Many configs have thread_stack_size configured explicitly, but that can cause rather bad trouble:
- if the stack inside a thread it’s too small, you can get segfault crashes (stack overflow, essentially). Particularly on 64-bit.
- if the stack is too large, your system cannot handle as many connections since it all eats RAM.
Let mysqld sort it out, on startup it does a calculation based on the CPU architecture, and that’s actually the most sensible. So for almost all setups, remove any thread_stack_size=… line you might have in my.cnf.