iMac works with ONE external display, but CRASHES with TWO!

[thefriendshipmachine]

My iMac works great with one external 4k monitor, but when I try and use two, then once or twice a day the entire screen will freeze, and if I leave it then the machine will show a kernel panic and reboot, or just reboot on its own. I have a:

iMac (Retina 5K, 27-inch, 2019)
Radeon Pro Vega 48 8 GB

things I tried:

1. Re-installing MacOS
2. Changing out the RAM sticks to see if one was faulty
3. Trying different external monitors
4. Trying different connectors (thunderbolt, HDMI, DP)

Unfortunately it's still happening. Sometimes it happens when the machine is in use, sometimes when it's sleeping, and I try and wake it and it doesn't wake. It seems completely random 😢

 

[CMMChris]

Please post the panic report. If it's watchdog timeout, this issue happens with a couple of AMD GPUs and started with the introduction of support for the first RDNA1 based GPUs. It has been reported to Apple by a lot of people including me but has never been entirely eliminated by Apple until today.

 

[Realityck]

My iMac works great with one external 4k monitor, but when I try and use two, then once or twice a day the entire screen will freeze, and if I leave it then the machine will show a kernel panic and reboot, or just reboot on its own. I have a:

iMac (Retina 5K, 27-inch, 2019)
Radeon Pro Vega 48 8 GB

things I tried:

1. Re-installing MacOS
2. Changing out the RAM sticks to see if one was faulty
3. Trying different external monitors
4. Trying different connectors (thunderbolt, HDMI, DP)

Unfortunately it's still happening. Sometimes it happens when the machine is in use, sometimes when it's sleeping, and I try and wake it and it doesn't wake. It seems completely random 😢

regarding

2. Changing out the RAM sticks to see if one was faulty

Go to https://www.memtest86.com/download.htm

Install the free memory test software called MemTest86 on a USB thumb drive, use Option during boot to select and launch into this memory testing environment. Now allow it to run the normal batch of tests. It will diffidently identify memory that might have errors when ran at full speed, to some memory pattern that it fails unlike Apple diagnostics provide because it does a very quick test comparably. Select the Image for creating bootable USB Drive for a Mac.

Only for x86 Macs.

 

[thefriendshipmachine]

Please post the panic report. If it's watchdog timeout, this issue happens with a couple of AMD GPUs and started with the introduction of support for the first RDNA1 based GPUs. It has been reported to Apple by a lot of people including me but has never been entirely eliminated by Apple until today.

 

[thefriendshipmachine]

Please post the panic report. If it's watchdog timeout, this issue happens with a couple of AMD GPUs and started with the introduction of support for the first RDNA1 based GPUs. It has been reported to Apple by a lot of people including me but has never been entirely eliminated by Apple until today.

And here's some watchdogs from when it happened:

 

[CMMChris]

Yes, it's well known window server freeze watchdog timeout issue that has been introduced in macOS Catalina 10.15.1. This issue can be randomly triggered by several things. Either multiple screens or encoding / decoding video on the GPU after sleep. It's a shame Apple still hasn't fixed this after such a long time. Open a bug report with Apple (which will join hundreds of other reports with no reaction from Apple). Nothing more you can do about that unfortunately.

 

[thefriendshipmachine]

Yes, it's well known window server freeze watchdog timeout issue that has been introduced in macOS Catalina 10.15.1. This issue can be randomly triggered by several things. Either multiple screens or encoding / decoding video on the GPU after sleep. It's a shame Apple still hasn't fixed this after such a long time. Open a bug report with Apple (which will join hundreds of other reports with no reaction from Apple). Nothing more you can do about that unfortunately.

Ah really? Would an eGPU fix it?

 

[CMMChris]

I don't know. Feel free to try it if you have one. Would be a waste of energy tho using an eGPU just run a second screen without crashes. Maybe a DisplayLink Dongle would be a more economic alternative.

 

[thefriendshipmachine]

I don't know. Feel free to try it if you have one. Would be a waste of energy tho using an eGPU just run a second screen without crashes. Maybe a DisplayLink Dongle would be a more economic alternative.

That's a good idea, I'll get one and see. Here's more crash logs. Are you certain it's a common error lots of folks get? I'm still debating if I should ask apple to swap out my logic board.

 

[CMMChris]

Sure, if it's still an option for you to get a free logic board replacement, try your luck with Apple. I can't 100% assure you that your panic has the same cause as other window server panics with watchdog timeout but it certainly looks similar and you have a GPU family that is affected by this driver bug. That being said, it could just as well be a hardware issue with your machine. If Apple is willing to replace it, try your luck. I guess it's better to play this option before trying any workarounds.