Socket timeout not respected with multiple threads in Java?

Question

I have a java program that spawns out 4 threads. In each thread, I have multiple socket timeouts. However, it seems that these timeouts are not respected ie the readLine() function might block for a longer period of time.

I want the following behavior: If I set a socket timeout to 300 ms, then I want the readLine() function to return within 300 ms from when the readLine() (ie the underlying select call) was invoked, no matter what. I understand that the OS scheduler would be putting the threads to sleep while doing processor sharing, but is there any way in Java to force the threads to always be woken up to ensure this behavior? Or is this just not the right way to think when doing multi-threaded programming?

Ideally, since I am spawning out 4 threads and running on a 6-core machine, each thread should be able to get its own CPU and run in parallel, and respect the select timeout... but that's perhaps too much to expect...

PS: I actually do use Thread.interrupt() to ensure that each of my threads exits within a certain time (I check the time elapsed in the master thread, and interrupt the child threads if its been too long). In each of my threads, I connect to a (different) server, make a request, and wait for a response. I do not know how long the response will be. So I keep on calling the readLine() method, till it times out with a SocketTimeoutException . I enforce a timeout of 300 ms, since I expect the server to start responding within this time. The reason I want to enforce this timeout is that the server behaves in a broadcast fashion, and sends responses to a request by a single client to all clients. So if I don't have a timeout, I will keep on getting data in response to requests by some other clients.

Answer 1

If I really understood your problem, you can always try to invoke Thread.interrupt() in the thread that is performing the readLine() operation. As you didn't provide any code, I leave this link for you to read. It's general, but it provides enough information about interrupting threads.

These two links may also be of use to you: How do you kill a thread in Java? and How to abort a thread in a fast and clean way in java? .

Regarding your question about the OS scheduler, you should be aware that in general purpose OSes you do not have total control on the way the OS schedules tasks. For instance, in Linux the interrupts are the highest priority tasks, then there are scheduling policies that enable you to put "some" determinism on how the tasks are scheduled. In Java, you can use the setPriority() method to change the priority of the thread, but in fact it is the same as using the nice command, and still you don't get any guarantees that this thread will be scheduled ahead of other OS threads.

I hope this helps.

Answer 2

You are making some wrong assumptions here:

• that the timeout will be exactly 300ms. In fact the timeout is that will be at least 300ms.
• the OS Scheduler does not do anything (rather than schedule the java os processes) within the java threads.
• having 6 core, does not mean that each one of your threads will run at separate core, it is not possible to bind thread->core at java
• for last, you consider that the jvm has only yours 4 threads running, but in fact there has more threads, by example, the garbage collector thread(s).

Asking your question: "is there any way in Java to force the threads to always be woken up to ensure this behavior? " Yes, depending how it is your code, if the thread is thread.sleep() , you can use thread.interrupt() (for readline() uses it) and handle the InterruptionException or if they are object.wait() , you can use object.notify() or object.notifyAll() .