Geeking Out

MacBook Air overheating with OS X Lion and FileVault

I noticed today that my Mid-2009 work-provied MacBook Air, which I recently freshly installed with Lion, was generating a ton of heat, and operating really slowly. I noticed that the kernel_task process was using between 130% and 150% CPU, and the CPU meter was completely full. Meanwhile, no other processes were using a significant amount of RAM and CPU (in fact, I have barely installed anything on this machine yet). I tried quitting all of my applications, unplugging all peripherals, and disabling wifi with no change. I rebooted with no change. Even researching the problem online was highly frustrating because things were going so slowly.

After reading a variety of message board postings I came to the conclusion that the kernel_task high CPU usage was a symptom, not a cause. The machine was overheating due to a combination of very high temperatures (high-90s F) and being seated on a surface that did not allow it to dissipate heat effectively. The kernel_task was running, apparently, to keep other processes from using the CPU.

Strange as this seemed, there was an easy test — I plopped the laptop, now running nothing else, in front of an air conditioner and hit it full blast. Within a few minutes the kernel_task process dropped to 70-80% CPU usage and things became responsive again. But even though the laptop was cool to the touch, the process never went down from there.

My next thought was that perhaps the new FileVault full-disk encryption was playing a role, since it was one of the only things running once I had disabled all third-party processes and quit all apps. So I set the hard drive to decrypting, which took only 20-30 minutes on SSD. Sure enough, as soon as it completed, the kernel_task processor usage dropped to almost nothing.

I’m not going to repeat the experiment to verify the results, so use this as one data point only. But if you are experiencing similar behavior, first cool the machine and move it to a hard, flat surface. That should alleviate most of the symptoms.

4 replies on “MacBook Air overheating with OS X Lion and FileVault”

  1. I suppose the new FileVault implementation is a dot-oh release, but still, that’s pretty surprising… good sleuthing. I’ll probably wait until 10.7.1 to muck about with FileVault on my current (aftermarket SSD mid-2010) MBP, just to be safe.

  2. Up until this problem I was running fine with FileVault on Lion DP3 in an office environment for a few weeks. It was only when I got to my hot non-air-conditioned home and was not running it on a solid table that I had problems. I’m also beginning to suspect that the fan in this particular machine may be faulty. So like I said, one data point, not a conclusion.

  3. I had the same issue, but was not running FileVault. I did however have my MBA on the desk with the cover closed connected to an external monitor, KB and TP. I slapped a Targus cool pad under it (Model AWE01US) and soon the CPU usage was minimal again and the Kernel_Task was down to 2% or so from 170.

    I thought it was a Lion problem, as I had just installed 10.8 on the computer, but I eventually figured out that it was just a heat thing since I never used it on the desk like this before (My iMac is in the shop getting a new video card…)

  4. I actually am seeing this problem with my Mac Pro. It’s about 3 years old and over the last 6 months-ish, I’ve noticed kernel_task climbing a lot. I tried just blowing the thing out and made sure that all pathways were clear, but no dice. It’s been even more prevalent with Lion – I thought that, perhaps after years of installing and uninstalling stuff the Tiger-then-Leopard-then-SL kernel was just messed up.

    I got smcFanControl and installed it when I got Lion. It reported a core temperature of about 95F. Thus far, I think I’m reliably seeing kernel_task get out of hand if the temperature is in the 90+ (f) zone. I have the fans manually running at a reasonable clip now and the internal temperature is in the 78-79 zone and it SEEMS AS THOUGH it’s going away. I also installed Lion 10.7.1 just today, that has a fix for iTunes using optical out – it was bumping the processor mercilessly and that seems to have also reduced load.

    I really like the air conditioner test. Nice one mate.

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