Question

If i have some code looks like this(Please ignore the syntax, i want to understand it without a specified language):

count = 0

def countDown():
  count += 1

if __name__ == '__main__':
    thread1(countDown)
    thread2(countDown)
    thread3(countDown)

Here i have a CPU with only one core, do i really need a lock to the variable count in case of it could be over-written by other threads.

I don’t know, but if the language cares a lot, please explain it under Java、C and Python, So many thanks.

---

Thanks guys, i now understand i do need a lock. But here’s another question, When do i need to use multi threads ?

Since the CPU will execute only one instructor, it seems that multi threads will take more time to manage the threads switch, and can’t save the calculation time.

Answer 1

Technically, in general yes. Maybe not in this particular example. But imagine your atomic function would consist of several instructions. The operating system can and does execute many threads at once. It executes some steps of one, then switches back to OS which chooses which process/thread to continue. It can start all of your threads and switch between them. Even on one CPU. Then all threads would operate on the same memory addresses and share variables.

Edit: Answer to 2nd question.
When you have one core I can imagine only one case when you would need multithreading. It is when one of your threads can lock and you need to monitor for it or do something else in this time. One practical example would be a sever. If you want to serve multiple clients at the same time you need to switch between them. If you served them in a queue one bad client could hang whole process.

If you are doing computations you might use it to split I/O and computation. But it would need to be a very extreme case to be useful or needed.