Hi Greg:

The main advantage for now is that OpenMP is broken with shared library builds on OSX, whereas GCD appears to work with shared libs. So we can now get parallelization where previously we could not. Also, when you run multiple parallel applications on the same multi-CPU system, you should system-wide load balancing with GCD, which you don't with OpenMP, so you could get better overall resource utilization.

In terms of efficiency, there should be little difference. To some degree, OpenMP is actually more idiot-proof, so for me, GCD may be less efficient in some cases due to poor application. As an example, when you parallelize a loop with many but cheap iterations, in OpenMP you simply put the usual pragma and that's it. In GCD, you want to compute a stride size for the loop first and thus cut it into larger blocks, otherwise each iteration will be submitted separately to a thread, which is inefficient in this scenario.

The final disadvantage of GCD so far is that, at least until I get my hands on XCode4, you cannot "dispatch" parallel sections from function templates or member functions of class templates, which means that some OpenMP-parallel code in CMTK right now cannot be ported to GCD at all. This affects, for example, the deblurring super-resolution volume reconstruction, which is in a class templated over the point spread function kernel shape.

TR