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Typically, my kernel_task process uses 1-4% CPU and 100-700MB of memory. However, sometimes when I have the power adapter plugged in, the CPU usage jumps to around 500% (8 cores) and stays there until I unplug the power adapter. I can plug it back in and everything is fine for a few seconds or a few days, then it eventually happens again. How can I stop this from happening?

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    Does/did this only happen when an external monitor is being used? I find I get overheating when I extend out to a second monitor. – Nic Cottrell Sep 14 '16 at 18:30
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In my experience this has always been due to a hardware fault. However before doing anything else reset the SMC as detailed here: http://support.apple.com/kb/HT3964 as its power related.

This appears to happen when the Mac is struggling to communicate with hardware. Which Mac is it? How old is the battery? Is your battery life still good? Has the Mac had any damage - dropped? Liquid? Have you tried a different power adapter?

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    MBP 2011, as old as the laptop, yes, no, no, yes. – Sparr Nov 25 '12 at 2:13
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I got this issue once on my 15-inch Mid 2012 MBP running 10.8.4 because the audio input had switched to the internal microphone and it was picking up the constant noise of my fan. Switched the audio input to Line In and cpu usage dropped from 524% to 3%

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    Disabling ambient noise cancelling seems to make it go away for me, in addition to removing power adapter (for now), will see if it continues to stay fixed. – ffledgling Apr 19 '15 at 12:58
  • What this during FaceTime or Skype or did this happen all th etime? – Nic Cottrell Sep 14 '16 at 18:24
  • This saved my life. I disabled ambient noise cancelling and it worked. Perfect storm of: plug in AC adapter, fan goes high to help with heat control, microphone software goes mad trying to cancel the noise of the fan. @nic-contrell, this happened all the time, even when playing a game that didn't need the microphone. – WattsInABox Dec 26 '18 at 5:29
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http://www.rdoxenham.com/?p=259 covers disabling the kernel extension responsible for triggering this misbehavior. However, doing so also disables some other thermal management functionality.

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    This is interesting, but you have to ask what is causing the problem in the first place. Something isn't right. As I said, everytime I've seen this, there has been a hardware fault. – Justsomeguy Nov 25 '12 at 2:46
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    For me this analysis doesn't explain correctly the origin of the problem. Moreover, I'm deeply convinced through other cases that the overheating and the kernel_task eating all the CPU are just correlated consequences of the same causing problem. Stupid loops within a kernel can only lead to overheating. – dan Aug 7 '13 at 15:43
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my kernel_task issue (cpu at -> 500% for ever) was blutooth related:

http://droolfactory.blogspot.com.es/2012/08/mac-os-x-lion-kerneltask-with-high-cpu.html

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    You should provide at least a short summary of the article you are referencing. The goal is to make Ask Different a self-sufficient resource. – Jawa Aug 8 '13 at 6:44
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Please have a look at this typical problem: kernel_task at 300% and the suggested method of analysis: diagnosis of a CPU hog.

The Activity Manager isn't acurate enough to help you see that there are other processes indirectly loading the Kernel. top will unveil these hogs.

To display processes sorted starting with the biggest CPU consummer:

top -o cpu

...sorted starting with the biggest virtual memory consummer:

top -o vsize

...sorted starting with the process making the more context switches (being started and stopped with a huge frequency, sometimes the result of 2 processes talking together, not letting the other processes the freedom to get the cpu):

top -o csw
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    How do you use top to unveil these hogs? – Erik B Oct 28 '13 at 10:03
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The folks who observed "AC power" contributes are on to something.

High chassis temperature from charging together with peripherals plugged in can cause this issue. Simply moving the charging cable from the left to the right ports can be enough to cool the hotspot and resolve the problem. On a machine with MagSafe try unplugging peripherals from the left ports until the battery is full.

CPU usage has nothing to do with this. A hot CPU is throttled by reducing its clockspeed not by scheduling no-op load.

See https://apple.stackexchange.com/a/363933/27135 for causal proof.

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