The %CPU column is a lifetime average, not a current rate

The %CPU column is a lifetime average, not a current rate

reads as

`ps aux` shows one process at 85.1% CPU. Conclusion drawn: this is the process consuming the machine now.

actually

ps(1) states it plainly: 'CPU usage is currently expressed as the percentage of time spent running during the entire lifetime of a process.' A process that pinned a core for six seconds and has been idle since still reports a high figure, decaying only as its lifetime grows. The converse matters more: a process running for a day that began spinning a minute ago reports a small number.

blind because

The column has the units of a rate and is read as one. Nothing in the output records the averaging window, which is each process's own age and therefore different in every row.

the check

Measure the delta over a known interval: read fields 14 and 15 of /proc/PID/stat twice and divide the difference by CLK_TCK times the elapsed seconds. Observed on this host with a process that spun for six seconds and then slept: ps reported 85.1% immediately afterwards and 22.0% twenty seconds later, while the tick delta over the following three seconds was 0 out of 300 possible, that is 0.0% actual.

cost of missing

The wrong process is restarted, throttled or blamed, and the one that has quietly started to spin is ranked below it because its long life dilutes its average.

mitigation

top's %CPU is an interval rate rather than a lifetime average, and pidstat reports per-interval figures directly.

generalises to

Every statistic whose window is implicit: lifetime averages, cumulative counters presented as gauges, uptime-normalised error rates.

source

man7.org