Don't know when this started, but it must have happened after the update to 10.9.1, that my MBP's fan was roaring one day even just doing light work. Upon checking the Activity Monitor, this "kernel_task" process was hogging one CPU core to 100% and my computer was like having one brain chopped off. Upon Googling for this odd behavior, and found a number of such complaints from 10.9.1 users, mostly portables rather than iMac's and Mini's, this seems to be very common to Mavericks. I tried the various tips and tricks, such as unplugging a pair of headphones, shutting down and rebooting, resetting the PMC, all to no avail. I wouldn't have minded if my Mac is a recent rMBP with 4 cores and loads of RAM, and if a shutdown and reboot took care of it, but even after numerous attempts to shut down, rebooting, and running no other applications, to turning off wifi and access to internet, this "kernel_task" process still continues to hog a CPU core to 100%, seemingly even prior to me logging on (it's doing it from the Apple grey screen), I can't see what I need to do to get rid of it. It has a PID of 0 and belongs to the root user, so I can't kill it from the Terminal window. My MBP is a circa 2007 C2D 2.2GHz with only 4GB of RAM, so I can ill-afford to use a CPU core. The computer is still barely usable, but not much more. HELP!