I just saw some benchmarks over at Phoronix comparing Snow Leopard and the developer preview of Lion. One of the differences is that the Lion kernel boots in 64-bit mode by default. One of their benchmarks measures compile time, and the results indicate that a 64-bit kernel might boost the speed of compiling code. Noticing that my MacBook Pro was running the 32-bit version of the Snow Leopard kernel, I decided to run some informal tests myself.

My test-case was a full build of the FolderGlance code base. I timed the build with my iPhone, so the results are not completely accurate. However, barring any measurement inaccuracies, the results were quite good:

Pretty good for simply switching to the 64-bit kernel! In case you are wondering: Switching to the 64-bit kernel can be done by either holding down the 6 and 4 keys while your Mac is booting, or you can execute the following command from Terminal to make the change persistent:

sudo systemsetup -setkernelbootarchitecture x86_64

Read more here: http://support.apple.com/kb/HT3773

This got me wondering if I could speed up compiles further using additonal processes. Xcode defaults to using the same number of sub-processes as the number of cores your Mac has. My MacBook is of the Intel Core i7 variety, with two cores, 8 gigs of RAM and an SSD disk. I figured I might try to up the ante, since compiles are often I/O-bound rather than CPU-bound, although I was skeptical since my Mac has an SSD disk.

I configured Xcode to use 3, 4 and 8 subprocesses, instead of the default 2. You can set the number of processes Xcode uses from Terminal as follows:

defaults write com.apple.Xcode PBXNumberOfParallelBuildSubtasks 4

These are the results I got (all on the 64-bit kernel):

So, for now I'll up the number of subprocesses Xcode uses to 4, since that seems to provide the best performance for this particular use case. The best number will also vary depending on the disk you use, how much memory you have available and the kind of code you are compiling.