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concurrency

Terminology for code reading

  • concurrency: simultaneous occurrence
  • CPU: piece of hardware executing code
  • OS: handles the scheduling of CPU among processes
  • Process: program executed
  • Thread: part of a process; a separate flow of execution
  • Daemon: a process that runs in the background
  • I/O bound: network, database, user inputs ops
  • CPU bound: task, jobs, calculation
  • time.sleep() artificial delay for switching between threads
## Multithreading in a nutshell:

 1 Process => 1 Interpreter => 1 GIL 

 __ Process ___________________________________________________         
|  _________               I/O              I/O                |
| |         |     lock     | |              | |     lock       |
| | Thread1 | ==========>  | |              | | ==========>    |
| |_________|              | |              | |                |
|  _________               | |              | |                |
| |         |              | |     lock     | |                |
| | Thread2 |              | | ==========>  | |                |
| |_________|              | |              | |                |
|______________________________________________________________|

I/O: something outside the interpreter domain;  while waiting for these external events,
the interpreter can switch and execute another thread.
lock: when the interpreter switch to a thread, GIL locks the interpreter and memory for that thread.
Despite the execution is slower, threads setup is faster ("light" processes)


## Multiprocessing in a nutshell:

n Processes => n Interpreters => n GILs

 __ (Parent) Process ___________________________________________
|  __________                                                   |
| |          |       lock                                       |
| | Process1 | =====================================>           |
| |__________|                                                  |
|  __________                                                   |
| |          |       lock                                       |
| | Process2 | =====================================>           |
| |__________|                                                  |
|_______________________________________________________________|                          

Each process has its own interpreter, memory share and lock. 
Despite the execution is faster, processes setup (instance generation) is slower.

The rigth tool for the job:
threads are useless in Python for parallel processing (CPU bound) but optimal for I/O bound cases.
Depending on what kind of concurrency is needed (tending toward I/O versus tending toward CPU)
multithreading or multiprocessing should be adopted.

In multithreading (multitasking), the way the threads take turn of execution (appearing to run at the same time)
allows differentiating further in pre-emptive multitasking and cooperative multitasking.

  • Pre-emptive: OS knows and anticipate each thread interrupting/starting different threads at any time.
  • Cooperative: Threads must cooperate by announcing when they are ready to be switched out.
      Concurrency Type       |   Number of CPU   |              Switching Decision                  |       Libs
_____________________________|___________________|__________________________________________________|___________________
                             |                   |                                                  |
    Pre-emptive multitasking |         1         |  OS decides when to switch tasks                 |  threading
    Cooperative multitasking |         1         |  Tasks decide when to give up control            |  asyncio
    Multiprocessing          |         N         |  N processes running at the same time on N CPUs  |  multiprocessing

In short:

if io_bound:
    if io_very_slow:
        print("Use Asyncio")
    else:
       print("Use Threads")
else:
    print("Use Multi Processing")
  • CPU Bound => multiprocessing
  • I/O Bound, Slow I/O, Many connections => asyncio
  • I/O Bound, Fast I/O, Limited Number of Connections => threading

asyncio

Specifically designed for server side network applications (spending a lot of time waiting for data to come in from the network).
It handles each client as a separate thread and uses coroutines to lightweight the threads burden (in terms of memory and resources).

  • coroutine: a function that can suspend its execution before reaching return, and can indirectly pass control to another coroutine for some time (use async and await syntax to return control to the event loop)
  • event loop: takes care of checking whether calls have completed and performing any subsequent tasks

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