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CPU usage is too high with Yosemite & an external monitor

I use Yosemite 10.10.4 on my MacBook Pro (2012).

I used to work with an external monitor: Apple Thunderbolt 27" monitor.

The issue is that, according to Activity Monitor, the kernel_task process consumes more than 600%of CPU even when no tasks are running! The computer becomes totally unusable.

The workaround is to disconnect the external monitor, but I really need this secondary screen to work.

Why didn't Apple fix this issue in the latest Yosemite update?

Is there a fix for this?

I'm having a similar issue with a 2011 13" MBP8,1 (2.7Ghz i7 CPU and HD3000 graphics), and to me it's pretty certain that it's provoked by video-intensive operations. Curiously, Hardware Monitor showed temperatures in the 70C range, quite far from the 90+C temperatures I've already seen while running big compile jobs.

I removed the bottom plate and removed far more very fine dust from the fan and its vents than I'd have expected, and blowing over the logic board dislodged even more. On the 13" model, the fan can be removed with just 3 screws after removing the bottom plate, so it's easy enough to clean (careful with the connector!). I haven't used the system much since, but 1st impressions were that the fan was much more effective again, and that the issue was gone.

Next steps will be running without the bottom plate and reducing my external screen's resolution from 1920*1080 to 1680*1080 (res. of my previous panel with which I never had problems, and a resolution that's clearly easier on the GPU for video playback).

I never understood why anyone would want to connect an external screen and NOT use the "internal" at the same time (saving battery? I just turn off the backlight in that case ...)

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I have a MacBook Pro Retina, 15-inch, Late 2013 2.3 GHz Intel Core i7 16 GB 1600 MHz DDR3 NVIDIA GeForce GT 750M 2048 MB Intel Iris Pro 1536 MB

With an external Sceptre X325BV monitor connected via HDMI. I was also seeing CPU percentages close to 500%.

I did three things that seem to have fixed this:

  1. I changed the laptop's retina monitor to be the main one.

  2. In System Preferences > Energy Saver, I unchecked "Automatic graphics switching."

  3. In System Preferences > Mission Control, I unchecked "Displays have separate Spaces."

Energy Saver Prefs Change Mission Control Prefs chnage

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This is a design feature documented by Apple. See if cooling your Mac and the processor can then speed up again and do the work you want. See this post on ideas for cooling while in clamshell mode.

You can now buy stands designed to cool your Mac.

If that doesn't work, try resetting the System Management Controller one time to ensure it's not a measurement error.

If you only need occasional cooling or to convince yourself, go ahead and open the clamshell and make sure there is good ventilation and you are not blocking the air inlet or outlet by the display hinges. The thermal management software reacts quite rapidly to changes in internal temperature. could even use Magnets to force the display to sleep while it's actually open and allowing the entire keyboard surface to be exposed and cooling the Mac that thinks it's still working in closed clamshell mode.

Activity Monitor may show that a process named kernel_task is using a large percentage of your CPU, and during this time you may notice a lot of fan activity. This process helps manage temperature by making the CPU less available to processes that are using the CPU intensely. In other words, kernel_task responds to conditions that cause your CPU to become too hot. When the temperature decreases, kernel_task automatically reduces its activity.

So, also quit the other tasks and apps you don't need if you can't make the physical arrangement of the Mac more cool or more conducive to radiating the heat away if convection isn't working for you. The GPU will cause lots of heat if it's pushing many pixels. Decreasing resolution might help in an edge case, but you'll want to attack cooling in general as a first step.

You could also get some advanced tools to peek into the CPU rate to confirm it's reacting to heat:

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