What is the "spool time" of Intel Turbo Boost?

Question

Just like a turbo engine has "turbo lag" due to the time it takes for the turbo to spool up, I'm curious what is the "turbo lag" in Intel processors.

For instance, the i9-8950HK in my MacBook Pro 15" 2018 (running macOS Catalina 10.15.7) usually sits around 1.3 GHz when idle, but when I run a CPU-intensive program, the CPU frequency shoots up to, say 4.3 GHz or so (initially). The question is: how long does it take to go from 1.3 to 4.3 GHz? 1 microsecond? 1 milisecond? 100 miliseconds?

I'm not even sure this is up to the hardware or the operating system.

This is in the context of benchhmarking some CPU-intensive code which takes a few 10s of miliseconds to run. The thing is, right before this piece of CPU-intensive code is run, the CPU is essentially idle (and thus the clock speed will drop down to say 1.3 GHz). I'm wondering what slice of my benchmark is running at 1.3 GHz and what is running at 4.3 GHz: 1%/99%? 10%/90%? 50%/50%? Or even worse?

Depending on the answer, I'm thinking it would make sense to run some CPU-intensive code prior to starting the benchmark as a way to "spool up" TurboBoost. And this leads to another question: for how long should I run this "spooling-up" code? Probably one second is enough, but what if I'm trying to minimize this -- what's a safe amount of time for "spooling-up" code to run, to make sure the CPU will run the main code at the maximum frequency from the very first instruction executed?

I think it partly depends on the energy_performance_preference (or ..._bias) setting you (or your OS) sets for the CPU's hardware frequency selection. (Linux Q&A: What are the implications of setting the CPU governor to "performance"?). But yes, well under 1 ms on Skylake-derived CPUs like yours which let the CPU choose its own frequency, maybe 10s of microseconds with an aggressive EPP setting. — Peter Cordes
Note that turbo frequency isn't the only kind of "warm up" needed for some benchmarks: if your benchmark touches an array, touching it first before your timed run is a good idea; the first access will cause page faults. See Idiomatic way of performance evaluation? for more — Peter Cordes

swineone swineone · Accepted Answer · 2020-10-08T01:18:21

I wrote some code to check this, with the aid of the Intel Power Gadget API. It sleeps for one second (so the CPU goes back to its slowest speed), measures the clock speed, runs some code for a given amount of time, then measures the clock speed again.

I only tried this on my 2018 15" MacBook Pro (i9-8950HK CPU) running macOS Catalina 10.15.7. The specific CPU-intensive code being run between clock speed measurements may also influence the result (is it integer only? FP? SSE? AVX? AVX-512?), so don't take these as exact numbers, but only order-of-magnitude/ballpark figures. I have no idea how the results translate into different hardware/OS/code combinations.

The minimum clock speed when idle in my configuration is 1.3 GHz. Here's the results I obtained in tabular form.

+--------+-------------+
| T (ms) | Final clock |
|        | speed (GHz) |
+--------+-------------+
| <1     | 1.3         |
| 1..3   | 2.0         |
| 4..7   | 2.5         |
| 8..10  | 2.9         |
| 10..20 | 3.0         |
| 25     | 3.0-3.1     |
| 35     | 3.3-3.5     |
| 45     | 3.5-3.7     |
| 55     | 4.0-4.2     |
| 66     | 4.6-4.7     |
+--------+-------------+

So 1 ms appears to be the minimum amount of time to get any kind of change. 10 ms gets the CPU to its nominal frequency, and from then on it's a bit slower, apparently over 50 ms to reach maximum turbo frequencies.

What is the "spool time" of Intel Turbo Boost?

2 Answers