Why does max number of threads decrease when I increase max heap size?

Question

This test shows the max number of threads that can be created in Java

    System.out.println("Max memory  " + Runtime.getRuntime().maxMemory() / 1024 / 1024 + "M");
    for (int i = 0;; i++) {
        Thread t = new Thread() {
            public void run() {
                try {
                    Thread.sleep(10000);
                } catch (InterruptedException e) {
                }
            };
        };
        try {
            t.start();
        } catch (Error e) {
            System.out.println("Max threads " + i);
            e.printStackTrace();
            System.exit(1);
        }
    }

When I run it with default heap size (256M) I get

Max memory  247M
Max threads 2247
java.lang.OutOfMemoryError: unable to create new native thread
    at java.lang.Thread.start0(Native Method)
    at java.lang.Thread.start(Thread.java:691)
    at test.Test1.main(Test1.java:19)

when I increase max heap size to 512M I get

Max memory  494M
Max threads 1906
...

when I increase max heap size to 1024M I get

Max memory  989M
Max threads 1162
...

That is, more heap memory fewer threads. Why is that?

Answer 1

Each thread requires a stack. The more memory you allocate to the heap, the less memory is left for the stacks.

This will be particularly acute if you're using a 32-bit JVM, since the process will have no more than 4GB of address space for everything (the code, the heap, the stacks etc). I cannot reproduce this behaviour on my 64-bit box, where "Max threads" remains the same regardless of how much memory I allocate to the heap.

It is worth noting that many operating systems enable you to tweak the size of the stack. On Unix this is done using ulimit -s.