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Is there a way to log the system activities, such as errors or cpu usage data, before user login?

I'm using macOS Big Sur 11.0.1 on MacBook Pro 16 inch - 2019, and I found the machine is hot and the fan runs at high speed at the login screen, indicating a high CPU usage. This situation continues forever until I logged in, at which time the fan slows down significantly, and the cpu usage is at normal level (below 3%). So everything is fine once logged in. I guess there should be some abnormal process(es) consuming huge amount of cpu when at the login screen, but I couldn't find any logs during that period because the system log I found starts only after login.

Any helpful information/clues will be appreciated!

Update:

I've tried these steps: -- reset SMC -- reset NVRAM -- enter safe mode -- reinstall macOS. Since none of these work, given that everything goes pretty well after login, I guess it's very likely to be caused by a software bug. If anyone could try to replicate the behaviour and comment the observations below, I will appreciate it very much! That will be very helpful to help confirm whether it is a bug.

To replicate the behaviour:

  1. Restart. Don't enter any password.

  2. Stay at the login screen. Within 2 or 3 mins, the fan should start. Within another 2 or 3 minutes, the fan could reach full speed because of the heat.

  3. The fan should slow down quickly after login.

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You should've waited a little longer for Apple to get their shit together with Big Sur imho, I would roll back to Catalina for another few months while they iron out the Big Sur. Better post this in Apple forums. Also, check this out: https://discussions.apple.com/thread/252033083

After every major upgrade there are processes that occur in the background such as new indexes and databases for such things as Photos or Search. These take time and the system needs to be up and running to resolve it. Try setting system NOT to log out or shut down for a day or so and you should see improvement.

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  • Thank you for the info. The post is helpful. That's a very similar issue, but slightly different from mine (my mac doesn't heat up after login or during idle. It only heats up at the login screen, before any password entered)
    – user12448123
    Commented Nov 24, 2020 at 22:51

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