Thanks for looking at it.

Possibility 1 is out of the question, it's a C2D E7300 - or it's damn good at pretending to be one. :)

Possibility 3 is also out, I think. I tried to replace the function that talks to R with one that simply calls one of my dumb C Mandelbrot renderers: the 2 threaded version finished in half time, confirming that the CPU is capable of running two processes in a truly parallel way. This also suggests that it is not my implementation that's at fault.

This leaves possibility 2, which seems to suggest that something fishy is going on with R when it comes to multithreading. Maybe it's as you say that there are two R interpreter instances, but only one backend?

I'll try to isolate the issue. Thanks again.