On Tue, Nov 30, 2010 at 4:38 PM, Simon Adler <boost@cg-effects.de> wrote:

If I use all threads all four cpus are in 100% usage - they are all together working but require more
than twice the time - Yes i am sure that every thread is just working on 2500 Elements.

Hmm, are you sure of that because the different threads are all producing log output, perhaps to std::cout?

I wonder if there's some unobvious shared resource (such as an ostream) that's being properly locked against concurrent access, but the locking is inducing the delay you're observing.