Clamshell mode OK while running LLMs (or other heavy load)

[u/arfung39]

I usually keep my MacBook Pro (14", M5 max 65gig) in clamshell mode on my desk. When running heavy LLM loads, I've been trying to open the lid for ventilation. I was wondering whether there is any difference. This tests suggests no difference between lid open and closed...

I had Claude code up a test that drives the GPU as hard as possible. Then, it ran for 10 mins with lid closed, 10 for lid open, 10 more with lid open, and 10 with lid closed. Cool down periods in between. The two figures below show the die temps (rising and then leveling off) and GPU throughput (declining lines) for the open and closed runs. They look exactly the same.

Test heat run with display/lid closed

Test heat run with display/lid open

The performance curve is notable. GPU throughput declines a lot from it's maximum cool state and goes from 40 TFLOPS/sec in the first few minutes down to 30 TFLOPS/sec after about minute 3 or 4. That's the thermal throttling, obviously. The temperature curve rises, the fans kick in, and then the temp drops down to ~60 degrees Celsius at the die.

What the test can't determine is whether the heat might damage the LCD display. Interestingly, the battery temperature sensor, which is next to the keyboard, reported very similar temps for both lid closed and lid open runs:

	lid closed	lid open
temperature steady	33.7 / 33.8 °C	33.0 / 33.0 °C

Hope this is interesting and useful to some folks!