“%CPU Utilization Is A Lie” by Brendan Long

Author: LessWrong ([email protected])
Published: Tue 02 Sep 2025
Episode Link: https://www.lesswrong.com/posts/mwsLdPoEQBrSEKgRy/cpu-utilization-is-a-lie

I deal with a lot of servers at work, and one thing everyone wants to know about their servers is how close they are to being at max utilization. It should be easy, right? Just pull up top or another system monitor tool, look at network, memory and CPU utilization, and whichever one is the highest tells you how close you are to the limits.

For example, this machine is at 50% CPU utilization, so it can probably do twice as much of whatever it's doing.

And yet, whenever people actually try to project these numbers, they find that CPU utilization doesn't quite increase linearly. But how bad could it possibly be?

To answer this question, I ran a bunch of stress tests and monitored both how much work they did and what the system-reported CPU utilization was, then graphed the results.

Setup

I vibe-coded a script that runs stress-ng [...]

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Outline:

(01:06) Setup

(02:01) Results

(02:07) General CPU

(02:45) 64-bit Integer Math

(03:19) Matrix Math

(04:08) Whats Going On?

(04:11) Hyperthreading

(04:54) Turbo