I had a Java program that I run thousand times based on a loop (according to the number of files to be compiled to build a linux kernel) in a bash script.
There was a performance problem since the jvm was started several times...
What i've done then is implementing a wrapper in java that does the same as my bash script, reads one line from a file and then calls the main of my previous program... This way, I only have one jvm running...
The problem now is that only one core of my box is used which is another performance issue... Do I have to start some threads or can I use the same method but maybe calling the "former" main in a different way ?
If i have to start some threads, how I dispatch them throughout the multiple cores ?
thanks...
Your java program needs to become multi-threaded, in order to take advantage of many cores.
For example, create a thread pool using java.util.concurrent.Executors, encapsulate your data items as a Runnable, and submit the Runnable to the threadpool.
At the risk of oversimplifying it, just have your old class implement Runnable, taking what was in main() and putting it in Run(), then create a new thread with that class and start the thread.
In reality it might be more complicated than that if the threads need to share data, but based on what you said you are doing here, it doesn't seem like they would. So it might actually be just that easy.
You will need to make your program multi-threaded. You will have to do some learning to do this and I recommend you start with the Java Concurrency Tutorial.
Related
My application is supposed to have a "realtime with pause" functionality. The user can pause execution, do some things that modify what's going to happen, then unpause and let stuff happen. Stuff happens at regular intervals as specified by the user, can be slow, can be fast.
My goal at using threading here is to improve performance on multicore systems. The amount of data that the application is supposed to crunch at the time intervals is supposed to be arbitrarily large (I expect lots and lots of loops over collections, modifying object properties and generating random numbers, but precious little disk access). I don't want the application to be constrained by the capacity of a single core, if it can use more to run faster.
Will this actually work this way?
I've run some tests (made a program crunch numbers a lot, and looked at CPU usage during its activity), but it's not really conclusive - usage is certainly in the proximity of 100% on my dual core machine, but hardly ever 100%. Does a single-threaded (main only) Java application use all available cores for computation?
Does a single-threaded (main only) Java application use all available cores for computation?
No, it will normally use a single core.
Making a program do computations in parallel with multiple threads may make it faster, but it's not a magical solution for any kind of problem. Whether this is a suitable solution for your program depends on what your program is doing exactly, and if the algorithm can be parallelized. If, for example, you are doing lots of computations where the next computation depends on the result of the previous computation, then making it multi-threaded will not help a lot, because you can't do the computations at the same time - the next one first has to wait for the answer of the previous one. So, you first have to think about what computations in your program could be run in parallel.
Java has a lot of support for multi-threading. You can program with threads directly, or use an executor service, or use the fork/join framework. Whatever is appropriate depends on what exactly you want to do.
Does a single-threaded (main only) Java application use all available cores for computation?
Not usually, but you could make use of some higher level apis in java that is actually using threads for you and youre not even usinfpg threads directly, more obviousiously fork/join and executors, less obvious the new Streams API on collections (ie parallelStream).
In general, though, to make use of all cores, you need to do some kind of concurrency. Further...its really hard to just observe you OS monitor to see what is going on (especially with only 2 cores)...your OS has other things going on (trying to manage itself, running your IDE, running crontab, running a browers to post to stackoverflow ;).
Finally, just implementing (concurrency) itself may not help, you have to do it "right" for your code/algorithm.
a java thread will run in a single cpu. to use multiple CPUs, you should have multiple threads.
Imagine that u have to do various tasks using your hand. You will do it slowly using one hand and more effciently using both your hands. Similarly, in java or in any other language multi threading provides the system with many hands. The good news is that you can have many threads to do different tasks. Running operations in a single thread will make the program sluggish and sometimes unresponsive. A good practice is to do long running tasks in a separate thread. For example loading large chunks of data from a database should be processed in a separate thread. Downloading data from the internet should also be processed in a separate thread. What happens if you do long running operations in the main thread? The program HANGS and will become unresponsive till the task gets completed and the user will think that there is someting wrong. I hope you get it
I have 4 java programs (all are alike), all programs have an infinite loop running in them. Now I want to execute all these programs in parallel from one more program. I want completely independent parallel processing. Is it possible?
It can be done using MultiThreading.From a single program,U can start your all other four programs as thread.The parallel execution will be there for those totally handled by Thread Scheduler.
Still you can make thread sleep and wait as per your requirement.
"A multithreaded program contains two or more parts that can run concurrently and each part can handle different task at the same time making optimal use of the available resources."
Does anybody know of a way to lock down individual threads within a Java process to specific CPU cores (on Linux)? I've done this in C, but can't find how to do this in Java. My instincts are that this will require a JNI call, but I was hoping someone here might have some insight or might have done it before.
Thanks!
You can't do this in pure java. But if you really need it -- you can use JNI to call native code which do the job. This is the place to start with:
http://ovatman.blogspot.com/2010/02/using-java-jni-to-set-thread-affinity.html
http://blog.toadhead.net/index.php/2011/01/22/cputhread-affinity-in-java/
UPD: After some thinking, I've decided to create my own class for this: ThreadAffinity.java It's JNA-based, and very simple -- so, if you want to use it in production, may be you should spent some time making it more stable, but for benchmarking and testing it works well as is.
UPD 2: There is another library for working with thread affinity in java. It uses same method as previously noted, but has another interface
I know it's been a while, but if anyone comes across this thread, here's how I solved this problem. I wrote a script that would do the following:
"jstack -l "
Take the results, find the "nid"'s of the threads I want to manually lock down to cores.
Taskset those threads.
You might want to take a look at https://github.com/peter-lawrey/Java-Thread-Affinity/blob/master/src/test/java/com/higherfrequencytrading/affinity/AffinityLockBindMain.java
IMO, this will not be possible unless you use native calls. JVM is supposed to be platform independent, any system calls done to achieve this will not result in a portable code.
It's not possible (at least with plain Java).
You can use thread pools to limit the amount of threads (and therefore cores) used for different types of work, but there is no way to specify a core to use.
There is even the (small) possibility that your Java runtime doesn't support native threading for your OS or hardware. In this case, green threads are used and only one core will be used for the whole JVM.
I am working on an application in which I want multiple tasks to be executed simultaneously.
I also want to be able to keep track of the number of such tasks being run in parallel, and sometimes add yet another task to be processed in parallel, in addition to the current set of tasks already being processed.
One more thing- I want to do the above, not only in a desktop app, but also in a cloud app, in which I initialise another virtual machine running Tomcat, and then repeat all of the above in that instance.
What is the best way to do this? If you can point me to the correct theory/guides on this subject, that would be great, although code samples are also welcome.
Concurrency is a huge topic in Java, please take your time for it
Lesson: Concurrency
Concurrency in a Java program is accomplished by starting your own Threads. Multiple processes can only be realized with multiple JVMs. When you are done with the basics, you want to take a look at Executors. They will help to implement your application in a structured way since they abstract from Threads to Tasks.
I don't know how much time you have planned for this, but if you are really at the start, get Java Concurrency in Practice, read it and write a kick-ass concurrent Java application.
Raising the whole thing to a distributed level is a whole other story. You cannot tackle that all at once.
Wow... What a series of steps. Start by extending Runnable, then using Thread to run and manage your Jobs. After that, you can get into Tomcat.
Does anybody know of a way to lock down individual threads within a Java process to specific CPU cores (on Linux)? I've done this in C, but can't find how to do this in Java. My instincts are that this will require a JNI call, but I was hoping someone here might have some insight or might have done it before.
Thanks!
You can't do this in pure java. But if you really need it -- you can use JNI to call native code which do the job. This is the place to start with:
http://ovatman.blogspot.com/2010/02/using-java-jni-to-set-thread-affinity.html
http://blog.toadhead.net/index.php/2011/01/22/cputhread-affinity-in-java/
UPD: After some thinking, I've decided to create my own class for this: ThreadAffinity.java It's JNA-based, and very simple -- so, if you want to use it in production, may be you should spent some time making it more stable, but for benchmarking and testing it works well as is.
UPD 2: There is another library for working with thread affinity in java. It uses same method as previously noted, but has another interface
I know it's been a while, but if anyone comes across this thread, here's how I solved this problem. I wrote a script that would do the following:
"jstack -l "
Take the results, find the "nid"'s of the threads I want to manually lock down to cores.
Taskset those threads.
You might want to take a look at https://github.com/peter-lawrey/Java-Thread-Affinity/blob/master/src/test/java/com/higherfrequencytrading/affinity/AffinityLockBindMain.java
IMO, this will not be possible unless you use native calls. JVM is supposed to be platform independent, any system calls done to achieve this will not result in a portable code.
It's not possible (at least with plain Java).
You can use thread pools to limit the amount of threads (and therefore cores) used for different types of work, but there is no way to specify a core to use.
There is even the (small) possibility that your Java runtime doesn't support native threading for your OS or hardware. In this case, green threads are used and only one core will be used for the whole JVM.