Stack Size of Threads

Several months ago, my boss assigned me to do performance tuning on several applications. One of them was a server application, which was for distributing data to clients. This application used a lot of threads, two threads per client -.-" It can support up to about 80 clients. If the number was over 80, the application generated a core dump. The core dump was due to out of memory. From top, I found the virtual memory size increases 4x MB for each newly connected client. It confused me. How could one client eat 4xMB?
Finally, I found the answer -- stack size of a thread. In the Linux system, the default stack size of a thread (in case of pthread library) is set to 20MB!
To change the stack size of a thread,

1. use pthread_attr_setstacksize (&attr, stacksize) to set the attribute during thread creation. For more details, you can refer to the following link: http://www.llnl.gov/computing/tutorials/pthreads/#Stack
2. use ulimit -s nnnn command to change the default stack size of pthread, where nnnn is the size in KBytes. (http://kbase.redhat.com/faq/FAQ_43_8710.shtm)