java.lang.OutOfMemoryError unable to create new native thread

Recently I have been doing some interesting work, improving performance and memory optimization on a custom inhouse built server designed to handle thousands of concurrent connections.  While running these server on my dev box (Win 32) I have been encountering

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

Interesting thing was I was not getting this error on our QA or Production boxes that were running 64 bit JVM on Solaris.

On my local dev box there was enough memory left (1G) on the Java heap of 1.5GB and still I was getting this error when I started to simulate 1000 threads.

Finally after some research I found that the more memory you give to the JVM the more likely you are to get java.lang.OutOfMemoryError: unable to create new native thread exceptions when you have thousands of threads.

The reason for this was when ever you create a thread the under lying operating system also creates a OS thread representing that thread. The operating system creates the thread with a thread stack what ever memory is left for its usage. Remember this memory is not taken from the Heap memory, infact its taken from the memory what ever is left after the allocation of Java Heap and other OS libraries (dlls).

Each thread in the JVM get’s a stack. The stack size will limit the number of threads that you can have, too big of a stack size and you will run out of memory as each thread is allocated more memory than it needs.

Solutions

1) Use 64Bit JVM and OS.

2) Decrease the stack size to reasonable value by using java  runtime argument -Xss determines the size of the stack: -Xss512k. If the stack space is too small, eventually you will see an exception Stack Over flow exception.

The default java stack size is 400k (un-confirmed)

Hope this helps