Don't start new threads unless you really need to. Starting threads is not cheap and for short running tasks starting the thread may actually take more time than executing the task itself. If you're on .NET take a look at the built in thread pool, which is useful in a lot of (but not all) cases. By reusing the threads the cost of starting threads is reduced.
EDIT: A few notes on creating threads vs. using thread pool (.NET specific)
Generally try to use the thread pool. Exceptions:
- Long running CPU bound tasks and blocking tasks are not ideal run on the thread pool cause they will force the pool to create additional threads.
- All thread pool threads are background threads, so if you need your thread to be foreground, you have to start it yourself.
- If you need a thread with different priority.
- If your thread needs more (or less) than the standard 1 MB stack space.
- If you need to be able to control the life time of the thread.
- If you need different behavior for creating threads than that offered by the thread pool (e.g. the pool will throttle creating of new threads, which may or may not be what you want).
There are probably more exceptions and I am not claiming that this is the definitive answer. It is just what I could think of atm.
