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?
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?
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%
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.
my kernel_task issue (cpu at -> 500% for ever) was blutooth related:
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