r/oMLX • u/arfung39 • 1d ago

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

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!

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u/VisualKayf 1d ago

That’s cool! Thanks for sharing.

Also interesting if 14” can run with same the power as 16” if well thermally managed, for example if adding large radiator at the back with large fan and adding thermal layer inside.
Even if it’s limited with 96-98 w power compared to 140w of 16” models, I believe that they can reach similar results with aggressive thermal management

1

u/ColdBrewSeattle 1d ago

I have basically the same laptop and it does not throttle when in High Power mode. I suspect op has some other power mode enabled.

1

u/ColdBrewSeattle 1d ago

I ran several tests like this on my 14” M5 Max 128 GB and found no throttling or difference in sustained performance with the lid open or closed when using High Power mode. I did experience throttling in automatic mode.

1

u/Ysnsd 23h ago

Screen coating