how many threads to run in java?

Question

I had this brilliant idea to speed up the time needed for generating 36 files: use 36 threads!! Unfortunately if I start one connection (one j2ssh connection object

Answer 1

A good practice would be to spawn threads equivalent to the number of cores in your processor. I normally use a Executors.fixedThreadPool(numOfCores) executor service and keep feeding it the jobs from my job queue, simple. :-)

Answer 2

Using more threads than the number of cores on your machine is going only to slow down the whole process. It will speed up till you reach this number.

Answer 3

First you have to find out where the bottle neck is.

• If it is the SSH connection, it usually does not help to open multiple connections in parallel. Better use multiple channels on one connection, if needed.

• If it is the disk IO, creating multiple threads writing (or reading) only helps if they are accessing different disks (which is seldom the case). But you could have another thread doing CPU-bound things while you are waiting on your disk IO in one thread.

• If it is the CPU, and you have enough idle cores, more threads can help. Even more, if they don't need to access common data. But still, more threads than cores (+ some threads doing IO) does not help. (Also take in mind that usually there are other processes on your server, too.)

Answer 4

You need to understand that if you create 36 threads you still have one or two processors and it would be switching between threats most of the time.

I would say you increment the threads a little bit, let's say 6 and see the behavior. And then go from there

Answer 5

Be sure you don't create more threads than you have processing units or you are likely to create more overhead with context switching than you gain in concurrency. Also remember that you only have 1 HDD and 1 HDD controller as a result, I doubt multithreading is going to help you at all here.

Answer 6

You could narrow it down to a more reasonable number of threads with an ExecutorService. You probably want to use something near the number of processor cores available, e.g:

int threads = Runtime.getRuntime().availableProcessors();
ExecutorService service = Executors.newFixedThreadPool(threads);
for (int i = 0; i < 36; i++) {
    service.execute(new Runnable() {
        public void run() {
            // do what you need per file here
        }
    });
}
service.shutdown();