Why is this C++11 code containing rand() slower with multiple threads than with one?

Question

I\'m trying around on the new C++11 threads, but my simple test has abysmal multicore performance. As a simple example, this program adds up some squared random numbers.

Answer 1

As you discovered, rand is the culprit here.

For those who are curious, it's possible that this behavior comes from your implementation of rand using a mutex for thread safety.

For example, eglibc defines rand in terms of __random, which is defined as:

long int
__random ()
{
  int32_t retval;

  __libc_lock_lock (lock);

  (void) __random_r (&unsafe_state, &retval);

  __libc_lock_unlock (lock);

  return retval;
}

This kind of locking would force multiple threads to run serially, resulting in lower performance.