I am writing a multithreaded app (Mac OS 10.5.8, Eigen 2.0.3) and getting slower performance with threads than without. There are lots of ways to go wrong in this but I thought I'd check just to make sure that there was nothing in Eigen that would cause undue contention between threads, i.e., nothing that would form a bottleneck that would cause hidden serialization or wait states.

Am I correct in this? or could it be the way malloc(), etc., is used in Eigen?

I have an older version of this app (Carbon, not Cocoa) that used BLAS/LAPACK and there the threading worked perfectly (2 threads gave twice the speed).

It's hard to be very specific here. Eigen doesn't do anything special in that respect, it doesn't do a lot of malloc's. It's true that it may use different malloc-like functions for aligned memory allocation, so if you think that's the issue, edit Eigen/src/Core/util/Memory.h to select another path and see if that helps (I doubt it would, but i'm no expert).