaks.
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Fixed size and byte order of basic types helps with
porting your software.
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.4/56
From C to Java
Structures now become Classes and have associated
Methods.
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
Easier to tell what functions can have access to a
structure.
No need to check the kind of structure another function
passed you - the O-O internal mechanisms do this
automatically.
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
O-O: How to let your compiler write your
switches. . .
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.5/56
From C to Java - II
Controlling access to methods, fields & classes themselves:
Introduction
What’s Java?
public
Java vs C/C++
From C to Java
private
Java Threads
Thread Example
Interrupts
protected
Monitors
Timers
JNI
CNI
Disadvantages
packages (only one class can be public in a
package)
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Other modifiers:
abstract / static / final / strictfp
Memory
Synchronisation
Asynchronous
synchronized (equivalent to a synchronized(this))
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.6/56
From C to Java - III
Introduction
What’s Java?
Java vs C/C++
unions & enums don’t exist - use polymorphism
(or compile time constants if it’s not worth the
abstraction)
From C to Java
Java Threads
Thread Example
multiple inheritance - use interfaces or composition
Interrupts
Monitors
Timers
JNI
Reflection - getClass, getMethods, getPackage,
isInstance
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.7/56
From C to Java - III
Introduction
What’s Java?
Java vs C/C++
unions & enums don’t exist - use polymorphism
(or compile time constants if it’s not worth the
abstraction)
From C to Java
Java Threads
Thread Example
multiple inheritance - use interfaces or composition
Interrupts
Monitors
Timers
JNI
Reflection - getClass, getMethods, getPackage,
isInstance
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Unlike C++, there are no object
destructors!
Cannot control when finalizers will be executed (if
they ever are. . . )
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.7/56
Concurrent Programming with Java
Java has:
Introduction
Threads
What’s Java?
Java vs C/C++
From C to Java
Monitors (& Condition Variables)
Java Threads
Thread Example
Interrupts
Monitors
Timers
Timers
JNI
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.8/56
Threads in Java
Introduction
What’s Java?
Threads implement the Runnable interface,
i.e., provide a run method
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Threads have priorities:
from MIN_PRIORITY to MAX_PRIORITY, default is
NORM_PRIORITY
Timers
JNI
CNI
Threads have lower priority than the GC (always)
Disadvantages
Speed
JIT or AOT?
Resource allocation is not (necessarily) priority-based
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Stack size can be declared,
but it can also be ignored. . .
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.9/56
Threads in Java - II
Dæmon threads (cannot keep an application running)
Introduction
What’s Java?
Java vs C/C++
From C to Java
Have methods: holdsLock, interrupt, join,
yield, sleep, destroy (not implemented)
Java Threads
Thread Example
Interrupts
No longer have: resume, stop, suspend
Monitors
Timers
JNI
Interrupts:
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
Threads poll for interrupts with one of:
wait, sleep, join, isInterrupted
RTSJ
Scheduling
Memory
The target thread can poll whenever it wants
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.10/56
Starting a Thread
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
class ERR {
static public void pr(String msg){
System.err.println(Thread.
currentThread().toString()+": "+msg)
;}}
class MyThread extends Thread {
Thread parent;
public MyThread() {parent = Thread.
currentThread();}
public void run() {
ERR.pr("Hello");
}
}
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.11/56
Starting a Thread - II
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
class Main {
public static void main(String[] argv)
{
MyThread aThread = new MyThread();
ERR.pr("this is main");
aThread.start(); // calls run()
try {aThread.join();}
catch (InterruptedException ie){ERR.pr
("Un-joinable"+ie);}
finally {ERR.pr("Bye");}
}
}
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.12/56
Interrupting a Thread
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
class MyThread extends Thread {
// As before ...
public void run() {
ERR.pr("Hello");
try {this.sleep(2);} catch (
InterruptedException ie) {}
parent.interrupt();
}
}
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.13/56
Interrupting a Thread - II
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
class Main {
public static void main(String[] argv)
throws InterruptedException {
MyThread aThread = new MyThread(Thread
.currentThread());
ERR.pr("this is main");
aThread.start();
// calls run()
try {
aThread.join();
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.14/56
Interrupting a Thread - III
if (Thread.currentThread().
isInterrupted()) {
ERR.pr("Was interrupted");
throw new InterruptedException("
Suicide...");}
} catch (InterruptedException ie) {
ERR.pr("Un-joinable:" + ie);
throw ie;
} finally {ERR.pr("Bye");}
ERR.pr("Never printed...");
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
}
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
}
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.15/56
Monitors
Each Java object has its own semaphore and its own
condition variable.
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
We use the synchronized(object){} construct to
denote a critical region block protected by a monitor on
the semaphore of the object.
Monitors
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
Inside a monitor, we use wait() & notify() to wait
on a condition & to flag it.
Can also use wait(/*timeout*/) and
notifyAll().
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
A break, continue, return, throw inside a monitor
will exit it after having unlocked the object.
Are you sure you do so in C/C++?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.16/56
Monitors - II
Entering a monitor is not priority-based.
Introduction
What’s Java?
Java vs C/C++
From C to Java
A timed wait can wait for more than the timeout
specified.
Java Threads
Thread Example
Interrupts
Monitors
Timers
Choosing a thread to notify is not priority-based.
No priority-sorted wait queues.
JNI
CNI
Disadvantages
No priority-inversion avoidance protocol.
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.17/56
Timers
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
java.lang.Timer & java.util.TimerTask
One-time execution;
Repeated execution.
fixed-delay
nth execution starts period milliseconds after the
(n − 1)th execution
(delays are therefore additive)
fixed-rate
nth execution starts n∗period milliseconds after
the first execution
(try to keep a constant rate - may execute many
times in rapid succession to catch up)
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.18/56
Timers - II
No Real-Time guarantees! – Using wait(/*timeout*/)
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
If a timer task takes too long, it can delay future
executions.
Not a dæmon thread by default
(can keep an application from terminating)
Timers
JNI
CNI
Disadvantages
Speed
When should I run if no delays?
scheduledExecutionTime()
JIT or AOT?
Real-Time GC
RTSJ
A timer can be associated with many timer tasks
Scheduling
Memory
Synchronisation
A timer task can be associated with many timers
Asynchronous
Other Proposals
Why Java?
cancel() a timer or a timer task
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.19/56
Timer Example
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
class MyTimerTask extends TimerTask {
private int MAX_TARDINESS = 15;
public void run() {
ERR.pr(toString()+": Started");
if (System.currentTimeMillis() scheduledExecutionTime() >=
MAX_TARDINESS)
return; // Too late
ERR.pr(toString()+": running @ " +
System.currentTimeMillis());
}
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.20/56
Timer Example - II
public static void main(String[] argv)
{
MyTimerTask aTimerTask = new
MyTimerTask();
Timer
aTimer = new Timer(false);
// Not a daemon timer
// Start=10 ms, period=30 ms
aTimer.scheduleAtFixedRate(aTimerTask
, 10L, 20L);
// aTimer.cancel();
}
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
}
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.21/56
Native Interface Example - Java
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
class HelloWorld {
private native void print();
static {
System.loadLibrary("HelloWorld");
}
public static void main(String[] args)
{
Thread.currentThread().setName("The
Main from Java");
new HelloWorld().print();
}
}
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.22/56
Native Interface
Declaring methods as native
Introduction
What’s Java?
Loading them up at start-up using static blocks
Java vs C/C++
From C to Java
Java Threads
Finding out signatures using javap -s class
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.23/56
Type Signatures
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
void
boolean
byte
int
float
V
Z
B
I
F
char
short
long
double
C
S
J
D
Object of class X :
LX;
Array of type X :
[X
ret foo(args) :
(args)ret
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
String[] foo(Thread[] arr, int i, long j):
?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.24/56
Type Signatures
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
void
boolean
byte
int
float
V
Z
B
I
F
char
short
long
double
C
S
J
D
Object of class X :
LX;
Array of type X :
[X
ret foo(args) :
(args)ret
Timers
JNI
CNI
String[] foo(Thread[] arr, int i, long j):
Disadvantages
Speed
JIT or AOT?
([Ljava/lang/Thread;IJ)[Ljava/lang/String;
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.24/56
JNI
Can do lock/unlock, new, etc.
Introduction
What’s Java?
It’s heavy - all queries use strings
Java vs C/C++
From C to Java
Java Threads
Cannot optimise code written with it
Thread Example
Interrupts
Monitors
Java threads are not necessarily system threads
Timers
JNI
CNI
Disadvantages
Speed
May have to use a finalizer
It will be run before the memory is reused. . .
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.25/56
CNI
Used with GCJ (front-end of GCC)
Introduction
What’s Java?
Java vs C/C++
Java classes are C++ classes (not vice versa)
From C to Java
Java Threads
Thread Example
Faster & can be optimised
Interrupts
Monitors
Timers
A lot simpler to write code
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.26/56
Disadvantages
Java is slow;
Introduction
What’s Java?
Java’s memory model is convoluted (Under revision);
Java vs C/C++
From C to Java
Java Threads
The GC is unpredictable;
Thread Example
Interrupts
Monitors
Scheduling is unpredictable;
Timers
JNI
CNI
Monitors can suffer from priority-inversion;
Disadvantages
Speed
JIT or AOT?
Notification is not priority-based;
Real-Time GC
RTSJ
Scheduling
Timers are not accurate;
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Design for portability
⇒ Cannot use any special OS mechanisms
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.27/56
Speed
It’s not Java, it’s the implementations;
Introduction
What’s Java?
Java vs C/C++
One can use a JIT or an AOT compiler;
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
It’s easier to optimise Java than C/C++
No (wild) casts, pointer-arithmetic, etc..
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.28/56
JIT or AOT?
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JIT:
can do anything a JVM does;
works on bytecodes;
needs resources;
cannot optimise much;
is unpredictable.
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
AOT:
can do aggressive optimisations;
no need for extra resources at the target system;
is predictable;
can always add a JVM for loading classes.
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.29/56
Real-Time GC
Interruptible, low latency, Real-Time GC’s exist
Introduction
What’s Java?
Java vs C/C++
Usually meant for large or non-critical systems
From C to Java
Java Threads
Thread Example
But there is also the memory region technique.
Interrupts
Monitors
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.30/56
RTSJ
Thread Scheduling & Dispatching
Introduction
What’s Java?
Java vs C/C++
Memory Management
From C to Java
Java Threads
Thread Example
Physical Memory Access
Interrupts
Monitors
Timers
Synchronisation & Resource Sharing
JNI
CNI
Disadvantages
Asynchronous Event Handling
Speed
JIT or AOT?
Real-Time GC
Asynchronous Transfer of Control
RTSJ
Scheduling
Memory
Asynchronous Thread Termination
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.31/56
Scheduling
Preemptive, Priority-based by default
At least 28 different priorities
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
RealtimeThread
Can access the heap
Lower priority than the GC
NoHeapRealtimeThread
No heap accesses
Higher priority than the GC
AsyncEventHandler
Depends on the noheap arg
Can be unbound or bound to a particular thread
It can block (holding resources)
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.32/56
Scheduling - II
Need to have a reference to a Scheduler obj.
Introduction
What’s Java?
Java vs C/C++
From C to Java
A Scheduler has params for priority, importance,
memory, etc.
Java Threads
Thread Example
Interrupts
Monitors
PeriodicParameters, AperiodicParameters,
SporadicParameters
Timers
JNI
CNI
Disadvantages
Speed
Processing group to treat non-periodic threads as a
single periodic one
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Max. Execution time, Deadline & overrun handlers
The execution overrun handler may be ignored & use
the duration only as a hint to the feasibility algorithm.
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.33/56
Scheduling - III
Introduction
What’s Java?
Java vs C/C++
Scheduler.addToFeasibility(Schedulable) to
add another task in the feasibility analysis performed
by the scheduler.
From C to Java
Java Threads
Thread Example
Interrupts
Scheduler.isFeasible() to find out if the current
system can be indeed scheduled
Monitors
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Changing the parameters of a task at run-time through
the setIfFeasible(Schedulable,
ReleaseParameters,
MemoryParameters)
Memory
Synchronisation
Asynchronous
Other Proposals
waitForNextPeriod() for periodic tasks.
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.34/56
Scedulable Objects
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
setScheduler(
Scheduler
sched,
SchedulingParameters sched_params,
ReleaseParameters
release_params,
MemoryParameters
mem_params,
ProcessingGroupParameters group)
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
defines
the
PriorityScheduler,
with
two
RTSJ
Scheduling
Memory
final int fields, MAX_PRIORITY and MIN_PRIORITY.
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.35/56
Scheduling Parameters
RTSJ defines two kinds of Scheduling parameters:
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
PriorityParameters(int priority)
and an additional one for declaring the importance of
the threads along with their priority
ImportanceParameters(int priority, int
importance)
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.36/56
Release Parameters - Periodic
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
PeriodicParameters(
HighResolutionTime start, /* null =
start when released */
RelativeTime period,
RelativeTime cost,
RelativeTime deadline, /* use period
when null */
AsyncEventHandler overrunHandler,
AsyncEventHandler missHandler)
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.37/56
Release Parameters - Aperiodic
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
AperiodicParameters(
RelativeTime cost,
RelativeTime deadline,
AsyncEventHandler overrunHandler,
AsyncEventHandler missHandler)
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.38/56
Release Parameters - Sporadic
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
SporadicParameters(
RelativeTime minInterarrival,
RelativeTime cost,
RelativeTime deadline,
AsyncEventHandler overrunHandler,
AsyncEventHandler missHandler)
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.39/56
Memory Parameters
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
MemoryParameters(
long maxMemoryArea,
// or MemoryParameters.NO_MAX (units=
bytes)
long maxImmortal,
// or NO_MAX (units=bytes)
long allocationRate)
// or NO_MAX (units=bytes/sec)
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.40/56
Thread Group
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
ProcessingGroupParameters(
HighResolutionTime start,
RelativeTime period,
RelativeTime cost,
RelativeTime deadline,
AsyncEventHandler overrunHandler,
AsyncEventHandler missHandler)
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.41/56
Creating a R-T Thread
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Scheduler sched= new PriorityScheduler();
PeriodicParameters pp = new
PeriodicParameters(
new RelativeTime(0,0),//when released
new RelativeTime(100, 10), // period
new RelativeTime(30, 0), // cost
new RelativeTime(60, 0), // deadline
null,
// no Overrun Handler
null);
// no Miss Period Handler
ImportanceParameters prio = new
ImportanceParameters(MAX_PRIORITY, 3);
MemoryArea ma = new LTMemory(1024, 1024);
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.42/56
Creating a R-T Thread - II
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
MemoryParameters mp = new
MemoryParameters(MemoryParameters.
NO_MAX,MemoryParameters.NO_MAX);
ProcessingGroupParameters gp = null;
RealtimeThread rt = new
RealtimeThread(prio, pp, mp, ma, gp,
new Runnable () {/*the run method...*/
});
rt.setScheduler(sched);
if (!rt.getScheduler().isFeasible())
throw new Exception("Not feasible");
rt.start();
// Release the thread
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.43/56
Finding a New Scheduler
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
class SchedExample {
public static Scheduler findSched(String
policy) {
String className = System.getProperty("
javax.realtime.scheduler."+policy);
Class clazz;
try {
if (null != className && null != (clazz
= Class.forName(className)))
return (Scheduler) clazz.getMethod("
instance",null).invoke(null,null);
} catch (/* lots of exceptions */) {}
return null;
}
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.44/56
Finding a New Scheduler - II
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Scheduler sched = findSched("EDF");
if (null != sched) {
RealtimeThread rt = new RealtimeThread(
/*...*/);
rt.setScheduler(sched);
if (!rt.getScheduler().isFeasible())
throw new Exception("Not feasible");
rt.start();
// Release the thread
}
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.45/56
Memory Management
Execution Stack, Heap & Memory Scopes
Scopes give bounds to the lifetime of objects allocated
within them
Introduction
What’s Java?
Java vs C/C++
From C to Java
Objects of a scope collected en masse after its
Runnable exits
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
A scope is collected before being reused
(not when exited, so finalizers still a problem)
Hierarchy of scopes - inner, outer (or unrelated)
Immortal memory : top-most scope, collected when
the JVM exits
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
Objects surviving current scope need be allocated to
the top-most scope which contains a reference to
these objects
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.46/56
Memory Management - II
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
LTMemory versus VTMemory
Allocation in LTMemory is linear with respect to
object’s size.
A VTMemory may be separately GC’ed and may be
augmented if it’s not big enough
Monitors
Timers
JNI
Assignment Rules
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Heap
Immortal
Scoped
Refs Heap Immortal
Yes
Yes
Yes
Yes
Yes
Yes
Scoped
No
No
Yes, if same
or higher scope
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.47/56
Memory Management - III
LTPhysicalMemory, VTPhysicalMemory,
ImmortalPhysicalMemory
Introduction
What’s Java?
SizeEstimator & reserve(class, instances)
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
RawMemoryAccess - Accessing a region interpreted
as byte, short, int, long or as an array of such
RawMemoryFloatAccess - Same but for float &
double
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Maximum preemption latency of a Real-Time thread
due to the GC is given by:
RealtimeSystem.currentGC().
getPreemptionLatency()
(returns a RelativeTime)
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.48/56
Synchronisation
Priority inheritance protocol supported by default
Introduction
What’s Java?
Java vs C/C++
Given the GC it’s impossible to implement PIP!
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
Disadvantages
3 Wait-free queues for avoiding this.
WaitFreeWriteQueue (non-blocking write),
WaitFreeReadQueue (non-blocking read),
WaitFreeDequeue (non-blocking write & read)
Speed
JIT or AOT?
Real-Time GC
Real-Time part of queue never blocks - no need for PIP.
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
If action is not possible (e.g., queue is full) you decide
(overwrite old value, ignore the new one)
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.49/56
Asynchronous Events
Introduction
What’s Java?
AsyncEvent, AsyncEventHandler,
handleAsyncEvent() ≡ run()
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
JNI
CNI
Disadvantages
bindTo(String), addHandler(), fire()
Data-less events
Can have multiple handlers per event, events per
handler
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Behave like RealtimeThread, or like
NoHeapRealtimeThread if given a scoped region
Can block
POSIXSignalHandler if OS supports P OSIX
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.50/56
Asynchronous Transfer of Control
Java’s longjmp
Introduction
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
Timers
A method must declare
AsynchronouslyInterruptedException,
otherwise it is postponed
Don’t break code which doesn’t expect to be
interrupted
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Also postponed if in a synchronized block
Don’t leave shared objects in an inconsistent state
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Use happened() to find out if the AIE you got is the
one you expected
Asynchronous
Other Proposals
Why Java?
doInterruptible() if it is, propagate it if not
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.51/56
ATC - II
Class Timed for Real-Time timers (throws an AIE)
Introduction
What’s Java?
Timer restarted after doInterruptible()
Java vs C/C++
From C to Java
Java Threads
resetTime() to use another duration
Thread Example
Interrupts
Monitors
Timers
disable, enable, fire, propagate, isEnabled,
happened
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Blocking java.io.* classes must know of AIE
(unblock, raise IOException, or complete
normally. . . )
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.52/56
Asynch. Termination
Allowing it everywhere, always is unsafe
Introduction
What’s Java?
So we use ATC to do it
Java vs C/C++
From C to Java
Java Threads
Thread Example
Interrupts
Monitors
An AIE is treated as a normal exception, but
a catch( ALL ) doesn’t catch it, unless we
specifically test for the AIE with happened
Timers
JNI
CNI
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.53/56
Other Proposals
RTCE & HIP- from the J-Consortium.org
Introduction
What’s Java?
Java vs C/C++
Ravenscar-Java- from the Ada community
From C to Java
Java Threads
Thread Example
JTRON- from Japan
Interrupts
Monitors
Timers
JNI
CNI
Java 2, Micro Edition (J2ME) - from SUN, for
embedded systems
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Scheduling
Memory
Synchronisation
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.54/56
Why Java?
Introduction
What’s Java?
Easier to write correct code (threads, monitors,
exceptions)
Java vs C/C++
From C to Java
Java Threads
Many libraries
Thread Example
Interrupts
Monitors
Strongly typed
Timers
JNI
CNI
No out-of-bound accesses, dangling pointers
Disadvantages
Speed
JIT or AOT?
Real-Time GC
RTSJ
Automatic GC- no more leaks
Easier to have it optimised
Scheduling
Memory
Synchronisation
It has all the buzzwords. . .
Asynchronous
Other Proposals
Why Java?
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.55/56
Why Java for Real-Time?
Others are already using it
Introduction
Tools are getting better (and more)
What’s Java?
Java vs C/C++
From C to Java
Java Threads
Thread Example
Using C/C++ is tedious, error-prone, difficult to
maintain and to have it validated
Interrupts
Monitors
Timers
JNI
Embedded systems is a huge market
CNI
Disadvantages
Speed
Software is replacing hardware
JIT or AOT?
Real-Time GC
RTSJ
Too complex for todays tools and languages
Scheduling
Memory
Synchronisation
Java promises to help us
Asynchronous
Other Proposals
Why Java?
Cannot fight a dream. . . So join it!
Java for Real-Time?
Christos K LOUKINAS
École IN2P3 d’informatique: Temps Réel – 29 May 2003 – p.56/56