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I recently ordered two 4K monitors (LG 27UD88). I connected them using Mini DisplayPort to DisplayPort Adapter cable to my MacBook Pro.

When I have the monitors connected, the CPU load is very high with fans running at 5500-6000 RPM and a processor temperature of 54°C. This does not seem healthy to me. It's kernel_task which is using absurd amounts of CPU (sometimes spikes up to 600%)

Things I've tried so far

  1. Before I used to have a high CPU load as well (with the additional monitors connected), but that got resolved after following the instructions in this article, CPU usage is too high with Yosemite and an external monitor, kernel_task spikes to 600%+.

  2. Disabling X86PlatformShim.kext

  3. Resetting the SMC.

This all seems very odd to be. I have a maxed out MacBook Pro which can't handle two monitors?

Here is the system configuration:

MacBook Pro (Retina, 15-inch, Mid 2015)
Processor: 2,8 GHz Intel Core i7
Memory: 16 GB 1600 MHz DDR3
Graphics: AMD Radeon R9 M370X 2 GB
          Intel Iris Pro 1536 MB
Storage: 500GB Flash Storage

Information from sensors:

enter image description here

UPDATE #1:

Here is the information on the GPU load. It seems that the GPU load is high, but that the processor of the AMD Radeon is not used. I'm currently looking into this, if you have any suggestions on how to make more use of the AMD Radeon GPU please do.

enter image description here

I do have Automatic graphics switching turned off (this should increase high-performance graphics).

Also the system preferences tell that the AMD Radeon is being used for the external display. Though the iStat data suggests otherwise.

enter image description here

Update #2:

I've installed gfxCardStatus and put it to discrete only. Looks like the AMD Radeon GPU is actively being used. This does not make a difference.

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Update #3:

I've ran iStat for a while now. Looks like the CPU has many spikes, even up to 90 degrees. This would obviously trigger the fans to start blowing (also the MacBook itself feels hot near the processor). Looks like the average temp at the moment is 70 degrees.

enter image description here

Here is the CPU frequency:

enter image description here

  • 1
    My first instinct is it's the GPU heating up and that doesn't sound too unreasonable with 2x 4k monitors. I should say that I am just guessing here... – Joonas May 9 '18 at 6:45
  • 1
    @Joonas Could well be. Still seems odd to me since I have a AMD Radeon R9 M370X with 2GB – Melvin Koopmans May 9 '18 at 6:47
  • What brand/model are the two 4K displays? – Monomeeth May 9 '18 at 9:24
  • @Monomeeth LG-27UD88. I will add it to the question as well. – Melvin Koopmans May 9 '18 at 9:27
  • 2
    IIRC, it’ll be a struggle for that rMBP to run 2 4K displays. One thing is it might not be using the discreet GPU automatically if you’re not using a GPU-heavy app. Try going to System Preferences -> Energy Saver and disable Automatic Graphics Switching. Does this improve the computer’s performance? Note this will increase power usage and thus decrease battery life. – NoahL May 9 '18 at 13:20

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