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Comment by scottlamb | Hacker News Reader

> I know it’s thermal throttling because I can see in iStat Menus that my CPU usage is 100% while the power usage in watts goes down.

There's another possibility. If your battery is low and you've mistakenly plugged it into a low-power USB-C source (phone charger), you will also see 100% CPU usage, low power usage, and terrible performance. Probably not the author's problem, but it's been mine more than once! It might be worth adding something to detect this case, too. You can see your charger power under "System Information"; I assume there's an API for it also.

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I have an M1 MacBook Air and do a once weekly virtual D&D session with some friends. I hook it up to my 4K monitor, and I assumed it had to do with that. It kept becoming a slideshow (unless I put an ice pack under it!) and I realized it’s because the battery life is so good it’s the only time of the week I charge the thing, so charging the battery was making the poor laptop a hot mess that was thermal throttling like crazy. This is with a nice dock that can push around 100W, so it isn’t necessarily an underprovisioned charger.

I started charging it an hour or two before our session, and the issues stopped.

Try cabinet fans under it, will be life changing.

While I have definitely done this a few times, one of my MacBooks could draw more power than the power supply could deliver and there was a particular computer game that I discovered I could “only” play for about five hours before the laptop shut itself off. Because it was having to draw supplemental power from the batteries to keep up.

IIRC the next generation of MacBook was the one that came with the larger power brick, which didn’t at all surprise me after that experience. Then they switched to GaN to bring the brick size back down.

> I know it’s thermal throttling because I can see in iStat Menus that my CPU usage is 100% while the power usage in watts goes down.

When I read this I wondered "Why isn't core temperature alone not a reliable indicator of thermal throttling?". Isn't that the state variable the thermal controller is directly aiming to regulate by not letting it exceed some threshold?

My M4 Max Macbook Pro can run for a while at like 105°C and fans to the max before throttling, when it starts throttling it doesn't exceed that threshold, and then the temperature goes down for a while before throttling stops

Interesting, yeah iStat Menus reports the wattage of the charger, sometimes I've charged my mac with like a 5 or 10W charger and I didn't have that issue But now that rings a bell, I think a coworker had that issue recently. I wonder why that happens

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