Fun data point though, I just ran three data points of the Phoronix nginx benchmark and got these results:
- Pinned to 6 cores: 28k QPS
- Pinned to 12 cores: 56k QPS
- All 24 cores: 62k QPS
I'm not sure how this applies to realistic workloads where you're using all of the cores but not maxing them out, but it looks like hyperthreading only adds ~10% performance in this case.
Yeah, the obvious thing with processors is to do something similar:
(1) Measure MIPS with perf (2) Compare that to max MIPS for your processor
Unfortunately, MIPS is too vague since the amount of work done depends on the instruction, and there's no good way to measure max MIPS for most processors. (╯°□°)╯︵ ┻━┻
The advantage of stress-ng is that it's easy to make it run with specific CPU utilization numbers. The tests where I run some number of workers at 100% utilization are interesting since they give such perfect graphs, but I think the version where I have 24 workers and increase their utilization slowly is more realistic for showing how production CPU utilization changes.
Some esoteric methods of measuring CPU utilizations are to calculate either the current power usage over the max available power, or the current temperature over the max operating temperature. Unfortunately these are typically even more non-linear than the standard metrics (but they can be useful sometimes).
Thanks for the feedback. I think you're right, so I changed a bunch of references and updated the description of the processor to 12 core / 24 thread. In some cases, I still think "cores" is the right terminology though, since my OS (confusingly) reports utilization as-if I had 24 cores.
A big part of this is that CPU utilization metrics are frequently averaged over a long period of time (like a minute), but if your SLO is 100 ms, what you care about is whether there's any ~100 ms period where CPU utilization is at 100%. Measuring p99 (or even p100) CPU utilization can make this a lot more visible.
To be fair, in most of these tests hyperthreading did provide a significant benefit (in the general CPU stress test, the hyperthreads increased performance by ~66%). It's just confusing that utilization metrics treat hyperthread usage the same as full physical cores.
At 51% reported CPU utilization, it's doing about 80% of the maximum requests per second, and it can't get above 80% utilization.
I also added a section: https://www.brendanlong.com/cpu-utilization-is-a-lie.html#bo...