If you're processor supports hyperthreading, it can be helpful for audio work, but if you're getting the notorious single core overload, you are actually overloading half of a core. It is possible to toggle this function on and off in os x depending on which works better for the particular project you're working on. This requires xcode to be installed.
1. Open the instruments app, which is located in /Applications/Xcode/Contents/Applications. To get there you can right click on the xcode app and choose "show package contents." You can create an alias of this for your applications folder or drag it to the dock if you like.
2. You should get a pop-up window, just click cancel. Go to preferences, and you should see a checkbox at the bottom to disable hyperthreading.
3. Restart logic, and you can now use the full frequency of each core. You can use the instruments app to toggle hyperthreading on and off as needed.
I hope this helps anyone having overloads with a fast processor. find I find it to be more reliable than logic's built in function, and much more convenient than disabling it in the bios. This helped me tremendously, i was getting overloads even on a 4.5 ghz quad core.
It would be really cool if anyone knows how to make an AppleScript or an automator action to do this that would be awesome.
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2011 i7 mbp, 16gb ram, os x 10.8.2, logic 9.1.8
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2006 MacBook, 4gb ram, lion server 10.7.4, logic node 9.1.8