Cooling and Overclocking a Raspberry Pi 4 Safely: Measure, Then Tune

#!/usr/bin/env python3
# thermal_test.py - repeatable thermal test for a Raspberry Pi 4
# Usage: python3 thermal_test.py 600 baseline.csv   (seconds, output file)
import csv
import subprocess
import sys
import time

DURATION = int(sys.argv[1]) if len(sys.argv) > 1 else 600
OUTFILE = sys.argv[2] if len(sys.argv) > 2 else 'thermal.csv'

def vc(*args):
    """Return the value part of a vcgencmd reply, e.g. '61.3' from temp=61.3'C."""
    out = subprocess.check_output(['vcgencmd', *args], text=True).strip()
    return out.split('=')[1]

def sample():
    temp = float(vc('measure_temp').rstrip("'C"))
    mhz = int(vc('measure_clock', 'arm')) // 1_000_000
    flags = vc('get_throttled')
    return temp, mhz, flags

# Start stress-ng on all 4 cores in the background
load = subprocess.Popen(['stress-ng', '--cpu', '4', '--timeout', f'{DURATION}s'],
                        stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)

start = time.time()
max_temp, min_mhz = 0.0, 10_000
with open(OUTFILE, 'w', newline='') as f:
    w = csv.writer(f)
    w.writerow(['seconds', 'temp_c', 'arm_mhz', 'throttled'])
    while load.poll() is None:              # until stress-ng exits
        t, mhz, flags = sample()
        elapsed = int(time.time() - start)
        w.writerow([elapsed, t, mhz, flags])
        max_temp, min_mhz = max(max_temp, t), min(min_mhz, mhz)
        print(f'{elapsed:4d}s  {t:5.1f} C  {mhz:5d} MHz  {flags}')
        time.sleep(2)

print(f'Peak temperature: {max_temp:.1f} C, lowest clock: {min_mhz} MHz')
print('Final flags:', vc('get_throttled'), '(0x0 means no throttling)')