Interview question

What is the Global Interpreter Lock (GIL) and why does it matter? Global Interpreter Lock (GIL) क्या है और यह क्यों important है?

Answer

The GIL is a mutex in CPython that allows only ONE thread to execute Python bytecode at a time, even on multi-core machines. It exists to make CPython's memory management simpler and thread-safe.

import threading
import time

def cpu_bound_task():
    count = 0
    for i in range(50_000_000):
        count += 1
    return count

# Running 2 threads for a CPU-bound task
start = time.time()
t1 = threading.Thread(target=cpu_bound_task)
t2 = threading.Thread(target=cpu_bound_task)
t1.start(); t2.start()
t1.join(); t2.join()
print(f'Threaded: {time.time() - start:.2f}s')
# Due to the GIL, this does NOT run twice as fast on a multi-core CPU -
# threads take turns holding the GIL, so CPU-bound work barely benefits

# Single-threaded equivalent runs at similar or even better speed
start = time.time()
cpu_bound_task()
cpu_bound_task()
print(f'Sequential: {time.time() - start:.2f}s')
# Often comparable to the threaded version, because of GIL contention overhead

Workload type	GIL impact
CPU-bound (heavy computation)	Threads don't help - GIL serializes execution
I/O-bound (network, disk, waiting)	Threads DO help - GIL is released during I/O waits

GIL CPython में एक mutex है जो एक समय में सिर्फ एक thread को Python bytecode execute करने देता है, multi-core machines पर भी। यह CPython की memory management को simpler और thread-safe बनाने के लिए है।

import threading
import time

def cpu_bound_task():
    count = 0
    for i in range(50_000_000):
        count += 1
    return count

start = time.time()
t1 = threading.Thread(target=cpu_bound_task)
t2 = threading.Thread(target=cpu_bound_task)
t1.start(); t2.start()
t1.join(); t2.join()
print(f'Threaded: {time.time() - start:.2f}s')
# GIL के कारण multi-core पर दोगुना तेज़ नहीं होता

start = time.time()
cpu_bound_task()
cpu_bound_task()
print(f'Sequential: {time.time() - start:.2f}s')
# अक्सर threaded version के बराबर या बेहतर

Workload type	GIL का असर
CPU-bound	Threads मदद नहीं करते
I/O-bound	Threads मदद करते हैं - I/O wait में GIL release होता है

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