I'm implementing a parallel, performance-critical algorithm with multiple threads. I assign all threads some data to work on. When all those threads have finished to work on their data, I assign all threads new data, and the cycle continues. (This is what I refer to as thread "clocking" since it's somewhat similar to CPU clocking.)
What I came up with so far is using a master thread that stores an integer. At the beginning of each cycle, I set the integer to the number of slave threads. When a slave thread is done, it decrements the master thread's integer. Once that integer reaches zero, I start a new cycle.
Is this a good approach, or are there more efficient ways of doing the same thing?
You'd be better off using a Phaser (if you have Java 7), or CyclicBarrier for Java 5+.
I would recommend looking at the newer classes in the java.util.concurrent package, especially ThreadPoolTaskExecutor. You might be reinventing the wheel if you haven't looked beyond java.lang.Thread.
Well. See CyclicBarrier (JavaDoc)
A better way is to use Thread.join(). In you main thread, you call join() on all the threads you are starting. The main thread will wait untill all joined threads are finished.
See for example http://javahowto.blogspot.com/2007/05/when-to-join-threads.html
An ExecutorService can do this for you.
ExecutorService executor = Executors.newFixedThreadPool(10);
do {
List<Callable> tasks = getNextTasksToExecute();
executor.invokeAll(tasks);
} while (tasks.size() > 0);
This will create a thread pool with 10 threads. It will then call getNextTasksToExecute() which you should implement yourself to return the next bunch of tasks that need doing. It will execute those tasks in parallel in the thread pool and then keep looping until getNextTasksToExecute() returns no more tasks.
Edit:
Code not tested, think there may be a compile error, but you can figure that out.
Related
I have a situation in which I have to run some 10,000 threads. Obviously, one machine cannot run these many threads in parallel. Is there any way by which we can ask Thread pool to run some specific number of threads in the beginning and as soon as one thread finishes, the threads which are left can start their processing ?
Executors.newFixedThreadPool(nThreads) is what most likely you are looking for. There will only be as many threads running at one time as the number of threads specified. And yes one machine cannot run 10,000 threads at once in parallel, but it will be able to run them concurrently. Depending on how resource intensive each thread is, it may be more efficient in your case to use
Executors.newCachedThreadPool() wherein as many threads are created as needed, and threads that have finished are reused.
Using Executors.newFixedThreadPool(10000) with invokeAll will throw an OutOfMemory exception with that many threads. You still could use it by submitting tasks to it instead of invoking all tasks at same time, that's I would say safer than just invokeAll.
For this use case. You can have a ThreadPollExecuter with Blocking Queue. http://howtodoinjava.com/core-java/multi-threading/how-to-use-blockingqueue-and-threadpoolexecutor-in-java/ this tutorial explains that very well.
It sounds like you want to run 10,000 tasks on a group of threads. A relatively simple approach is to create a List and then add all the tasks to the list, wrapping them in Runnable. Then, create a class that takes the list in the constructor and pops a Runnable of the list and then runs it. This activity must be synchronized in some manner. The class exits when the list is empty. Start some number of threads using this class. They'll burn down the list and then stop. Your main thread can monitor the length of the list.
I read a great article about the fork-join framework in Java 7, and the idea is that, with ForkJoinPool and ForkJoinTask, the threads in the pool can get the sub tasks from other tasks, so it's able to use less threads to handle more tasks.
Then I tried to use a normal ExecutorService to do the same work, and found I can't tell the difference, since when I submit a new task to the pool, the task will be run on another available thread.
The only difference I can tell is if I use ForkJoinPool, I don't need to pass the pool to the tasks, because I can call task.fork() to make it running on another thread. But with normal ExecutorService, I have to pass the pool to the task, or make it a static, so inside the task, I can call pool.submit(newTask)
Do I miss something?
(You can view the living code from https://github.com/freewind/fork-join-test/tree/master/src)
Although ForkJoinPool implements ExecutorService, it is conceptionally different from 'normal' executors.
You can easily see the difference if your tasks spawn more tasks and wait for them to complete, e.g. by calling
executor.invoke(new Task()); // blocks this thread until new task completes
In a normal executor service, waiting for other tasks to complete will block the current thread. There are two possible outcomes: If your executor service has a fixed number of threads, it might deadlock if the last running thread waits for another task to complete. If your executor dynamically creates new threads on demand, the number of threads might explode and you end up having thousands of threads which might cause starvation.
In opposite, the fork/join framework reuses the thread in the meantime to execute other tasks, so it won't deadlock although the number of threads is fixed:
new MyForkJoinTask().invoke();
So if you have a problem that you can solve recursively, think of using a ForkJoinPool as you can easily implement one level of recursion as ForkJoinTask.
Just check the number of running threads in your examples.
As far as I understand Executors help handling the execution of runnables. E.g. I would choose using an executor when I have several worker threads that do their job and then terminate.
The executor would handle the creation and the termination of the Threads needed to execute the worker runnables.
However now I am facing another situation. A fixed number of classes/objects shall encapsulate their own thread. So the thread is started at the creation of those objects and the Thread shall continue running for the whole life time of these objects.
The few objects in turn are created at the start of the programm and exist for the whole run time.
I guess Threads are preferable over Executors in this situation, however when I read the internet everybody seems to suggest using Executors over Threads in any possible situation.
Can somebody please tell me if I want to choose Executors or Threads here and why?
Thanks
You're somewhat mixing things. Executor is just an interface. Thread is a core class. There's nothing which directly implies that Executor implementations execute tasks in separate threads.
Read the first few lines of the JavaDoc.
Executor
So if you want full control, just use Thread and do things on your own.
Without knowing more about the context, it's hard to give a good answer, but generally speaking I'd say that the situations that calls for using Thread are pretty few and far between. If you start trying to synchronize your program "manually" using synchronized I bet things will get out of hand quickly. (Not to mention how hard it will be to debug the code.)
Last time I used a thread was when I wanted to record some audio in the background. It was a "start"/"stop" kind of thing, and not "task oriented". (I tried long and hard to try to find an audio library that would encapsulate that for me but failed.)
If you choose to go for a thread-solution, I suggest you try to limit the scope of the thread to only execute within the associated object. This will to an as large extent as possible avoid forcing you to think about happens-before relations, thread-safe publishing of values etc throughout the code.
ExecutorService can have thread pool
It optimizes performance, because creating a Thread is expensive.
ExecutorService has life cycle control
shutdown(), shutdownNow() etc are provided.
ExecutorService is flexible
You could invoke variety of behaviors: customize ThreadFactory, set thread pool size, delay behavior ScheduledThreadPoolExecutor etc...
I am a newbie to Java concurrency and am a bit confused by several concepts and implementation issues here. Hope you guys can help.
Say, I have a list of tasks stored in a thread-safe list wrapper:
ListWrapper jobs = ....
'ListWrapper' has synchronized fetch/push/append functions, and this 'jobs' object will be shared by multiple worker threads.
And I have a worker 'Runnable' to execute the tasks:
public class Worker implements Runnable{
private ListWrapper jobs;
public Worker(ListWrapper l){
this.jobs=l;
}
public void run(){
while(! jobs.isEmpty()){
//fetch an item from jobs and do sth...
}
}
}
Now in the main function I execute the tasks:
int NTHREADS =10;
ExecutorService service= Executors.newFixedThreadPool(NTHREADS);
//run threads..
int x=3;
for(int i=0; i<x; i++){
service.execute(new Worker(jobs) );
}
I tested this code with 'x=3', and I found that only 3 threads are running at the same time; but as I set 'x=20', I found that only 10 (=NTHREADS) are running at the same time. Seems to me the # of actual threads is the min of the two values.
Now my questions are:
1) Which value ('x' or 'NTHREADS') should I set to control the number of concurrent threads? Or it doesn't matter in either I choose?
2) How is this approach different from simply using the Producer-Consumer pattern --creating a fixed number of 'stud' threads to execute the tasks(shown in the code below)?
Thread t1= new Worker(jobs);
Thread t2= new Worker(jobs);
...
t1.join();
t2.join();
...
Thank you very much!!
[[ There are some good answers here but I thought I'd add some more detail. ]]
I tested this code with 'x=3', and I found that only 3 threads are running at the same time; but as I set 'x=20', I found that only 10 (=NTHREADS) are running at the same time. Seems to me the # of actual threads is the min of the two values.
No, not really. I suspect that the reason you weren't seeing 20 threads is that threads had already finished or had yet to be started. If you call new Thread(...).start() 20 times then you will get 20 threads started. However, if you check immediately none of them may have actually begun to run or if you check later they may have finished.
1) Which value ('x' or 'NTHREADS') should I set to control the number of concurrent threads? Or it doesn't matter in either I choose?
Quoting the Javadocs of Executors.newFixedThreadPool(...):
Creates a thread pool that reuses a fixed number of threads operating off a shared unbounded queue. At any point, at most nThreads threads will be active processing tasks.
So changing the NTHREADS constant changes the number of threads running in the pool. Changing x changes the number of jobs that are executed by those threads. You could have 2 threads in the pool and submit 1000 jobs or you could have 1000 threads and only submit 1 job for them to work on.
Btw, after you have submitted all of your jobs, you should then shutdown the pool which stops all of the threads if all of the jobs have been run.
service.shutdown();
2) How is this approach different from simply using the Producer-Consumer pattern --creating a fixed number of 'stud' threads to execute the tasks(shown in the code below)?
It differs in that it does all of the heavy work for you.
You don't have to create a ListWrapper of the jobs since you get one inside of the ExecutorService. You just submit the jobs to the ExecutorService and it keeps track of them until the threads are available to run them.
You don't have to create any threads or worry about them throwing exceptions and dying because the ExecutorService starts/restarts the threads for you.
If you want your tasks to return information you can make use of the submit(Callable) method and use the Future to get the results of the jobs. Etc, etc..
Doing this code yourself is going to be harder to get right, more code to maintain, and most likely will not perform as well as the code in the JDK that is battle tested and optimized.
You shouldn't create threads by yourself when using a threadpool. Instead of WorkerThread class you should use a class that implements Runnable but is not a thread. Passing a Thread object to the threadpool won't make the thread run actually. The object will be passed to a different internal thread, which will simply execute the run method of your WorkerThread class.
The ExecutorService is simply incompatible with the way you want to write your program.
In the code you have right now, these WorkerThreads will stop to work when your ListWrapper is empty. If you then add something to the list, nothing will happen. This is definitely not what you wanted.
You should get rid of ListWrapper and simply put your tasks directly into the threadpool. The threadpool already incorporates an internal list of jobs shared between the threads. You should just submit your jobs to the threadpool and it will handle them accordingly.
To answer your questions:
1) Which value ('x' or 'NTHREADS') should I set to control the number of concurrent threads? Or it doesn't matter in either I choose?
NTHREADS, the threadpool will create the necessary number of threads.
2) How is this approach different from simply using the Producer-Consumer pattern --creating a fixed number of 'stud' threads to execute the tasks(shown in the code below)?
It's just that ExecutorService automates a lot of things for you. You can choose from a lot of different implementations of threadpools and you can substitute them easily. You can use for instance a scheduled executor. You get extra functionality. Why reinvent the wheel?
For 1) NTHREADS is the maximum threads that the pool will ever run concurrently, but that doesn't mean there will always be that many running. It will only use as many as is needed up to that max value... which in your case is 3.
As the docs say:
At any point, at most nThreads threads will be active processing tasks. If additional tasks are submitted when all threads are active, they will wait in the queue until a thread is available
http://docs.oracle.com/javase/8/docs/api/java/util/concurrent/Executors.html#newFixedThreadPool-int-
As for 2) using Java's concurrent executors framework is preferred with new code. You get a lot of stuff for free and removes the need for having to handle all of the fiddly thread work yourself.
The number of threads passed into newFixedThreadPool is at most how many threads could be running executing your tasks. If you only have three tasks ever submitted I'd expect the ExecutorService to only create three threads.
To answer your questions:
You should use the number you pass into the constructor to control how many threads are going to be used to execute your tasks.
This differs because of the extra functionality the ExecutorService gives you, as well as the flexibility it gives you such as in the case you need to change your ExecutorService type or number of tasks you'll run (less lines of code to change).
All that is happening is the executor service is only creating as many threads as it needs. NTHREADS is effectively the maximum number of threads it'll create.
There is no point creating ten threads up front if it only has 3 tasks to complete, the other 7 will just be hanging around consuming resources.
If you submit more than NTHREADS number of tasks then it will process that number concurrently and the rest will wait on a queue until a thread becomes free.
This isn't any different from creating a fixed set of your own threads, except the thread management and scheduling is handled for you. The executor service also restarts threads if they are killed by rogue exceptions in your task which you'd otherwise have to code for.
See: The Javadoc on Executorservice.newFixedThreadPool
I'd like to have a ScheduledThreadPoolExecutor which also stops the last thread if there is no work to do, and creates (and keeps threads alive for some time) if there are new tasks. But once there is no more work to do, it should again discard all threads.
I naivly created it as new ScheduledThreadPoolExecutor(0) but as a consequence, no thread is ever created, nor any scheduled task is ever executed.
Can anybody tell me if I can achieve my goal without writing my own wrapper around the ScheduledThreadpoolExecutor?
Thanks in advance!
Actually you can do it, but its non-obvious:
Create a new ScheduledThreadPoolExecutor
In the constructor set the core threads to the maximum number of threads you want
set the keepAliveTime of the executor
and at last, allow the core threads to timeout
m_Executor = new ScheduledThreadPoolExecutor ( 16,null );
m_Executor.setKeepAliveTime ( 5, TimeUnit.SECONDS );
m_Executor.allowCoreThreadTimeOut ( true );
This works only with Java 6 though
I suspect that nothing provided in java.util.concurrent will do this for you, just because if you need a scheduled execution service, then you often have recurring tasks to perform. If you have a recurring task, then it usually makes more sense to just keep the same thread around and use it for the next recurrence of the task, rather than tearing down your thread and having to build a new one at the next recurrence.
Of course, a scheduled executor could be used for inserting delays between non-recurring tasks, or it could be used in cases where resources are so scarce and recurrence is so infrequent that it makes sense to tear down all your threads until new work arrives. So, I can see cases where your proposal would definitely make sense.
To implement this, I would consider trying to wrap a cached thread pool from Executors.newCachedThreadPool together with a single-threaded scheduled executor service (i.e. new ScheduledThreadPoolExecutor(1)). Tasks could be scheduled via the scheduled executor service, but the scheduled tasks would be wrapped in such a way that rather than having your single-threaded scheduled executor execute them, the single-threaded executor would hand them over to the cached thread pool for actual execution.
That compromise would give you a maximum of one thread running when there is absolutely no work to do, and it would give you as many threads as you need (within the limits of your system, of course) when there is lots of work to do.
Reading the ThreadPoolExecutor javadocs might suggest that Alex V's solution is okay. However, doing so will result in unnecessarily creating and destroying threads, nothing like a cashed thread-pool. The ScheduledThreadPool is not designed to work with a variable number of threads. Having looked at the source, I'm sure you'll end up spawning a new thread almost every time you submit a task. Joe's solution should work even if you are ONLY submitting delayed tasks.
PS. I'd monitor your threads to make sure your not wasting resources in your current implementation.
This problem is a known bug in ScheduledThreadPoolExecutor (Bug ID 7091003) and has been fixed in Java 7u4. Though looking at the patch, the fix is that "at least one thread is started even if corePoolSize is 0."