Programming Support in Java™, Ada and POSIX

Programming Support in Java™, Ada and POSIX
Concurrency and Real-Time Programming Support in Java™, Ada and POSIX Tutorial for SIGAda 2001 October 1, 2001 Bloomington, MN Unclear why I’m in the “Wireless, Mobile, Peer-to-Peer, Distributed Computing” track The only thing wireless is that I’m not doing an on-screen ppt presentation Ben Brosgol 79 Tobey Road Belmont, MA USA (Voice) (FAX)

Topics Concurrency issues Basic model / lifetime Mutual exclusion
Coordination / communication Asynchrony Interactions with exception handling Real-time issues Memory management / predictability Scheduling and priorities (priority inversion avoidance) Time / periodic activities Java approach Java language specification Real-Time Specification for Java (Real-Time for Java Expert Group) Core Extensions to Java (J-Consortium) Ada 95 approach Core language Systems Programming and Real-Time Annexes POSIX approach Pthreads (1003.1c) Real-time extensions (1003.1b) For each issue, we present / compare the languages’ approaches Basically this tutorial is a matrix -- along one axis are the technical issues, along the other axis are the three languages Choice of approach: 1- Language by language (Ada, Java, Posix) 2- Topic by topic (lifetime, mutual exclusion, ...) Decided on 2- a) More “object oriented” b) Easier to focus on comparison c) Make sure people don’t walk out when I get to the Ada part

Concurrency Granularity / Terminology
“Platform” Hardware + OS + language-specific run-time library “Process” Unit of concurrent execution on a platform Communicates with other processes on the same platform or on different platforms Communication / scheduling managed by the OS (same platform) or CORBA etc (different platforms) Concurrency on a platform may be true parallelism (multi-processor) or multiplexed (uniprocessor) Per-process resources include stack, memory, environment, file handles, ... Switching/communication between processes is expensive “Thread” (“Task”) Unit of concurrent execution within a process Communicates with other threads of same process Shares per-process resources with other threads in the same process Per-thread resources include PC, stack Concurrency may be true parallelism or multiplexed Communication / scheduling managed by the OS or by language-specific run-time library Switching / communication between threads is cheap Our focus: threads in a uniprocessor environment Not formal definitions Platform: what’s needed to run a binary OS may be RTOS, kernel, or nothing Concurrent = potential or actual parallelism Process and program are both loadable executables, they differ in their external communication process can create other processes

Summary of Issues Concurrency Basic model / generality
Lifetime properties Creation, initialization, (self) termination, waiting for others to terminate Mutual exclusion Mechanism for locking a shared resource, including control over blocking/awakening a task that needs the resource in a particular state Coordination (synchronization) / communication Asynchrony Event / interrupt handling Asynchronous Transfer of Control Suspension / resumption / termination (of / by others) Interactions with exception handling Libraries and thread safety Real-Time Predictability (time, space) Scheduling policies / priority Range of priority values Avoidance of “priority inversion” Clock and time-related issues and services Range/granularity, periodicity, timeout Libraries and real-time programming predictability - time (WCET); space (no leaks, no fragmentation)

Overview of Java Concurrency Support (1)
Java Preliminaries Smalltalk-based, dynamic, safety-sensitive OO language with built-in support for concurrency, exception handling Dynamic data model Aggregate data (arrays, class objects) on heap Only primitive data and references on stack Garbage Collection required Two competing proposals for real-time extensions Sun-sponsored Real-Time for Java Expert Group J-Consortium Basic concurrency model Unit of concurrency is the thread A thread is an instance of the class java.lang.Thread or one of its subclasses run() method = algorithm performed by each instance of the class Programmer either extends Thread, or implements the Runnable interface Override/implement run() All threads are dynamically allocated If implementing Runnable, construct a Thread object passing a Runnable as parameter

Example of simple thread Lifetime properties Constructing a thread creates the resources that the thread needs (stack, etc.) “Activation” is explicit, by invoking start() Started thread runs “concurrently” with parent Thread terminates when its run method returns Parent does not need to wait for children to terminate Restrictions on “up-level references” from inner classes prevent dangling references to parent stack data public class Writer extends Thread{ final int count; public Writer(int count){this.count=count;} public void run(){ for (int i=1; i<=count; i++){ System.out.println("Hello " + i); } } public static void main( String[] args ) throws InterruptedException{ Writer w = new Writer(60); w.start(); // New thread of control invokes w.run() w.join(); // Wait for w to terminate } }

Mutual exclusion Shared data (volatile fields) synchronized blocks/methods Thread coordination/communication Pass data to new thread via constructor Pulsed event - wait() / notify() Broadcast event - wait() / notifyAll() join() suspends caller until the target thread completes Asynchrony interrupt() sets a bit that can be polled Asynchronous termination stop() is deprecated destroy() is discouraged suspend() / resume() have been deprecated RTJEG, J-C proposals include event / interrupt handling, ATC, asynchronous termination Interaction with exception handling No asynchronous exceptions in “baseline Java” Async exceptions for ATC in RTJEG, J-C Various thread-related exceptions Thread propagating an unhandled exception Terminates, but first calls uncaughtException Other functionality Thread group, dæmon threads, thread local data run can propagate checked exception via newInstance, other intraspection stuff

Overview of Ada Concurrency Support (1)
Ada 95 preliminaries Pascal-based ISO Standard reliable OO language with built-in support for packages (modules), concurrency, exception handling, generic templates, ... Traditional data model (“static” storage, stack(s), heap) Aggregate data (arrays, records) go on the stack unless dynamically allocated Implementation not required to supply Garbage Collection “Specialized Needs Annexes” support systems programming, real-time, several other domains Basic concurrency model Unit of concurrency (thread) is the task Task specification = interface to other tasks Often simply just the task name Task body = implementation (algorithm) Comprises declarations, statements Task type serves as a template for task objects performing the same algorithm Tasks and task types are declarations and may appear in “global” packages or local scopes Tasks follow normal block structure rules Each task has own stack Task body may refer (with care :-) to data in outer scopes, may declare inner tasks Task objects may be declared or dynamically allocated

Example of declared task object Lifetime properties Declared task starts (is activated) implicitly at the begin of parent unit Allocated task starts at the point of allocation Task statements execute “concurrently” with statements of parent Task completes when it reaches its end “Master” is suspended when it reaches its end, until each of its dependent tasks terminates Prevents dangling references to local data No explicit mechanism (such as Java’s join()) to wait for another task to terminate with Ada.Text_IO; procedure Example1 is Count : Integer := 60; task Writer; -- Specification task body Writer is -- Body begin for I in 1..Count loop Ada.Text_IO.Put_Line( "Hello" & Integer'Image(I)); delay 1.0; -- Suspend for at least 1.0 second end loop; end Writer; begin -- Writer activated null; -- Main procedure suspended until Writer terminates end Example1;

Example of task type / dynamic allocation Mutual exclusion Shared data, pragma Volatile / Atomic Protected objects / types Data + “protected” operations that are executed with mutual exclusion “Passive” task that sequentializes access to a data structure via explicit communication (rendezvous) Explicit mutex-like mechanism (definable as protected object/type) that is locked and unlocked with Ada.Text_IO; procedure Example2 is task type Writer(Count : Natural); -- Specification type Writer_Ref is access Writer; Ref : Writer_Ref; task body Writer is -- Body begin for I in 1..Count loop Ada.Text_IO.Put_Line( "Hello" & I'Img); delay 1.0; -- Suspend for at least 1.0 second end loop; end Writer; begin Ref := new Writer(60); -- activates new Writer task object -- Main procedure suspended until Writer object terminates end Example2;

Coordination / communication Pass data to task via discriminant or rendezvous Suspension_Object Binary semaphore with 1-element “queue” Rendezvous Explicit inter-task communication Implicit wait for dependent tasks Asynchrony Event handling via dedicated task, interrupt handler Asynch interactions subject to “abort deferral” abort statement Asynchronous transfer of control via timeout or rendezvous request Hold / Continue procedures (suspend / resume) Interaction with exception handling No asynchronous exceptions Tasking_Error raised at language-defined points Task propagating an (unhandled) exception terminates silently Other functionality Per-task attributes Restrictions for high-integrity / efficiency-sensitive applications Ravenscar Profile

Overview of POSIX Concurrency Support (1)
Basic concurrency model A thread is identified by an instance of (opaque) type pthread_t Threads may be allocated dynamically or declared locally (on the stack) or statically Program creates / starts a thread by calling pthread_create, passing the addresses of the thread id, an “attributes” structure, the function that the thread will be executing, and the function’s arguments Thread function takes and returns void* Return value passed to “join”ing thread Example Notation: POSIX call in upper-case is a macro whose expansion includes querying the error return code #include <pthread.h> #include <stdio.h> void *tfunc(void *arg){ // thread function int count = *( (int*)arg ); int j; for (j=1; j <= count; j++){ printf("Hello %d\n", j); } return NULL; int main(int argc, char *argv[]){ // main thread pthread_t pthread; int pthread_arg = 60; PTHREAD_CREATE( &pthread, NULL, tfunc, (void*)&pthread_arg); PTHREAD_JOIN( pthread, NULL );

Lifetime properties Thread starts executing its thread function as result of pthread_create, concurrent with creator Termination A thread terminates via a return statement or by invoking pthread_exit Both deliver a result to a “join”ing thread, and both invoke cleanup handlers A terminated thread may continue to hold system resources until it is recycled Detachment and recycling A thread is detachable if It has been the target of a pthread_join or a pthread_detach (either before or after it has terminated), or it was created with its detachstate attribute set A terminated detachable thread is recycled, releasing all system resources not released at termination No hierarchical relationship among threads Created thread has a pointer into its creator’s memory  danger of dangling reference Main thread is special in that when it returns it terminates the process, killing all other threads To avoid this mass transitive threadicide, main thread can pthread_exit rather than return

Mutual exclusion Mutexes (pthread_mutex_t type) with lock / unlock functions Coordination / communication Condition variables (pthread_cond_t type) with pulsed and broadcast events Semaphores Data passed to thread function at pthread_create, result delivered to “joining” thread at return or pthread_exit Asynchrony Thread cancellation with control over immediacy and ability to do cleanup Interaction with exception handling Complicated relationship with signals Consistent error-return conventions The result of each pthread function is an int error code (0  normal) If the function needs to return a result, it does so in an address (“&”) parameter No use of errno in Pthreads functions Per-thread errno used when a thread calls a function that sets errno Other Thread-specific data area “pthread once” functions

Comparison: Basic Model / Lifetime
Points of difference Nature of unit of concurrency: class, task, function Implicit (Ada, POSIX) versus explicit (Java) activation How parameters are passed / how result communicated Methodology / reliability Ada and Java provide type checking, prevent dangling references Flexibility / generality All three provide roughly the same expressive power POSIX allows a new thread to be given its parameters explicitly on thread creation POSIX allows a thread to return a value to a “join”ing thread Ada lacks an explicit mechanism for one task to wait for another task to terminate In particular, waiting for an allocated task to terminate Efficiency Ada requires run-time support to manage task dependence hierarchy

Mutual Exclusion in Ada via Shared Data
Example: One task repeatedly updates an integer value Another task repeatedly displays it Advantage Efficiency Need pragma Atomic to ensure that Integer reads/writes are atomic Optimizer does not cache Global Drawbacks Methodologically challenged Does not scale up (e.g. aggregate data, more than one updating task) with Ada.Text_IO; procedure Example3 is Global : Integer := 0; pragma Atomic( Global ); task Updater; task Reporter; task body Updater is begin loop Global := Global+1; delay 1.0; -- 1 second end loop; end Updater; task body Reporter is Ada.Text_IO.Put_Line( Global'Img ); delay 2.0; -- 2 seconds end Reporter; null; end Example3; Note: the assignment statement is not atomic Methodology- - no encapsulation - communication via shared data e.g. if two variables, it doesn't generalize nor if more than 1 updating task

Mutual Exclusion in Java via Shared Data
Java version of previous example Comments Same advantages and disadvantages as Ada version Need volatile to prevent hostile optimizations public class Example4{ static volatile int global = 0; public static void main(String[] args){ Updater u = new Updater(); Reporter r = new Reporter(); u.start(); r.start(); } } class Updater extends Thread{ public void run(){ while(true){ Example4.global++; ... sleep( 1000 ); ... // try block omitted } } } class Reporter extends Thread{ public void run(){ while(true){ System.out.println(Example4.global); } sleep( 2000 ); ... // try block omitted } } } POSIX - likewise use volatile

Mutual Exclusion in Ada via Protected Object
with Ada.Integer_Text_IO; procedure Example5 is type Position is record X, Y : Integer := 0; end record; protected Global is procedure Update; function Value return Position; private Data : Position; end Global; protected body Global is procedure Update is begin Data.X := Data.X+1; Data.Y := Data.Y+1; end Update; function Value return Position is return Data; end Value; task Updater; task Reporter; task body Updater is loop Global.Update; delay 1.0; -- 1 second end loop; end Updater; task body Reporter is P : Position; P := Global.Value; Ada.Integer_Text_IO.Put (P.X); Ada.Integer_Text_IO.Put (P.Y); delay 2.0; -- 2 seconds end Reporter; null; end Example5; Interface Implementation Executed with mutual exclusion

Basic Properties of Ada Protected Objects
A protected object is a data object that is shared across multiple tasks but with mutually exclusive access via a (conceptual) “lock” The rules support “CREW” access (Concurrent Read, Exclusive Write) Form of a protected object declaration Encapsulation is enforced Client code can only access the protected components through protected operations Protected operations illustrated in Example5 Procedure may “read” or “write” the components Function may “read” the components, not “write” them The protected body provides the implementation of the protected operations Comments on Example5 Use of protected object ensures that only one of the two tasks at a time can be executing a protected operation Scales up if we add more accessing tasks Allows concurrent execution of reporter tasks Data may only be in the private part protected Object_Name is { protected_operation_specification ; } [ private { protected_component_declaration } ] end Object_Name;

Mutual Exclusion in Java via Synchronized Blocks
global u r pu pr x y Position Updater Reporter class Position{ int x=0, y=0; } public class Example6{ public static void main(String[] args){ Position global = new Position(); Updater u = new Updater( global ); Reporter r = new Reporter( global ); u.start(); r.start(); } } class Updater extends Thread{ private final Position pu; Updater( Position p ){ pu=p; } public void run(){ while(true){ synchronized(pu){ pu.x++; pu.y++; } sleep( 1000 ); } } } class Reporter extends Thread{ private final Position pr; Reporter( Position p ){ pr=p; } public void run(){ while(true){ synchronized(pr){ System.out.println(pr.x); System.out.println(pr.y); } sleep( 2000 ); } } }

Semantics of Synchronized Blocks
Each object has a lock Suppose thread t executes synchronized(p){...} In order to enter the {...} block, t must acquire the lock associated with the object referenced by p If the object is currently unlocked, t acquires the lock and sets the lock count to 1, and then proceeds to execute the block If t currently holds the lock on the object, t increments its lock count for the object by 1, and proceeds to execute the block If another thread holds the lock on the object, t is “stalled” Leaving a synchronized block (either normally or “abruptly”) t decrements its lock count on the object by 1 If the lock count is still positive, t proceeds in its execution If the lock count is zero, the threads “locked out” of the object become eligible to run, and t stays eligible to run But this is not an official scheduling point If each thread brackets its accesses inside a synchronized block on the object, mutually exclusive accesses to the object are ensured No need to specify volatile

Mutual Exclusion in Java via Synchronized Methods
class Position{ private int x=0, y=0; public synchronized void incr(){ x += 1; y += 1; } public synchronized int[] value(){ return new int[2]{x, y} } } global u r pu pr x y Position Updater Reporter public class Example7{ public static void main(String[] args){ Position global = new Position(); Updater u = new Updater( global ); Reporter r = new Reporter( global ); u.start(); r.start(); } } class Updater extends Thread{ private final Position pu; Updater( Position p ){ pu=p; } public void run(){ while(true){ pu.incr(); sleep( 1000 ); } } } class Reporter extends Thread{ private final Position pr; Reporter( Position p ){ pr=p; } public void run(){ while(true){ int[] arr = pr.value(); System.out.println(arr[0]); System.out.println(arr[1]); sleep( 2000 ); } } }

Comments on Synchronized Blocks / Methods
Effect of synchronized instance method is as though body of method was in a synchronized(this) block Generally better to use synchronized methods versus synchronized blocks Centralizes mutual exclusion logic For efficiency, have a non-synchronized method with synchronized(this) sections of code Synchronized accesses to static fields A synchronized block may synchronize on a class object The “class literal” Foo.class returns a reference to the class object for class Foo Typical style in a constructor that needs to access static fields A static method may be declared as synchronized Constructors are not specified as synchronized Only one thread can be operating on a given object through a constructor Invoking obj.wait() releases lock on obj All other blocking methods (join(), sleep(), blocking I/O) do not release the lock class MyClass{ private static int count=0; MyClass(){ synchronized(MyClass.class){ count++; } } }

Mutual Exclusion in POSIX via Mutex
A mutex is an instance of type pthread_mutex_t Initialization determines whether a pthread can successfully lock a mutex it has already locked PTHREAD_MUTEX_INITIALIZER (“fast mutex”) Attempt to relock will fail PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP (“recursive mutex”) Attempt to relock will succeed Operations on a mutex pthread_mutex_lock(&mutex) Blocks caller if mutex locked Deadlock condition indicated via error code pthread_mutex_trylock(&mutex) Does not block caller pthread_mutex_unlock(&mutex) Release waiting pthread pthread_mutex_destroy(&mutex) Release mutex resources Can reuse mutex if reinitialize how know when safe to destroy a mutex

Monitors In most cases where mutual exclusion is required there is also a synchronization* constraint A task performing an operation on the object needs to wait until the object is in a state for which the operation makes sense Example: bounded buffer with Put and Get Consumer calling Get must block if buffer is empty Producer calling Put must block if buffer is full The monitor is a classical concurrency mechanism that captures mutual exclusion + state synchronization Encapsulation State data is hidden, only accessible through operations exported from the monitor Implementation must guarantee that at most one task is executing an operation on the monitor Synchronization is via condition variables local to the monitor Monitor operations invoke wait/signal on the condition variables A task calling wait is unconditionally blocked (in a queue associated with that condition variable), releasing the monitor A task calling signal awakens one task waiting for that variable and otherwise has no effect Proposed/researched by Dijkstra, Brinch-Hansen, Hoare in late 1960s and early 1970s Mutual exclusion is the opposite of synchronization: need to prevent two things from happening at the same time * “Synchronization” in the correct (versus Java) sense

Monitor Example: Bounded Buffer
monitor Buffer {Pascal-like syntax} export Put, Get, Size; const Max_Size = 10; var Data : array[1..Max_Size] of Whatever; Next_In, Next_Out : 1..Max_Size; Count : 0..Max_Size; NonEmpty, NonFull : condition; procedure Put(Item : Whatever); begin if Count=Max_Size then Wait( NonFull ); Data[Next_In] := Item; Next_In := Next_In mod Max_Size + 1; Count := Count + 1; Signal( NonEmpty ); end {Put}; procedure Get(Item : var Whatever); if Count=0 then Wait( NonEmpty ); Item := Data[Next_Out]; Next_Out := Next_Out mod Max_Size + 1; Count := Count - 1; Signal( NonFull ); end {Get}; function Size : Integer; Size := Count; end {Size}; Count := 0; Next_In := 1; Next_Out := 1; end {Buffer}; Next_Out Next_In Max_Size 1 Data Count: 4 Snapshot of data structures after inserting 5 elements and removing 1

Monitor Critique Semantic issues
If several tasks waiting for a condition variable, which one is unblocked by a signal? Longest-waiting, highest priority, unspecified, ... Which task (signaler or unblocked waiter) holds the monitor after a signal Signaler? Unblocked waiter? Then when does signaler regain the monitor Require signal either to implicitly return or to be the last statement? Depending on semantics, may need while vs if in the code that checks the wait condition Advantages Encapsulation Efficient implementation Avoids some race conditions Disadvantages Sacrifices potential concurrency Operations that don’t affect the monitor’s state (e.g. Size) still require mutual exclusion Condition variables are low-level / error-prone Programmer must ensure that monitor is in a consistent state when wait/signal are called Nesting monitor calls can deadlock, even without using condition variables Race condition avoided: condition vbl local to monitor thus can only be waited/signaled by a task already holding the monitor. Otherwise a signal could awaken a late coming waiter

Monitors and Java Every object is a monitor in some sense
Each object obj has a mutual exclusion lock, and certain code is executed under control of that lock Blocks that are synchronized on obj Instance methods on obj’s class that are declared as synchronized Static synchronized methods for obj if obj is a class But encapsulation depends on programmer style Non-synchronized methods, and accesses to non-private data from client code, are not subject to mutual exclusion No special facility for condition variables Any object (generally the one being accessed by synchronized code) can be used as a condition variable via wait() / notify() But that means that there is only one condition directly associated with the object To invoke wait() or notify() on an object, the calling thread needs to hold the lock on the object Otherwise throws a run-time exception The notifying thread does not release the lock Waiting threads thus generally need to do their wait in a while statement versus a simple if No guarantee which waiting thread is awakened by a notify

Bounded Buffer in Java Notes
Essential for each wait() condition to be in a while loop and not simply an if statement Important to signal via notifyAll() versus simply notify() A producer and a consumer thread may be in the object’s wait set at the same time! public class BoundedBuffer{ public static final int maxSize=10; private final Object[] data = new Object[maxSize]; private int nextIn=0, nextOut=0; private volatile int count=0; public synchronized void put(Object item) throws InterruptedException{ while (count == max) { this.wait(); } data[nextIn] = item; nextIn = (nextIn + 1) % max; count++; this.notifyAll(); } public synchronized Object get() while (count == 0) { this.wait(); } Object result = data[nextOut]; data[nextOut] = null; nextOut = (nextOut + 1) % max; count--; return result; public int size(){ // not synchronized return count; count is volatile since otherwise size might be inlined and count stored in a cache for the calling thread

Monitors and Ada Protected Objects
Encapsulation enforced in both Data components are inaccessible to clients Mutual exclusion enforced in both All accesses are via protected operations, which are executed with mutual exclusion (“CREW”) Condition variables A protected entry is a protected operation guarded by a boolean condition (“barrier”) which, if false, blocks the calling task Barrier condition can safely reference the components of the protected object and also the “Count attribute” E'Count = number of tasks queued on entry E Value does not change while a protected operation is in progress (avoids race condition) Barrier expressions are Ada analog of condition variables, but higher level (wait and signal implicit) Caller waits if the barrier is False (and releases the lock on the object) Barrier conditions for non-empty queues are evaluated at the end of protected procedures and protected entries If any are True, queuing policy establishes which task is made ready Protected operations (unlike monitor operations) are non-blocking Allows efficient implementation of “lock”

Bounded Buffer in Ada Evaluate barriers package Bounded_Buffer_Pkg is
Max_Length : constant := 10; type W_Array is array(1 .. Max_Length) of Whatever; protected Bounded_Buffer is entry Put( Item : in Whatever ); entry Get( Item : out Whatever ); function Size; private Next_In, Next_Out : Positive := 1; Count : Natural := 0; Data : W_Array; end Bounded_Buffer; end Bounded_Buffer_Pkg; package body Bounded_Buffer_Pkg is protected body Bounded_Buffer is entry Put( Item : in Whatever ) when Count < Max_Length is begin Data(Next_In) := Item; Next_In := Next_In mod Max_Length + 1; Count := Count+1; end Put; entry Get( Item : out Whatever ) when Count > 0 is Item := Data(Next_Out); Next_Out := Next_Out mod Max_Length + 1; Count := Count-1; end Get; function Size is return Count; end Size; end Bounded_Buffer; end Bounded_Buffer_Pkg; Evaluate barriers

Monitors and POSIX: Mutex + Condition Variables
POSIX supplies type pthread_cond_t for condition variables Always used in conjunction with a mutex Avoids race conditions such as a thread calling wait and missing a signal that is issued before the thread is enqueued May be used to simulate a monitor, or simply as an inter-thread coordination mechanism Initialized via PTHREAD_COND_INITIALIZER or via pthread_cond_init function Operations Waiting operations pthread_cond_wait( &cond_vbl, &mutex ) pthread_cond_timedwait(&cond_vbl, &mutex, &timeout) Signaling operations pthread_cond_signal( &cond_vbl ) Pulsed event No guarantee which waiter is awakened pthread_cond_broadcast (&cond_vbl ) Broadcast event All waiters awakened Initialization pthread_cond_init( &cond_val ) Resource release pthread_cond_destroy( &cond_vbl )

Bounded Buffer in POSIX (*)
#include <pthread.h> #define MAX_LENGTH 10 #define WHATEVER float typedef struct{ pthread_mutex_t mutex; pthread_cond_t non_full; pthread_cond_t non_empty; int next_in, next_out, count; WHATEVER data[MAX_LENGTH]; } bounded_buffer_t; void put( WHATEVER item, bounded_buffer_t *b ){ PTHREAD_MUTEX_LOCK(&(b->mutex)); while (b->count == MAX_LENGTH){ PTHREAD_COND_WAIT(&(b->non_full), &(b->mutex)); } ... /* Put data in buffer, update count and next_in */ PTHREAD_COND_SIGNAL(&(b->non_empty)); PTHREAD_MUTEX_UNLOCK(&(b->mutex)); void get( WHATEVER *item, bounded_buffer_t *b ){ while (b->count == 0){ PTHREAD_COND_WAIT(&(b->non_empty), &(b->mutex)); } /* Get data from buffer, update count and next_out */ PTHREAD_COND_SIGNAL(&(b->non_full)); int size( bounded_buffer_t *b ){ int n; n = b->count; return n; } /* Initializer function also required poor man’s generic (*) Based on example in Burns & Wellings, Real-Time Systems and Programming Languages, pp

Comparison of Mutual Exclusion Approaches
Points of difference Expression of mutual exclusion in program Explicit code markers in POSIX (lock/unlock mutex) Either explicit code marker (synchronized block) or encapsulated (synchronized method) in Java Encapsulated (protected object) in Ada No explicit condition variables in Java (or Ada) Blocking prohibited in protected operations (Ada) Locks are implicitly recursive in Java and Ada, programmer decides whether “fast” or recursive in POSIX Methodology / reliability All provide necessary mutual exclusion Ada entry barrier is higher level than condition variable Absence of condition variable from Java can lead to clumsy or obscure style Main reliability issue is interaction between mutual exclusion and asynchrony, described below Flexibility / generality Ada: protected operations need to be non-blocking Efficiency Ada provides potential for concurrent reads Ada does not require queue management, but barrier (re)evaluation entails overhead

Coordination / Communication Mechanisms
Pulsed Event Waiter blocks unconditionally Signaler awakens exactly one waiter (if one or more), otherwise event is discarded Broadcast Event Signaler awakens all waiters (if one or more), otherwise event is discarded Persistent Event (Binary Semaphore) Signaler allows one and only one task to proceed past a wait Some task that already has, or the next task that subsequently will, call wait Counting semaphore A generalization of binary semaphore, where the number of occurrences of signal are remembered Simple 2-task synchronization Persistent event with a one-element queue Direct inter-task synchronous communication Rendezvous, where the task that initiates the communication waits until its partner is ready

Pulsed Event Java Any object can serve as a pulsed event via wait() / notify() Calls on these methods must be in code synchronized on the object wait() releases the lock, notify() doesn’t wait() can throw InterruptedException An overloaded version of wait() can time out, but no direct way to know whether the return was normal or via timeout Ada Protected object / type can model a pulsed event Can time out on any entry via select statement Can’t awaken a blocked task other than via abort POSIX Condition variable can serve as pulsed event Arnie movie- I'd like to awaken you, but then I'd have to kill you protected type Pulsed_Event is entry Wait; procedure Signal; private Signaled : Boolean := False; end Pulsed_Event; protected body Pulsed_Event is entry Wait when Signaled is begin Signaled := False; end Wait; procedure Signal is Signaled := Wait'Count>0; end Signal; end Pulsed_Event;

Broadcast Event Java Any object can serve as a broadcast event via wait() / notifyAll() Calls on these methods must be in code synchronized on the object Ada Protected object / type can model a broadcast event Protected object can model more general forms, such as sending data with the signal, to be retrieved by each awakened task Locking protocol / barrier evaluation rules prevent race conditions POSIX Condition variable can serve as broadcast event protected type Broadcast_Event is entry Wait; procedure Signal; private Signaled : Boolean := False; end Broadcast_Event; protected body Broadcast_Event is entry Wait when Signaled is begin Signaled := Wait'Count>0; end Wait; procedure Signal is end Signal; end Broadcast_Event;

Semaphores (Persistent Event)
Binary semaphore expressible in Java J-Consortium spec includes binary and counting semaphores Binary semaphore expressible in Ada POSIX Includes (counting) semaphores, but intended for inter-process rather than inter-thread coordination public class BinarySemaphore { private boolean signaled = false; public synchronized void await() throws InterruptedException{ while (!signaled) { this.wait(); } signaled=false; } public synchronized void signal(){ signaled=true; this.notify(); } } protected type Binary_Semaphore is entry Wait; procedure Signal; private Signaled : Boolean := False; end Binary_Semaphore; protected body Binary_Semaphore is entry Wait when Signaled is begin Signaled := False; end Wait; procedure Signal is Signaled := True; end Signal; end Binary_Semaphore;

Simple Two-Task Synchronization
Java, POSIX No built-in support Ada Type Suspension_Object in package Ada.Synchronous_Task_Control Procedure Suspend_Until_True(SO) blocks caller until SO becomes true, and then atomically resets SO to false Procedure Set_True(SO) sets SO’s state to true “Bounded error” if a task calls Suspend_Until_True(SO) while another task is waiting for SO procedure Proc is task Setter; task Retriever; SO : Suspension_Object; Data : array ( ) of Float; task body Setter is begin Initialize Data Set_True(SO); ... end Setter; task body Retriever is Suspend_Until_True(SO); Use data null; end Proc;

Direct Synchronous Inter-Task Communication (1)
Calling task (caller) Requests action from another task (the callee), and blocks until callee is ready to perform the action Called task (callee) Indicates readiness to accept a request from a caller, and blocks until a request arrives Rendezvous Performance of the requested action by callee, on behalf of a caller Parameters may be passed in either or both directions Both caller and callee are unblocked after rendezvous completes Java No direct support Can model via wait / notify, but complicated POSIX Same comments as for Java T1 T2 “T2, do action A”  Wait for T2 to start action A (T2 does action A) Wait for T2 to complete action A “Accept request for action A [from T1]”  Wait for request for action A [from T1] Do action A Awaken caller Rendezvous

Ada “Action” is referred to as a task’s entry Declared in the task’s specification Caller makes entry call, similar syntactically to a procedure call Callee accepts entry call via an accept statement Caller identifies callee but not vice versa Many callers may call the same entry, requiring a queue Often callee is a “server” that sequentializes access to a shared resource Sometimes protected object is not sufficient, e.g. if action may block In most cases the server can perform any of several actions, and the syntax needs to reflect this flexibility Also in most cases the server is written as an infinite loop (not known in advance how many requests will be made) so termination is an issue Ada provides special syntax for a server to automatically terminate when no further communication with it is possible Caller and/or callee may time out Timeout canceled at start of rendezvous

Ada example task Sequentialized_Output is entry Put_Line( Item : String ); entry Put( Item : String ); end Sequentialized_Output; task body Sequentialized_Output is begin loop select accept Put_Line( Item : String ) do Ada.Text_IO.Put_Line( Item ); end Put_Line; or accept Put( Item : String ) do Ada.Text_IO.Put( Item ); end Put; terminate; end select; end loop; end Sequentialized_Output; task Outputter1; task body Outputter1 is begin; ... Sequentialized_OutPut. Put("Hello"); end Outputter1; task Outputter2; task body Outputter2 is begin; ... Sequentialized_Output. Put("Bonjour"); end Outputter2;

Comparison of Coordination/Communication Mechanisms
Points of difference Different choice of “building blocks” Ada: Suspension_Object, protected object, rendezvous Java, POSIX: pulsed/broadcast events Java allows “interruption” of blocked thread by throwing an exception, Ada and POSIX allow only cancellation Methodology / reliability Ada’s high-level feature (rendezvous) supports good practice Potential for undetected bug in Ada if a task calls Suspend_Until_True on a Suspension_Object that already has a waiting task Flexibility / generality Major difference among the languages is that Ada is the only one to provide rendezvous as built-in communication mechanism Efficiency No major differences in implementation efficiency for mechanisms common to the three approaches Ada’s Suspension_Object has potential for greater efficiency than semaphores

Asynchrony Mechanisms
Setting/Polling Setting a datum in a task/thread that is polled by the affected task/thread Asynchronous Event Handling Responding to asynchronous events generated internally (by application threads) or externally (by interrupts) Resumptive: “interrupted” thread continues at the point of interruption, after the handler completes Combine with polling or ATC to affect the interrupted thread Asynchronous Termination Aborting a task/thread Immediacy: are there regions in which a task / thread defers requests for it to be aborted? ATC Causing a task to branch based on an asynchronous occurrence Immediacy: are there regions in which a task / thread defers requests for it to have an ATC? Suspend/resume Causing a thread to suspend its execution, and later causing the thread to be resumed Immediacy: are there regions in which a task / thread defers requests for it to be suspended?

Setting / Polling Not exactly asynchronous (since the affected task/thread checks synchronously) But often useful and arguably better than asynchronous techniques Ada No built-in mechanism, but can simulate via protected object or pragma Atomic variable global to setter and poller Java t.interrupt() sets interruption status flag in the target thread t Static Thread method boolean interrupted() returns current thread’s interruption status flag and resets it Boolean method t.isInterrupted() returns target thread’s interruption status flag If t.interrupt() is invoked on a blocked thread t, t is awakened and an InterruptedException (a checked exception) is thrown Each of the methods thr.join(), Thread.sleep(), and obj.wait() has a “throws InterruptedException” clause POSIX No built-in mechanism

Asynchronous Event Handling
Ada No specific mechanism for asynch event handling Interrupt handlers can be modeled by specially identified protected procedures, executed (at least conceptually) by the hardware Other asynch event handlers modeled by tasks Java (RTSJ) Classes AsyncEvent (“AE”), AsyncEventHandler (“AEH”) model asynchronous events, and handlers for such events, respectively Programmer overrides one of the AEH methods to define the handler’s action Program can register one or more AEHs with any AE (listener model) An AEH is a schedulable entity, like a thread (but not necessarily a dedicated thread) When an AE is fired, all registered handlers are scheduled based on their scheduling parameters Program needs to manage any data queuing Methods allow dealing with event bursts Scales up to large number of events, handlers J-Consortium proposal has analogous mechanism POSIX Messy interaction between signals (originally a process-based mechanism) and threads

Asynchronous Termination (1)
Ada Abort statement sets the aborted task’s state to abnormal, but this does not necessarily terminate the aborted task immediately For safety, certain contexts are abort-deferred; e.g. Accept statements Protected operations Real-Time Annex requires implementation to terminate an abnormal task as soon as it is outside an abort-deferred region Java Language Spec No notion of abort-deferred region Invoke t.stop(Throwable exc) or t.stop() Halt t asynchronously, and throw exc or ThreadDeath object in t Then effect is as though propagating an unchecked exception Deprecated (data may be left in an inconsistent state if t stopped while in synchronized code) Invoke t.destroy() Halt t, with no cleanup and no release of locks Not (yet :-) deprecated but can lead to deadlock Invoke System.exit(int status) Terminates the JVM By convention, nonzero status  abnormal termination By declaring a controlled object and appropriately defining Finalize, can have an aborted task perform its last wishes

Java Language Spec (cont’d.) Recommended style is to use interrupt() Main issue is latency RTSJ Synchronized code, and methods that do not explicitly have a throws clause for AIE, are abort deferred To abort a thread, invoke t.interrupt() and have t do its processing in an asynchronously interruptible method class Boss extends Thread{ Thread slave; Boss(Thread slave){ this.slave=slave; } public void run(){ ... if (...){ slave.interrupt(); // abort slave } } class PollingSlave extends Thread{ while (!Thread.interrupted()){ // main processing } ... // pre-shutdown actions

J-Consortium abort() method aborts a thread Synchronized code is not necessarily abort-deferred May need to terminate a deadlocked thread that is in synchronized code Synchronized code in objects that implement the Atomic interface is abort deferred POSIX A pthread can set its cancellation state (enabled or disabled) and, if enabled, its cancellation type (asynchronous or deferred) pthread_set_cancelstate(newstate, &oldstate) PTHREAD_CANCEL_DISABLE PTHREAD_CANCEL_ENABLE pthread_set_canceltype(newtype, &oldtype) PTHREAD_CANCEL_ASYNCHRONOUS PTHREAD_CANCEL_DEFERRED Default setting: enabled, deferred cancellation Deferred  cancel at next cancellation point Minimal set of cancellation points defined by standard, others can be added by implementation pthread_cancel( &pthr ) sends cancellation request Cleanup handlers give the cancelled thread the opportunity to consistentize data, unlock mutexes

Asynchronous Transfer of Control (“ATC”)
What is it A mechanism whereby a triggering thread (possibly an async event handler) can cause a target thread to branch unconditionally, without any explicit action from the target thread Controversial facility Triggering thread does not know what state the target thread is in when the ATC is initiated Target thread must be coded carefully in presence of ATC Implementation cost / complexity Interaction with synchronized code Why provide support? User community requirement Useful for certain idioms Time out of long computation when partial result is acceptable Abort an iteration of a loop Terminate a thread ATC may have shorter latency than polling

Asynchronous Transfer of Control (1)
Ada Allow controlled ATC, where the effect is restricted to an explicit syntactic context Restrict the ATC triggering conditions Time out Acceptance of an entry call Defer effect of ATC until affected task is outside abort-deferred region Java (RTSJ) ATC based on model of asynchronous exceptions, thrown only at threads that have explicitly enabled them ATC deferred in synchronized code and in methods that lack a “throws AIE” clause Timeout is a specific kind of AIE function Eval(Interval : Duration) return Float is X : Float := 0.0; pragma Atomic( X ); begin select delay Interval; return X; then abort while ... loop ... X := ...; ... end loop; end select; end Eval; 1 2 3a 3b

Asynchronous Transfer of Control (2)
abstract class Func{ abstract double f(double x) throws AIE; volatile double current; // assumes atomic } class MyFunc extends Func{ double f(double x) throws AIE { current = ...; while(...){ ... current = ...; } return current; class SuccessiveApproximation{ static boolean finished; static double calc(Func func, double arg, long ms){ double result = 0.0; new Timed( new RelativeTime(ms, 0) ).doInterruptible( new Interruptible(){ public void run(AIE e) throws AIE{ result = func.f(arg); finished = true; public void interruptAction(AIE e){ result = func.current; finished = false; }); return result; public static void main(String[] args){ MyFunc mf = new MyFunc(); double answer = calc(mf, 100.0, 1000); // run mf.f(100.0) for at most 1 second System.out.println(answer); System.out.println("calc completed? " + finished );

Suspend / Resume Ada Real-Time Annex defines a package Ada.Asynchronous_Task_Control with procedures Hold, Continue Hold(T) conceptually sets T’s priority less than that of the idle task Effect deferred during protected operations, rendezvous Continue(T) restores T’s pre-held priority Java t.suspend() suspends t, without releasing locks t.resume() resumes t These methods have been deprecated If a thread t is suspended while holding a lock required by the thread responsible for resuming t, the threads will deadlock Arguably this programming bug should not have caused the methods to be deprecated POSIX Not supported

Comparison of Asynchrony Mechanisms
Points of difference Ada attempts a minimalist approach, whereas the real-time Java specs (and to some extent POSIX) provide more general models Methodology / reliability Asynchronous operations are intrinsically dangerous, the goal is to minimize / localize the code that needs to be sensitive to disruption Regular Java’s interrupt mechanism, though requiring polling, is a reasonable approach Java RTSJ has nice model for asynchronous event handling POSIX cancellation semantics allows thread owning a mutex to cleanly deal with cancellation request Ada ATC constrains the effect of an asynchronous request to a clearly identified syntactic region, and defines orderly cleanup POSIX signal interactions are messy Flexibility / generality Java RTSJ offers a general ATC model based on asynchronous exceptions Efficiency ATC may incur distributed overhead in Java RTSJ (check on method returns)

Scheduling and Priorities: Introduction
Scheduler decides which ready task to run (“dispatching”), which task to unblock when a resource with a queue of waiters is available Variety of dispatching policies, including: Priority-based fixed priority(*), FIFO within priority Run until blocked (non-preemptive) Run until blocked or preempted Run until blocked or preempted or timeslice expires Priority-based non-fixed priority Priority aging Earliest deadline first Variety of queue service policies, such as: FIFO ignoring priorities FIFO within priorities Unspecified Finer levels of detail also arise When thread is preempted, or when its priority is modified, where in its ready queue is it placed? Scheduling policies affect predictability and throughput, goals which are in conflict Real-time programs generally require predictability at expense of throughput run risk of starvation (*) “Fixed priority”  scheduler does not implicitly change a thread’s priority except to avoid priority inversions; program can change a thread’s priority

Priority Inversion What is a “priority inversion”?
A higher-priority thread is blocked / stalled while a lower-priority thread is running It is sometimes necessary When the lower priority thread holds a lock that is needed by the higher priority thread Scheduling policy affects worst case blocking time A high priority thread may be blocked (stalled on a lock) during execution of a lower-priority thread not holding the lock - “unbounded priority inversion” Mars lander mission in 1999 Priority Inheritance and Highest Lockers (Priority Ceiling) considerably reduce worst-case blocking time, at expense of throughput Priority inheritance When a thread H attempts to acquire a lock that is held by a lower-priority thread L, L inherits H’s priority as long as it is holding the lock Applied transitively if L is waiting for a lock held by a yet-lower-priority thread Highest lockers (Priority ceiling) While holding a lock, a thread executes at a priority higher than or equal to that of any thread that needs the lock How on Earth does a prio inv happen? It might not, but it does in space higher than or equal affects time slicing and queue management in queueless case

Priority Inversion Example
H is a high-priority thread, M a medium priority thread, and L a low-priority thread  L awakens and starts to run (the other two threads are blocked, waiting for the expiration of delays)  L starts to use a mutually-exclusive resource Enters a monitor, locks a mutex  H awakens and preempts L  H tries to use the resource held by L and is blocked, thus allowing L to resume This priority inversion is necessary  M awakens and preempts L This “unbounded” priority inversion is evil, since M is indirectly preventing H from running  M completes, and L resumes  L releases the mutually exclusive resource and is preempted by H, which can then use the resource  H releases the resource  H completes execution, allowing L to resume  L completes execution H M L          

Priority Inheritance H M
 L awakens and starts to run at priority L  L starts to use a mutually-exclusive resource  H awakens, preempts L and runs at priority H  H tries to use the resource held by L and is blocked, thus allowing L to resume At this point L inherits H’s priority (H)  M awakens but does not preempt L This avoids the unbounded priority inversion  L releases the mutually exclusive resource, reverts to its pre-inheritance priority L, and is preempted by H, which can then use the resource  H releases the resource  H completes execution, allowing M (the higher priority of the two ready threads) to execute  M completes, allowing L to resume  L completes execution Effect of Priority Inheritance A thread holding a lock executes at the maximum priority of all threads currently requiring that lock H M L          L H M 

Priority Ceilings (Highest Lockers)
 L awakens and starts to run at priority L  L starts to use a mutually-exclusive resource with ceiling H' > H, and runs at priority H' This will prevent unbounded priority inversion  H awakens but does not preempt L  M awakens but does not preempt L  L releases the mutually exclusive resource, reverts to its pre-ceiling priority L, and is preempted by H (the higher-priority of the two ready tasks) which then runs at priority H  H starts to use the resource with ceiling H' > H, and runs at priority H'  H releases the resource and reverts to priority H  H completes execution, allowing M (the higher priority of the two ready threads) to execute  M completes, allowing L to resume  L completes execution Effect of Priority Ceiling A thread holding a lock executes at a priority higher than that of any thread that might need the lock H M L     L H´     M H  

Priority Inversion Avoidance Techniques
Priority Inheritance Supported by many RTOSes Only change priority when needed (thus no cost in common case when resource not in use) Thread may be blocked once for each lock that it needs (“chained blocking”) Implementation may be expensive Thread’s priority is being changed as a result of an action external to the task Ceiling Priorities If no thread can block while holding the lock on a given shared object, then a queue is not needed for that object In effect, the processor is the lock Prevents deadlock (on uniprocessor) Ensures that a thread is blocked only once each period, by one lower priority thread holding the lock Fixed ceilings not appropriate for applications where priorities need to change dynamically Requires check and priority change at each call Overhead even if object not locked But this is inconsequential in the queueless case If ceiling high, effect  disabling thread switching Both sacrifice responsiveness for predictability A thread may be prevented from running in order to guarantee that deadlines are met overall Can use queueless impl for wait-notify objects since the lock is released when the calling thread calls wait()

Java for Real-Time Programming: Language Features and Issues
Scheduling/priorities sleep(millis) suspends the calling thread Priority is in range 1..10 Thread can change or interrogate its own or another thread’s priority yield() gives up the processor Thread model Priority range (1..10) too narrow Priority semantics are implementation dependent and fail to prevent unbounded priority inversion Relative sleep() not sufficient for periodicity Memory management Predictable, efficient garbage collection appropriate for real-time applications is not (yet) in the mainstream Java lacks stack-based objects (arrays and class instances) Heap used for exceptions thrown implicitly as an effect of other operations Run-time semantics Dynamic class loading is expensive, and it is not easy to see when it will occur Array initializers  run-time code OOP for real-time programming? Dynamic binding complicates analyzability Garbage Collection defeats predictability

Regular Java Semantics for Scheduling
Section of the Java Language Specification “Every thread has a priority. When there is competition for processing resources, threads with higher priority are generally executed in preference to threads with lower priority. Such preference is not, however, a guarantee that the highest priority thread will always be running, and thread priorities cannot be used to reliably implement mutual exclusion.” Problems for real-time applications This rule makes it impossible to guarantee that deadlines will be met for periodic threads No guarantee that priority is used for selecting a thread to unblock when a lock is released No prevention of priority inversion High priority thread may be blocked for longer than desired when it is waiting to acquire a lock No guarantee that priority is used for selecting which thread is awakened by a notify(), or which thread awakened by notifyAll() is selected to run

Garbage Collection and Real-Time Programming
No Garbage Collection Require that all allocations be performed at system initialization Common in many kinds of real-time applications Difficult in Java since all non-primitive data are dynamically allocated Real-Time Garbage Collector Techniques exist that have predictable / bounded costs Incremental or concurrent, vs. mark-sweep But programmer still needs to ensure that allocation rate does not exceed rate at which GC can reclaim space Also, in the absence of specialized hardware, such techniques tend to introduce high latencies GC needs to run at high priority or with the heap locked, to prevent an application thread from referencing an inconsistent heap Hybrid approach For low latency, allow a thread to preempt GC if the thread never references the heap In absence of optimization, need run-time check on each heap reference Allow a thread to allocate objects in a scope-associated area Area flushed at end of scope/thread

Real-Time Specification for Java - Scheduling and Priority Support (1)
Basics Class RealtimeThread extends java.lang.Thread Flexible scheduling framework + default scheduler + priority inversion avoidance Memory management Garbage-Collected heap Immortal memory Scoped memory Assignment rules prevent dangling references NoheapRealtimeThread can preempt GC Initial default scheduler At least 28 distinct priority values, beyond the 10 for regular Java threads Fixed-priority preemptive, FIFO within priority Implementation defines where in ready queue a preempted thread goes User may replace with a different scheduler General concept of schedulable object Classes RealtimeThread, NoHeapRealtimeThread, AsyncEventHandler Constructors for these classes take different kinds of “parameters” objects SchedulingParameters (priority, importance) ReleaseParameters (cost, deadline, period, ...) MemoryParameters (memory area, ...) Kinds of memory areas The 10 regular priority values have arbitrary mapping to native priorities Scheduler only changes a thread’s priority for priority inversion avoidance User can modify priority

Real-Time Specification for Java - Scheduling and Priority Support (2)
Priority Inversion avoidance Priority inheritance protocol by default for synchronization locks Priority ceiling emulation (with queuing) also available Programmer can set monitor control either locally (per object) or globally Synchronization between no-heap real-time threads and regular Java threads needs some care Use non-blocking queues Support for feasibility analysis Implementation can use data in “parameters” objects to determine if a set of schedulable objects can satisfy some constraint Example: Rate-Monotonic Analysis Methods to add/remove a schedulable object to/from feasibility analysis Implementation not required to support feasibility analysis Flexibility Implementation can install arbitrary scheduling algorithms and feasibility analysis Users can replace these dynamically, can have different schedulers for different schedulable objects Supports earliest deadline first - esmertec

J-Consortium’s Real-Time Core Extensions - Scheduling and Priority Support
Concurrency Class CoreTask (method work())  Thread.run Fixed-priority preemptive scheduler + priority inversion avoidance Memory management GC heap for baseline objects, non-GC “allocation contexts” for Core objects Per-task allocation context, implicitly freed On-the-fly allocation contexts, explicitly freed Stackable objects Base scheduler 128 task priorities, above the 10 from regular Java Fixed-priority, preemptive dispatching Timeslicing allowed within highest priority Priority inversion avoidance Priority Inheritance for regular synchronized code Priority Ceiling (without queues) for synchronization on objects whose classes implement the PCP interface (blocking not allowed) Priority Inheritance for Mutex objects, which can be locked and unlocked around code that needs mutually exclusive access to some resource Queue management A task t goes to head of ready queue for its priority when it is preempted by a higher-priority task, or when it loses inherited priority Do the 128 need to be distinct?

Ada Scheduling / Priority Support (Real-Time Annex)
Priorities Priority range must include at least 30 values, and at least one higher value for interrupt handlers Dynamic_Priorities package Concepts of base versus active priority Subprograms to set / get a task’s base priority Deferral of priority changes in certain contexts Scheduling-related policies - per partition (program) pragma Dispatching_Policy(policy-id) affects selection of which ready task to run FIFO_Within_Priority Run until blocked or preempted Implies Ceiling_Locking locking policy Preempted task, or task which loses inherited priority, or task whose timeslice expires, goes to head of ready queue Default dispatching policy not specified pragma Locking policy(policy-id) for priority inversion avoidance on protected objects Ceiling_Locking Default locking policy implementation defined pragma Queuing_Policy(policy-id) for entry queues FIFO_Queuing (default) Priority_Queuing Implementation may add further policies “delay 0.0;”  yield processor (scheduling point) Impl policies- Priority inheritance locking policy Queue-managed priority ceiling locking policy (e.g. to allow protected operations to block) Timeslicing and/or priority aging dispatching policies

POSIX Scheduling / Priority Support
Real-time scheduling is optional facility Check if _POSIX_THREAD_PRIORITY_SCHEDULING is defined If so, then struct sched_param structure is provided, with at least a sched_priority member Scheduling policies SCHED_FIFO  run until blocked or higher priority thread is ready, FIFO within highest priority SCHED_RR  similar to SCHED_FIFO but with time slice (“round robin” within highest priority) SCHED_OTHER  implementation defined Basic properties Priority range is implementation defined Set a thread’s scheduling policy / priority on creation (via attribute) and/or dynamically When creating a thread, set the inheritsched attribute to control whether scheduling properties are inherited from creator With SCHED_FIFO or SCHED_RR, priority dictates which ready thread runs, including after a mutex is unlocked or a condition variable is signaled or broadcast Other properties pthread_yield voluntarily relinquishes processor Contention scope: system vs process Allocation domain: relevant for multiprocessors functions to query min and max priority

Priority Inversion Avoidance in POSIX
Optionally provided support for priority ceiling and priority inheritance protocols, for mutexes Set protocol in an attribute that is passed to a mutex creation function Priority Ceiling Protocol Available if _POSIX_THREAD_PRIO_PROTECT defined Set priority ceiling in attribute passed to mutex creation function Ceiling should be >= priority of any locker Locker at priority <= ceiling runs at ceiling priority while holding lock Locker at priority > ceiling runs at own priority but may get priority inversion Ceiling can be reset dynamically Priority Inheritance Protocol Available if _POSIX_THREAD_PRIO_INHERIT defined A mutex locker’s priority is boosted dynamically to the priority of a higher priority thread that attempts to lock the mutex, and is reset when the mutex is unlocked Transitive if the lock holder is itself blocked on another mutex These protocols apply only to mutexes and not to condition variables or semaphores No “owner” of a condition variable or semaphore not for semaphores since sema is a signaling mechanism set by one thread and awaited by another Prio inversion in PCP: mutex has ceiling 10 and is locked; caller has prio 15 and is blocked; thread at prio 12 can preempt current locker

Clock- and Time-Related Features (1)
Time and clock (range, granularity) Java JLS System.currentTimeMillis() returns milliseconds (long) since epoch Range is epoch (00:00:00 UTC, 1/1/1970)  263 milliseconds RTSJ HighResolutionTime measured in (long milliseconds, int nanoseconds) and subclasses for AbsoluteTime (relative to epoch), RelativeTime, RationalTime Support for multiple clocks J-Consortium Time represented as long (nanoseconds) relative to most recent system start Ada Ada.Real_Time.Time reflects monotonically non-decreasing time values since implementation-defined origin (“epoch”) Range of time values must be at least from program start to 50 years later Clock tick  1msec, time unit  20 sec POSIX Time value structure: seconds and nanosec Realtime clock requires 20 msec resolution

Delay / sleep Java JLS Relative sleep methods Thread.sleep(), taking a long (millis) or a long (millis) and an int (nanos) RTJEG Overloadings of sleep() taking a HighResolutionTime (which may be absolute) J-Consortium Absolute sleepUntil(Time time) method Ada delay expr;  relative delay, where expr is of type Duration delay until expr;  absolute delay, where expr is of a time type POSIX Relative delay via unsigned int sleep(unsigned int seconds ) which suspends for seconds seconds Returns 0 if suspended for the specified duration, else the time remaining (if awakened by a signal)

Timeout Java Timeouts allowed on wait, join (but not on entering synchronized code) Ada Timeouts (including “conditional” calls that check and continue without blocking) allowed on entry calls, but not for acquiring a lock POSIX Timeouts on wait and join Periodic / sporadic real-time tasks / threads RTJEG Via release parameters for real-time thread constructor, with control over deadline miss / budget overrun J-Consortium Via event handlers Via loop on absolute delay (or rendezvous from dispatching task) Via loop on relative sleep method

Periodic RealtimeThread in Real-Time Specification for Java
class Position{ double x, y; } class Sensor extends RealtimeThread{ final Position ps; Sensor(Position p){ super( new PriorityParameters( PriorityScheduler.instance().getMinPriority() + 15 ), new PeriodicParameters( null, // when to start (null means now) new RelativeTime(100, 0), // 100 ms period new RelativeTime(20, 0), // 20 ms cost new RelativeTime(90, 0), // 90 ms deadline null, // no overrun handler null ) // no miss handler ); ps = p; } public void run(){ while ( true ){ double x = InputPort.read(1); // application class double y = InputPort.read(2); // application class synchronized(ps){ ps.x=x; ps.y=y;} // update position try { this.waitForNextPeriod(); } catch (InterruptedException e) { return; } } } } class Test{ public static void main(String[] args){ Position p = new Position(); Sensor s = new Sensor(p); s.start(); ... s.interrupt(); // terminate s need lighter weight notation for setting a priority

Periodic Task in Ada type Proc_Ref is access procedure;
task type Periodic is entry Init(Prio : System.Priority; Period : Ada.Real_Time.Time_Span; Action : Proc_Ref; Start : Ada.Real_Time.Time); end Periodic; task body Periodic is Prio : System.Priority; Period : Ada.Real_Time.Time_Span; Action : Proc_Ref; Next_Time : Time; begin accept Init(Prio : System.Priority; Start : Ada.Real_Time.Time) do Periodic.Prio := Prio; Periodic.Period := Period; Periodic.Action := Proc_Ref; Next_Time := Start; end Init; Set_Priority(Prio); loop delay until Next_Time; Action.all; Next_Time := Next_Time + Period; end loop;

Other Real-Time Support
Java RTJEG Access to raw memory, physical memory J-Consortium Low-Level I/O Unsigned integer conversions / comparisons Ada Storage management Not an issue as in Java, since GC not required Programmer can arrange reclamation via Unchecked_Deallocation or memory pools Controlled types (user-defined finalization) possible but may compromise predictability Restrictions that facilitate more efficient or high-integrity run-time library POSIX Control over per process or per system thread contention Process-oriented concurrency mechanisms

Comparison of Real-Time Support
Points of difference Real-time scheduling support is optional for a POSIX implementation RTSJ provides an extensible framework J-Consortium spec provides flexible scheduling options Both sets of real-time Java extensions need to cope with storage management, what to do about garbage collection Methodology / reliability All address priority inversion “Absolute” delay in Ada and the two RT Java specs helps meet deadlines Flexibility / generality Ada not as general as POSIX or the real-time Java specs Policies are partition-wide Focus on priority ceiling protocol Efficiency Ada’s queueless protected objects can be implemented efficiently Generality of RTSJ comes at run-time cost

Conclusions - Ada Advantages Software engineering
Portability / standardization Encapsulation Abort-deferred region Flexibility Comprehensive / general set of features Only one of the three languages to include rendezvous Practical concerns Implementations exist Efficiency Disadvantages Ada not as popular as other languages Some run-time error conditions not required to be detected Common idioms should be in standard Conservative mechanisms may be restrictive Per-partition scheduling policies Non-blocking protected operations Ada is like Woody Allen movies - cult following but never a blockbuster (or perhaps Ingmar Bergman) Designed from the start to address concurrency and real-time issues Abort deferred region, ATC allow programmer control over latency of response to asynch events

Conclusions - Java Advantages Language popularity
Applicable to dynamic real-time domains RTJEG Flexible, dynamic scheduling framework Support for periodic activities with overrun / miss detection and handling, async events Control over memory areas J Consortium Good performance Certain constructs require analyzable code Disadvantages Not a standard Real-Time support not yet implemented Performance questions Requires programmer to pay attention to memory allocations ATC is complex Model is not easy to grasp (kernel-like facilities external to Java Virtual Machine) Relationship to the Java language not clear Ada is like Woody Allen movies - cult following but never a blockbuster (or perhaps Ingmar Bergman) Designed from the start to address concurrency and real-time issues Abort deferred region, ATC allow programmer control over latency of response to asynch events

Conclusions - POSIX and Recommendations
Advantages Language independent, in principle Implementations exist Attention to resource cleanup Flexible approach to thread cancellation C-based spec has large potential audience Disadvantages Many opportunities for undetected errors Dangling references Type mismatches (casts to/from void*) Nonportabilities Implementation dependences Optional or incompatibly supported features Clash of process and thread oriented features Bottom line- If you need something that works today: Ada or POSIX If you need something that reduces the likelihood of undetected programmer error: Ada or Java If you need something in wide use: POSIX (and perhaps some day one of the Java RT specs) If you need code portability: Ada or Java If you need something flexible / dynamic: Java (especially the RTSJ)

References (1) General A. Burns and A. Wellings; Real-Time Systems and Programming Languages (3rd ed.); Addison Wesley, 2001; ISBN Comparison Papers B. Brosgol and B. Dobbing; “Real-Time Converg-ence of Ada and Java”; Proc. of SIGAda 2001 Conference, Bloomington, MN; October 2001 B. Brosgol; “A Comparison of the Concurrency and Real-Time Features of Ada and Java”; Proc. of Ada UK Conference, Bristol, UK; October 1998. Ada Ada 95 Reference Manual, International Standard ANSI/ISO/IEC-8652:1995; Jan. 1995 Ada 95 Rationale (The Language, The Standard Libraries); January 1995 J. Barnes; Programming in Ada 95 (2nd ed.); Addison-Wesley, 1998; ISBN Current research reported in proceedings of annual ACM SIGAda and Ada Europe Conferences General Ada Web site: Best overall resource

References (2) Java J. Gosling, B. Joy, G. Steele, G. Bracha; The Java Language Specification (2nd ed.); Addison Wesley, 2000; ISBN S. Oaks and H. Wong; Java Threads (2nd edition); O’Reilly, 1999; ISBN D. Lea; Concurrent Programming in Java (2nd ed.); Addison Wesley; 2000; ISBN G. Bollella, J. Gosling, B. Brosgol, P. Dibble, S. Furr, D. Hardin, M. Turnbull; The Real-Time Specification for Java; Addison Wesley, 2000; ISBN International J Consortium Specification; Real-Time Core Extensions, Draft , September Available at POSIX ISO/IEC : 1996 (ANSI/IEEE Standard , 1996 Edition); POSIX Part 1: System Application Program Interface (API) [C Language] D. Butenhof; Programming with POSIX Threads; Addison Wesley, 1997; ISBN

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