Java - relationship between threads and CPUs

Question

I am pretty new to multithreading, and I am working on a project where I am trying to utilize 4 CPUs in my Java program. I wanted to do something like

int numProcessors = Runtime.getRuntime().availableProcessors();
ExecutorService e = Executors.newFixedThreadPool(numProcessors);

Will this guarantee that I will have one thread working per CPU? At the time that I create the threads, the system will not be busy, however some time afterwards it will be extremely busy. I thought that the OS will pick the least busy CPU to create the threads, but how does it work if none of them are particularly busy at the time of creation?

Also, the thread pool service is supposed to reuse threads, but if it sees there is more availability on another CPU, will it kill the thread and spawn a new one there?

Answer 1

Will this guarantee that I will have one thread working per CPU?

If you have four tasks which need to be executed at the same time, you can expect them have a thread each. In the HotSpot JVM, it creates the Thread object which are proxies for the actual thread when you create the pool. When the actual threads are created, and how, does matter to you.

At the time that I create the threads, the system will not be busy, however some time afterwards it will be extremely busy. I thought that the OS will pick the least busy CPU to create the threads, but how does it work if none of them are particularly busy at the time of creation?

Threads are created by the OS and it is added to the list of threads to schedule.

Also, the thread pool service is supposed to reuse threads, but if it sees there is more availability on another CPU, will it kill the thread and spawn a new one there?

Java has no say in the matter. The OS decides. It doesn't kill and restart threads.

Threads are not tied to CPUs in the way you suggest. The OS passes threads between CPUs based on which one threads need to run and which CPUs are free.