How to disable HyperThreading on Mac OS X Lion?

Download and install Xcode. It includes a Processor.prefPane (not sure where it's located exactly) that allows you to disable hyperthreading.

With Xcode installed, you can also disable hyperthreading using the following terminal command:

hwprefs cpu_ht=false

I don't believe this setting is retained after restarting your Mac, so you might want to create an AppleScript to run that command on startup.

Any luck with the following? Requires a reboot to take.

$ sudo nvram SMT=0

Might be something you can put in boot-args as well but I couldn't find it.

$ sudo nvram boot-args=""

Ref: Technical Q&A QA1141 Disabling Processor Cores on a Multi-Core System