max thread per process in linux

Question

I wrote a simple program to calculate the maximum number of threads that a process can have in linux (Centos 5). here is the code:

int main()
{
    pthread_t thrd[400];
    for(int i=0;i<400;i++)
    {
        int err=pthread_create(&thrd[i],NULL,thread,(void*)i);
        if(err!=0)
            cout << "thread creation failed: " << i <<" error code: " << err << endl;
    }
    return 0;
}

void * thread(void* i)
{
    sleep(100);//make the thread still alive
    return 0;
}

I figured out that max number for threads is only 300!? What if i need more than that? I have to mention that pthread_create returns 12 as error code.

Thanks before

Answer 1

There is a thread limit for linux and it can be modified runtime by writing desired limit to /proc/sys/kernel/threads-max. The default value is computed from the available system memory. In addition to that limit, there's also another limit: /proc/sys/vm/max_map_count which limits the maximum mmapped segments and at least recent kernels will mmap memory per thread. It should be safe to increase that limit a lot if you hit it.

However, the limit you're hitting is lack of virtual memory in 32bit operating system. Install a 64 bit linux if your hardware supports it and you'll be fine. I can easily start 30000 threads with a stack size of 8MB. The system has a single Core 2 Duo + 8 GB of system memory (I'm using 5 GB for other stuff in the same time) and it's running 64 bit Ubuntu with kernel 2.6.32. Note that memory overcommit (/proc/sys/vm/overcommit_memory) must be allowed because otherwise system would need at least 240 GB of committable memory (sum of real memory and swap space).

If you need lots of threads and cannot use 64 bit system your only choice is to minimize the memory usage per thread to conserve virtual memory. Start with requesting as little stack as you can live with.