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I have a Dell Optiplex 7070 running Ubuntu 18.04.5 LTS, with 2 monitors connected via DisplayPort.

Under normal operation (RDC, Firefox, Video Streaming), both monitors remain connected and the CPU temperature (reported by coretemp-isa-0000) is in the high 40s/low 50s:

coretemp-isa-0000
Adapter: ISA adapter
Package id 0:  +50.0°C  (high = +86.0°C, crit = +100.0°C)
...
$ xrandr -q 
Screen 0: minimum 320 x 200, current 3840 x 1080, maximum 16384 x 16384
DP-1 connected primary 1920x1080+1920+0 (normal left inverted right x axis y axis) 598mm x 336mm
   1920x1080     60.00*+  50.00    59.94  
...
HDMI-1 disconnected (normal left inverted right x axis y axis)
DP-2 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 598mm x 336mm
   1920x1080     60.00*+  50.00    59.94  
...
HDMI-2 disconnected (normal left inverted right x axis y axis)
...

But when I have zoom meetings, I observe an increase in CPU temp up to the mid 60s to high 70s, and one of the monitors turns off.

Temperature Observation:

coretemp-isa-0000
Adapter: ISA adapter
Package id 0:  +67.0°C  (high = +86.0°C, crit = +100.0°C)

I can see from xrandr that DP-2 has been disconnected:

$ xrandr -q 
Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 16384 x 16384
DP-1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 598mm x 336mm
   1920x1080     60.00*+  50.00    59.94  
...
HDMI-1 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)
...

My assumption is that some sort of temperature cutoff is being triggered, but I havent been able to identify the process/mechanism thats making that decision. I dont see any event in syslog, other than gdm-x-session reacting to DP-2 being disconnected and reprinting the available Modelines.

How can I identify the mechanism that's causing DP-2 to disconnect? How can I understand the thresholds its set at?

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    The temperature seems fine and shouldnt be the cause of your problems. Does it exclusively happen when you use Zoom ? Have you tried load testing your CPU with something like Prime95 to observe similar issues ? Could you try a livecd of a more recent dist and see if the problem persists ? – Silbee May 28 at 7:35
  • Not exclusively during Zoom, although it happens almost always when a zoom video call is in progress. Anecdotally, it seems like I can induce it to happen by playing fast-changing videos full-screen, but it seems strongly correlated to temperature crossing 60C. I will try testing the load another way, and with a 20.04 live boot to see what that shows - thanks – user2242865 May 28 at 12:58
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    The typical test for thermal problems would be to open the computer case and let it run with a big fan placed before the open case. It it is a thermal problem it might not be controlled by Linux, instead a chip may internally decide to power down because of high temperature. Then you would only see the effect but not the trigger as the temp sensor is not accessible to Linux. – Robert May 28 at 13:14
  • Good call - I popped the case off and have a few zoom calls lined up so I will observe what happens. I think you put your finger on it - the frustrating thing is not having a clear indicator of why the switchoff happens. – user2242865 May 28 at 15:02

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