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Program not always terminating? [duplicate]

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Why is i++ not atomic?
(10 answers)
What is a debugger and how can it help me diagnose problems?
(2 answers)
Closed 4 years ago.
I wanted to test out multithreading for a project of mine, trying to also develop a solution in case something goes wrong.
So I made this small test:
main
public class main
{
static int addToCounter;
static int addToErrorCounter;
public static void main(String[] args) throws InterruptedException
{
int threads = 10;
Executor exec = new Executor();
for (int i = 0; i < threads; i++)
{
double error = Math.random();
testClass aldo = new testClass();
Thread thread = aldo.getThread(300, error);
exec.execute(thread);
}
while (threads != (addToCounter + addToErrorCounter))
{
System.out.println("Not all threads finished, number of finished threads is: " + (addToCounter + addToErrorCounter));
Thread.sleep(50);
}
System.out.println("Number of Threads that finished correctly: " + addToCounter);
}
}
testClass
import test1.main;
public class testClass
{
public Thread getThread(long time, double error)
{
Thread thread = new Thread()
{
public void run()
{
try
{
Thread.sleep(time);
}
catch (InterruptedException e)
{
// TODO Auto-generated catch block
e.printStackTrace();
}
if (error > 0.5)
{
main.addToErrorCounter++;
throw new java.lang.Error("HELLO");
}
System.out.println("I DID THIS!");
main.addToCounter++;
}
};
return thread;
}
}
(you'll have to fix the imports, also I use a custom class Executor, although that's only a wrapper for ExecutorService)
The weird behaviour is that sometimes it works properly, and sometimes it doesn't (total terminated thread count is 9, although I can see clearly it printed "I DID THIS!" and the error exactly 10 times).
Any fix?

The Problem might be a racecondition.
the "++" operator is not atomic.
Imageine the following scenario. There are two Threads at the same time. both want to increase a number and finish.
The initial value of the number is 0.
Thread 0 reads the number, knows now it is 0.
Thread 1 reads the number, knows now it is 0.
Thread 0 (who knew it was 0) now writes 1 to the memory.
Thread 1 does not know, that the number has changed, and still believes the number is 0 so he also writes a 1 to the memory.
You need something like a synchronizing mechanisim, something like a lock, or a semaphore or something else.
have a look at this for more information: http://winterbe.com/posts/2015/04/30/java8-concurrency-tutorial-synchronized-locks-examples/
for your example you could use the "synchronized" example from that link.
add a method to your main class looking like this to increment the addToCounter and also to the addToErrorCounterto remove the effects from your error counter:
synchronized AddToError(int e){
addToError += e;
}
synchronized IncCounter(){
addToCounter++;
}
call those methods in your threads in the testclass instead of incrementing them unsynchronized.

My guess is that the postfix operator (main.addToCounter++) is not atomic. This line of code is probably equivalent to something like:
int temp = main.addToCounter;
main.addToCounter = temp + 1;
return temp;
With multiple threads doin this at the same time, two threads could obtain the same value for temp (because both peform the first line in the above pseudo-code before either performs the second), and hence the counter total will be too small once all threads are complete. See Why is i++ not atomic? for more information.
A quick fix in this situation is to make addToCounter an AtomicInteger, then use addToCounter.incrementAndGet() in place of addToCounter++.

Mutual exclusion code

I'm trying to convert this code to java and using thread to implement it
turn = 0 // shared control variable
while (turn != i);
// CS
turn = (turn + 1) % n;
I'm really tried hard to reach to right code but I failed this is my code
/*
* Mutual exclusion using thread
*/
class gV{
int turn=0;
}
class newThread extends Thread{
static int i;
int n=10;
newThread(gV obj){
this.i=obj.turn;
start();
}
public void run(){
while(obj.turn!=i&&obj.turn<n);
criticalSection(i);
obj.turn=(obj.turn+1);
i++;
}
public void criticalSection(int numOfProcess){
System.out.println("Process " + numOfProcess + " done!!");
}
}
class MutualExclusion{
public static void main(String args[]){
gV obj = new gV();
new newThread(obj);
}
}
I know my code has some mistakes. Thank you for the help!

Use an AtomicInteger.
Atomic means that any operation on it will fully complete before any other thread can see the result. Meaning that you won't have two simultaneous operations 'clobber' it. For example, imagine if you had a non atomic integer and two threads attempted to increment it simultaneously - say it had value 1, they both read it as 1 and attempt to set it to 2. They both incremented it once - but instead of it becoming 3, it became 2! AtomicInteger solves this problem by giving you IncrementAndGet, which guarantees no other thread can access the AtomicInteger's value before the increment completes.
In particular, use these methods:
http://docs.oracle.com/javase/6/docs/api/java/util/concurrent/atomic/AtomicInteger.html#get()
http://docs.oracle.com/javase/6/docs/api/java/util/concurrent/atomic/AtomicInteger.html#incrementAndGet()
You might notice that this increments it, but it doesn't take it modulo n. Well, you can take it modulo n whenever you read its value, you don't need it to be stored that way.
EDIT: By the way, doing something like this:
while (turn != i);
is called busy-waiting, and it's a bad idea because it means that CPU usage will be 100%, checking the variable hundreds of thousands of times per second. In this kind of scenario, instead of making each thread check as often as possible, you want to have threads wait and be notifyed by another thread when it is that thread's turn to continue execution.
I believe in Java that using lock and synchronized to implement mutual exclusion will also give you this property, e.g. if you try to lock on something or enter a synchronized block but it is already in use then the thread goes to sleep and is woken up when it is its turn. So, you can look into this as well.

Java Synchronized Method and Block

I'm trying to understand synchronization of multiple threads in Java more fully. I understand the high level idea behind the use of the synchronized keyword, and how it provides mutual exclusion among threads.
The only thing is that most of the examples I read online and in my textbook still work correctly even if you remove the synchronized keyword which is making this topic more confusing than I think it needs to be.
Can anyone provide me with a concrete example of when not including the synchronized keyword will produce erroneous results? Any information would be much appreciated.

You can usually trigger a race condition by increasing the number of iterations. Here's a simple example that works with 100 and 1,000 iterations but fails (at least on my quad-core box) at 10,000 iterations (sometimes).
public class Race
{
static final int ITERATIONS = 10000;
static int counter;
public static void main(String[] args) throws InterruptedException {
System.out.println("start");
Thread first = new Thread(new Runnable() {
#Override
public void run() {
for (int i = 0; i < ITERATIONS; i++) {
counter++;
}
}
});
Thread second = new Thread(new Runnable() {
#Override
public void run() {
for (int i = 0; i < ITERATIONS; i++) {
counter++;
}
}
});
first.start();
second.start();
first.join();
second.join();
System.out.println("Counter " + counter + " should be " + (2 * ITERATIONS));
}
}
>>> Counter 12325 should be 20000
This example fails because access to counter is not properly synchronized. It can fail in two ways, possibly both in the same run:
One thread fails to see that the other has incremented the counter because it doesn't see the new value.
One thread increments the counter between the other thread reading the current value and writing the new value. This is because the increment and decrement operators are not atomic.
The fix for this simple program would be to use an AtomicInteger. Using volatile isn't enough due to the problem with increment, but AtomicInteger provides atomic operations for increment, get-and-set, etc.

The thing about race conditions is that they don't necessarily happen if you don't do proper synchronization -- indeed, quite frequently it'll work just fine -- but then one year later, in the middle of the night, your code will crash with a completely unpredictable bug that you can't reproduce, because the bug only appears randomly.
Race conditions are so insidious precisely because they don't always make your program crash, and they trigger more or less randomly.

Java strange performance inconsistency

I have a simple recursive method, a depth first search. On each call, it checks if it's in a leaf, otherwise it expands the current node and calls itself on the children.
I'm trying to make it parallel, but I notice the following strange (for me) problem.
I measure execution time with System.currentTimeMillis().
When I break the search into a number of subsearches and add the total execution time, I get a bigger number than the sequential search. I only measure execution time, no communication or sync, etc. I would expect to get the same time when I add the times of the subtasks. This happens even if I just run one task after the other, so without threads. If I just break the search into some subtasks and run the subtasks one after the other, I get a bigger time.
If I add the number of method calls for the subtasks, I get the same number as the sequential search. So, basically, in both cases I do the same number of method calls, but I get different times.
I'm guessing there's some overhead on initial method calls or something else caused by a JVM mechanism. Any ideas what could it be?
For example, one sequential search takes around 3300 ms. If I break it into 13 tasks, it takes a total time of 3500ms.
My method looks like this:
private static final int dfs(State state) {
method_calls++;
if(state.isLeaf()){
return 1;
}
State[] children = state.expand();
int result = 0;
for (int i = 0; i < children.length; i++) {
result += dfs(children[i]);
}
return result;
}
Whenever I call it, I do it like this:
for(int i = 0; i < num_tasks; i++){
long start = System.currentTimeMillis();
dfs(tasks[i]);
totalTime += (System.currentTimeMillis() - start);
}
Problem is totalTime increases with num_tasks and I would expect to stay the same because the method_calls variable stays the same.

You should average out the numbers over longer runs. Secondly the precision of currentTimeMillis may not be sufficient, you can try using System.nanoTime().

As in all the programming languages, whenever you call a procedure or a method, you have to push the environment, initialize the new one, execute the programs instructions, return the value on the stack and finally reset the previous environment. It cost a bit! Create a thread cost also more!
I suppose that if you enlarge the researching tree you will have benefit by the parallelization.

Adding system clock time for several threads seems a weird idea. Either you are interested in the time until processing is complete, in which case adding doesn't make sense, or in cpu usage, in which case you should only count when the thread is actually scheduled to execute.
What probably happens is that at least part of the time, more threads are ready to execute than the system has cpu cores, and the scheduler puts one of your threads to sleep, which causes it to take longer to complete. It makes sense that this effect is exacerbated the more threads you use. (Even if your program uses less threads than you have cores, other programs (such as your development environment, ...) might).
If you are interested in CPU usage, you might wish to query ThreadMXBean.getCurrentThreadCpuTime

I'd expect to see Threads used. Something like this:
import java.util.concurrent.Executor;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class Puzzle {
static volatile long totalTime = 0;
private static int method_calls = 0;
/**
* #param args
*/
public static void main(String[] args) {
final int num_tasks = 13;
final State[] tasks = new State[num_tasks];
ExecutorService threadPool = Executors.newFixedThreadPool(5);
for(int i = 0; i < num_tasks; i++){
threadPool.submit(new DfsRunner(tasks[i]));
}
try {
threadPool.shutdown();
threadPool.awaitTermination(1, TimeUnit.SECONDS);
} catch (InterruptedException e) {
System.out.println("Interrupted");
}
System.out.println(method_calls + " Methods in " + totalTime + "msecs");
}
static final int dfs(State state) {
method_calls++;
if(state.isLeaf()){
return 1;
}
State[] children = state.expand();
int result = 0;
for (int i = 0; i < children.length; i++) {
result += dfs(children[i]);
}
return result;
}
}
With the runnable bit like this:
public class DfsRunner implements Runnable {
private State state;
public DfsRunner(State state) {
super();
this.state = state;
}
#Override
public void run() {
long start = System.currentTimeMillis();
Puzzle.dfs(state);
Puzzle.totalTime += (System.currentTimeMillis() - start);
}
}

Why does an IllegalThreadStateException occur when Thread.start is called again

public class SieveGenerator{
static int N = 50;
public static void main(String args[]){
int cores = Runtime.getRuntime().availableProcessors();
int f[] = new int[N];
//fill array with 0,1,2...f.length
for(int j=0;j<f.length;j++){
f[j]=j;
}
f[0]=0;f[1]=0;//eliminate these cases
int p=2;
removeNonPrime []t = new removeNonPrime[cores];
for(int i = 0; i < cores; i++){
t[i] = new removeNonPrime(f,p);
}
while(p <= (int)(Math.sqrt(N))){
t[p%cores].start();//problem here because you cannot start a thread which has already started(IllegalThreadStateException)
try{
t[p%cores].join();
}catch(Exception e){}
//get the next prime
p++;
while(p<=(int)(Math.sqrt(N))&&f[p]==0)p++;
}
//count primes
int total = 0;
System.out.println();
for(int j=0; j<f.length;j++){
if(f[j]!=0){
total++;
}
}
System.out.printf("Number of primes up to %d = %d",f.length,total);
}
}
class removeNonPrime extends Thread{
int k;
int arr[];
public removeNonPrime(int arr[], int k){
this.arr = arr;
this.k = k;
}
public void run(){
int j = k*k;
while(j<arr.length){
if(arr[j]%k == 0)arr[j]=0;
j=j+arr[k];
}
}
}
Hi I'm getting an IllegalThreadStateException when I run my code and I've figured it's because I am trying to start a thread that has already been started. So how could I kill
or stop the thread each time, to get around this problem?

how could I kill or stop the thread each time, to get around this problem?
The answer is, you can't. Once started, a Thread may not be restarted. This is clearly documented in the javadoc for Thread. Instead, what you really want to do is new an instance of RemoveNonPrime each time you come around in your loop.
You have a few other problems in your code.
First, you need to increment p before using it again:
for(int i = 0; i < cores; i++){
t[i] = new removeNonPrime(f,p); //<--- BUG, always using p=2 means only multiples of 2 are cleared
}
Second, you might be multithreaded, but you aren't concurrent. The code you have basically only allows one thread to run at a time:
while(p <= (int)(Math.sqrt(N))){
t[p%cores].start();//
try{
t[p%cores].join(); //<--- BUG, only the thread which was just started can be running now
}catch(Exception e){}
//get the next prime
p++;
while(p<=(int)(Math.sqrt(N))&&f[p]==0)p++;
}
Just my $0.02, but what you are trying to do might work, but the logic for selecting the next smallest prime will not always pick a prime, for example if one of the other threads hasn't processed that part of the array yet.
Here is an approach using an ExecutorService, there are some blanks (...) that you will have to fill in:
/* A queue to trick the executor into blocking until a Thread is available when offer is called */
public class SpecialSyncQueue<E> extends SynchronousQueue<E> {
#Override
public boolean offer(E e) {
try {
put(e);
return true;
} catch (InterruptedException ex) {
Thread.currentThread().interrupt();
return false;
}
}
}
ExecutorService executor = new ThreadPoolExecutor(cores, cores, new SpecialSyncQueue(), ...);
void pruneNonPrimes() {
//...
while(p <= (int)(Math.sqrt(N))) {
executor.execute(new RemoveNonPrime(f, p));
//get the next prime
p++;
while(p<=(int)(Math.sqrt(N))&&f[p]==0)p++;
}
//count primes
int total = 0;
System.out.println();
for(int j=0; j<f.length;j++){
if(f[j]!=0){
total++;
}
}
System.out.printf("Number of primes up to %d = %d",f.length,total);
}
class RemoveNonPrime extends Runnable {
int k;
int arr[];
public RemoveNonPrime(int arr[], int k){
this.arr = arr;
this.k = k;
}
public void run(){
int j = k*k;
while(j<arr.length){
if(arr[j]%k == 0)arr[j]=0;
j+=k;
}
}
}

You could implement Runnable instead and use new Thread( $Runnable here ).start() or use a ExecutorService to reuse threads.

* It is never legal to start a thread more than once.
* In particular, a thread may not be restarted once it has completed
* execution.
*
* #exception IllegalThreadStateException if the thread was already started
*/
public synchronized void start() {
In Android, document still mention that we will get IllegalThreadStateException if the thread was already started.
However for some device it will not throw this exception (tested on Kyocera 7.0). In some popular device like Samsung, HTC, it throw throw the exception normally
I answer here because the Android question is mark as duplicated to this question.

Why does an IllegalThreadStateException occur when Thread.start is
called again
Because JDK/JVM implementers coded Thread.start() method that way. Its a reasonable functional expectation to be able to restart a thread after a thread has completed its execution and that is what being suggested in chrisbunney's answer ( and I have put in a comment in that answer ) but if you look at Thread.start() implementation , the very first line is ,
if (threadStatus != 0)
throw new IllegalThreadStateException();
where threadStatus == 0 means NEW state so my guess is that implementation doesn't resets this state to zero after execution has completed & thread is left in TERMINATED state ( non - zero state ). So when you create a new Thread instance on same Runnable , you basically reset this state to zero.
Also, I noticed the usage of word - may & never in same paragraph as different behavior is being pointed out by Phan Van Linh on some OSes,
It is never legal to start a thread more than once. In particular, a
thread may not be restarted once it has completed execution.
I guess what they are trying to say in above Javadoc that even if you don't get IllegalThreadStateException on certain OS, its not legal in Java/Thread class way & you might get unexpected behavior.
The famous thread state diagrams depict the same scenario - no going back from dead state to new.

ThreadPools can be used for delivering tasks to set number of threads. When initiating you set the number of threads. Then you add tasks for the pool. And after you can block until all tasks have finished processing. Here is some sample code.

I am not at all sure I understand the question. All the methods for stopping threads that are executed from other threads are deprecated; the way to stop a thread is to have it check a variable that it and another thread can access (perhaps a volatile variable), and have the running thread check it occasionally to see if it should exit on its own.
I cannot tell why/whether you want to eliminate the running thread and use another one, and I cannot see how the different threads are going to help execute your overall goal any faster. But it's possible I'm just not understanding the math.

The Thread.isAlive() method can tell you if the Thread has already been started. Simply do this where you want to start your thread:
if(!t[p%cores].isAlive()){
t[p%cores].start();
}