Macbook Air Overheating and Core Shutdown

How do we stop core shutdown and improve performance?
2008-05-25

It turns out that my original glowing review of the Macbook Air was a bit premature. To say I have buyer's remorse is an understatement.

After spending some time using my Macbook Air I noticed that as time went on, the laptop seemed to slow down. Simultaneously I noticed that the machine would get hot. I was originally very happy with how cool the machine ran. Over time though it started getting noticably warmer.

I did some Googling, and sure enough, I was not alone. As soon as the machine gets pretty hot, it goes from being a 1.8Ghz Intel Core Duo to a 1Ghz(due to throttling) Core Uno. Its infuriatingly easy to reproduce, too:

• Open up Activity Monitor to watch it happen.
• Go to Hulu.
• Watch a video fullscreen.

5-6 minutes in you will see your second core shut off. Even at the Apple Store. It will be rather obvious this has happened, as the video will get very choppy. This is a very easily reproducible demo which doesn't illustrate real-world performance issues. Trust me tho, these issues are very real.

The proposed solution revolves around a utility called CoolBook. After buying the program and installing it I can vouch that it does exactly that. CoolBook deactivates the Apple/Intel SpeedStep driver, and installs its own custom cpu power driver. This custom driver will not shut down your second core.

But first a quick note on the current generation of processors. Gone are the days when CPUs had one speed and one core. As computers get faster, they run hotter. And its been a challenge for engineers to work out how to get faster chips to the masses without melting your lap.

Enter technologies such as SpeedStep and PowerNow. Not only do these chips focus on efficiency, they can offer a higher top speed, which looks good to the marketing department. Yes, your 2.2Ghz Macbook Pro isn't actually able to sustain 2.2Ghz for long periods of time. As soon as it gets hot, it will throttle back to 1.2Ghz or so in order to stay cool. So in reality its more of a 1.2Ghz chip that can sustain 2Ghz for short periods of time.

The point of CoolBook is to allow you more fine-grained control over these CPU power features. It also provides the ability to send the processor less electricity so that it extends battery life and reduces heat.

But I'm not happy to slow down my computer in order to keep it from crapping out. I want to actually use my computer to do stuff. Important stuff, like Hulu. By default CoolBook will improve the performance simply by deactivating SpeedStep. But if you increase the speed settings, you can get the advertised performance at the expense of high heat output.

For the 'adapter' settings in CoolBook I removed all entries but one: "1800Mhz / 1.0000V". This has the effect of making sure that your processor will be at max speed at all times necessary. It will still throttle down to 1.2Ghz when idle, but it will not throttle down under heavy load. Needless to say this is not a good idea. The maker of CoolBook, Magnus Lundholm, specifically says you should not do this, and had this to say when I asked him to elaborate:

'An Apple MacBook is not designed to run at full speed for longer periods of time. It must lower the performance after a while, otherwise it will overheat. The MBP is better, and the "Penryns" should work better as well. My previous C2D MB couldn't run CPUTest for that many minutes before stepping down.'

So cautionary tale kids...pegging your 1.8Ghz CPU can very possibly damage your machine. Since I'm going to do something similar this week, I'll let you know how it pans out. According to Magnus, use the temp limit feature and multiple settings to keep you Mac from overheating, similar to the default settings.

As a last note, Magnus, while not having seen the core shutdown himself, seemed skeptical that this was the issue:

'I don't think the shutting down of one core is the reason for the choppiness, one core is more than enough for the UI. Instead I think the clock modulation is changed, or the execution is halted when the temperature reaches over a certain level. This can happen to the MB and MBP as well.'

I can see the results of my core shutting down are correlated to performance and stuttering. But I am not an expert in CPU power management software. According to Magnus its not the core shutdown, but the throttling of the cpu. I can say that regardless of the cause, using CoolBook has made my Air usable under heavy load.