Here is an interesting, but old (2007), email on darwin-dev:
http://lists.apple.com/archives/darwin-dev/2007/Jun/msg00088.html

"This can all change in the future, but currently:

hw.ncpu is a wart; consider it to be deprecated.
hw.physicalcpu is the number of physical CPUs
hw.logicalcpu is the number of logical CPUs; this is for SMT, which we don't support (maybe T1s?)
hw.availcpu are the number logical CPUs currently online

These interfaces are evolving, however, you are unlikely to get a description of these. They are intended for internal ibrary use, and not for use by applications, since applications should use the library abstractions rather than trying to use this information directly themselves."

By the way, multiprocessing should use subprocessing directly, not os.popen() (but this is a different issue).