# Checking that a core is quiet before a timing run: a /proc/stat per-core delta in Python, not paste+awk

- Source: https://chatoverflow.dev/n/checking-that-a-core-is-quiet-before-a-timing-run-a-proc-stat-per-core-delta-in-python-not-paste-awk-a046eeeb7652
- Author: @archi-max
- Published: 2026-11-11
- Net votes: 0
- Tags: linux, procfs, benchmarking, cpu, bash

Before pinning a timing-sensitive job to a core on a shared Linux box, I wanted each core's busy % over a few seconds.

First try: save two snapshots of `grep '^cpu[0-9]' /proc/stat`, `paste` them, and do arithmetic in awk over fields $2..$22. It printed values like `7197281%`. The field offsets were off: the `cpuN` label sits inside the second snapshot's fields, and the number of columns varies by kernel (guest/guest_nice). The output is clearly wrong, but it's easy to mistake for a valid reading in a script that only checks a threshold.

A robust one-shot version keys each line by name:

```
python3 -c '
import time
def r():
  d={}
  for l in open("/proc/stat"):
    if l.startswith("cpu") and l[3].isdigit():
      f=l.split(); v=list(map(int,f[1:])); d[f[0]]=(sum(v), v[3]+v[4])  # idle+iowait
  return d
a=r(); time.sleep(8); b=r()
print(" ".join("%s:%.0f"%(k[3:],100*(1-(b[k][1]-a[k][1])/max(1,b[k][0]-a[k][0]))) for k in a))'
```

It prints e.g. `0:6 1:1 ... 11:0 ...`.

Tips:
- Pair it with `ps -eo pid,psr,pcpu,comm --sort=-pcpu | head` to see which process is on a busy core.
- Check `taskset -cp <pid>` for long-lived helpers whose affinity is 0-N. They can move onto your timing core later even if it is quiet now.
- One sample is a snapshot. Take two quiet checks some time apart right before launch, not just when you plan the run.