MacBook Air Core Shutdown
I have been using a MacBook Air as my sole laptop for a month or so now. In general I feel very positive about my experience with it. However, there is one issue that seems to be plaguing myself and other MacBook Air owners. When taxing the CPU for extended periods, the MacBook Air shuts down one of it’s cores to prevent overheating. The easiest way to reproduce the problem is to watch a few YouTube videos. After approximately 15 minutes the Mac OS X Activity Monitor clearly shows only one core running. Within a few minutes of the CPU load being lightened, the dormant core springs back to life.
While running on only one CPU core, the MacBook Air is barely usable. The UI becomes jittery and the onscreen trackpad pointer jumps around.
While trying to find a solution to the problem I came across this forum post. The author suggests an application called ‘Coolbook‘ which purports to underclock the laptops CPU by lowering the voltage supplied to the processor and by more agressively throttling the speed of the CPU. My initial reaction was one of extreme skepticism. Alarm bells were ringing. While I consider myself a geek I have never dabbled with overclocking of CPU’s and have never even heard of underclocking. But after reading five pages of positive responses I decided to take the plunge and install ‘Coolbook’.
Firstly, let’s describe the applications UI as abysmal. There is no getting away from it. Maybe it would make more sense to someone with experience in underclocking but the whole point in a GUI is to assist the uninitiated in achieving their goals without having to resort to technical manuals. Needless to say I had to resort to the manual which is thankfully quite good.
After tinkering with the various parameters and following the advice of others on the forum I settled on the following settings…
When power adaptor is plugged in…
600HMz = 0.9V
800MHz = 0.9V
1200MHz = 0.9V
1400Mhz = 0.9V
1600Mhz = 0.95V
When the MacBook is running on battery…
600MHz = 0.9V
800Mhz = 0.9V
1200Mhz = 0.9V
This lowers the voltage supplied to the CPU by 0.2V. Also of note is the removal of the 1400MHz and 1600MHz setting when running on batteries. This has had the following results…
Approx CPU temperature under light load 52C
Approx CPU temperature under heavy load 75C
Fan speed under light load 2500rpm
Fan speed under heavy load 6500rpm
Battery life under light load – 3.35
Approx CPU temperature under light load 43C
Approx CPU temperature under heavy load 62C
Fan speed under light load 2500rpm
Fan speed under heavy load 4250rpm
Battery life under light load – 4.25
Since installing Coolbook, my MacBook Air has suffered no core shutdowns, it has run significantly cooler to the touch, it is quieter (as a result of the lower fan speeds) and it lasts almost an hour longer than usual when running on battery power. As of yet I have found no downsides and I have not experienced any instability.
There is some debate about the possibilities of damaging the CPU by underclocking it. My research leads me to believe that such damage is unlikely and that using too low a voltage will simply result in OS instability. However, I am not advising anyone else rushes into underclocking their MacBook Air. All I offer is my experience with ‘Coolbook‘ for those who might be interested I will update this post should my experience change over the coming weeks.
Filed under: Apple, Hardware, MacBook Air | 72 Comments