How to Monitor Process RSS Memory in Python

import os
import time
import subprocess
import sys

def get_rss_mb(pid):
    """Return RSS memory in MB for a given process ID."""
    try:
        with open(f"/proc/{pid}/status", "r") as f:
            for line in f:
                if line.startswith("VmRSS:"):
                    return int(line.split()[1]) / 1024  # kB to MB
    except (FileNotFoundError, ProcessLookupError):
        return None
    return None

def monitor_process(pid, interval=1.0, threshold=100.0, max_cycles=10):
    """Watch RSS of a process and alert if it grows beyond threshold."""
    print(f"Monitoring PID {pid} every {interval}s (threshold: {threshold} MB)")
    previous_rss = None
    for cycle in range(max_cycles):
        rss = get_rss_mb(pid)
        if rss is None:
            print(f"Cycle {cycle+1}: Process {pid} no longer exists.")
            return
        growth = ""
        if previous_rss is not None:
            delta = rss - previous_rss
            growth = f" (Δ{delta:+.1f} MB)"
        print(f"Cycle {cycle+1}: RSS = {rss:.1f} MB{growth}")
        if rss > threshold:
            print(f"ALERT: RSS exceed threshold {threshold} MB")
            return
        previous_rss = rss
        time.sleep(interval)

if __name__ == "__main__":
    if len(sys.argv) > 1:
        target_pid = int(sys.argv[1])
    else:
        # Use another instance of this script as a demo target
        child = subprocess.Popen([sys.executable, "-c", "import time; time.sleep(15)"])
        target_pid = child.pid
        print(f"Demo target spawned with PID {target_pid}")
    monitor_process(target_pid, interval=0.5, threshold=200.0, max_cycles=5)