Đa luồng & đa tiến trình

race.py - bug LATENT (tiềm ẩn)

import threading
counter = 0
def inc(n):
    global counter
    for _ in range(n):
        counter += 1          # doc -> cong -> ghi: KHONG nguyen tu

ts = [threading.Thread(target=inc, args=(200_000,)) for _ in range(4)]
[t.start() for t in ts]; [t.join() for t in ts]
print(counter)               # ky vong 800000 - nhung co the it hon

800000   # ... hoac 799_xxx - THAT THUONG

lock.py

import threading
counter = 0
lock = threading.Lock()

def tang(n):
    global counter
    for _ in range(n):
        with lock:            # vung gang: moi luc chi 1 luong vao
            counter += 1

ts = [threading.Thread(target=tang, args=(200_000,)) for _ in range(4)]
[t.start() for t in ts]; [t.join() for t in ts]
print(counter)               # LUON 800000 (on dinh)

800000

mp.py - đo tăng tốc CPU-bound

import time
from concurrent.futures import ProcessPoolExecutor

def nang(n):
    s = 0
    for i in range(n): s += i * i
    return s

if __name__ == "__main__":            # bat buoc tren Windows/macOS
    N = 20_000_000
    t = time.perf_counter(); [nang(N) for _ in range(4)]; seq = time.perf_counter() - t
    t = time.perf_counter()
    with ProcessPoolExecutor(max_workers=4) as ex:
        list(ex.map(nang, [N] * 4))
    par = time.perf_counter() - t
    print(f"seq={seq:.2f}s  4 tien trinh={par:.2f}s  -> {seq/par:.1f}x")

seq=3.38s  4 tien trinh=1.06s  -> 3.2x

queue_demo.py - producer/consumer

import threading, queue

viec = queue.Queue()
ket_qua = queue.Queue()

def tho():                          # nhieu tho cung lay viec
    while True:
        x = viec.get()
        if x is None: break         # tin hieu dung
        ket_qua.put(x * x)
        viec.task_done()

thos = [threading.Thread(target=tho) for _ in range(3)]
[t.start() for t in thos]
for i in range(10): viec.put(i)
viec.join()                         # cho lam het
for _ in thos: viec.put(None)       # bao dung
[t.join() for t in thos]
print(sorted(ket_qua.queue))

[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

futures.py

from concurrent.futures import ThreadPoolExecutor   # I/O-bound
# from concurrent.futures import ProcessPoolExecutor # CPU-bound: doi DUNG dong nay

def tai(url):
    ...                      # goi mang (I/O-bound)
    return url

urls = ["a", "b", "c", "d"]
with ThreadPoolExecutor(max_workers=4) as ex:
    ket_qua = list(ex.map(tai, urls))   # giu thu tu; hoac ex.submit -> future