Where have all the threads gone?

Recently, we got a nasty error in the log file:

java.lang.OutOfMemoryError: unable to create new native thread

Although we had specified -XX:HeapDumpOnOutOfMemoryError, this does not include dumping the heap to disk when the heap is not involved in an OutOfMemoryError. In this case, the JVM was unable to create new threads as the limit of threads was reached.

So, the question is: How many threads can be created?
The answer is, as usual: It depends!

It depends on the amount of memory of the operating system, the architecture of the platform and the size of each thread and how much memory it takes in the stack.

To show you how it depends on the total memory assigned to the JVM and that it's an inversely proportional function, i ran a tiny test. The test created Threads with an unlimited Thread.sleep(Integer.MAX_VALUE) and had no additional data, so the overhead is minimal/default. I ran the test on my business notebook, which has (according to Windows Task-Manager) 3.66GB of physical memory. The java version is Java 32Bit 1.6.0_13, client vm, mixed mode, sharing. The operating system is Windows XP 32-Bit.

The results show a very unintuitive, but logical behaviour: The more memory is available to Java, the less is available for OS Threads and thus the total number of threads which can be created decreases:

Created 5605 threads with avail. memory 63MB
Created 5478 threads with avail. memory 127MB
Created 5096 threads with avail. memory 254MB
Created 4285 threads with avail. memory 508MB
Created 2661 threads with avail. memory 1016MB
Created 1201 threads with avail. memory 1488MB
Created 909 threads with avail. memory 1587MB
Created 890 threads with avail. memory 1593MB

What can you do about it?
Well, if your bound to hardware resources, decrease the amount of memory available to the JVM or inspect the software components that they're releasing threads properly. Monitor your application with a JMX Monitor such as JConsole or the wonderful VisualVM.

Links:
How to fix "java.lang.OutOfMemoryError: unable to create new native thread"
IBM: Thanks for the memory
The magic /3GB switch of IBM JVMs

WHERE HAVE ALL THE THREADS GONE 
words and music by Pete Seeger
performed by Pete Seeger and Tao Rodriguez-Seeger
modified by mike

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