All, I have found a very strange behaviour (for me at least!) in boost.Thread. The same dummy code: boost::mutex M; void functionA(int j) { for(int i=0; i<j; ++i) { boost::mutex::scoped_lock lock(M); sqrt(static_cast<double>(i)); } } void functionB(int j) { for(int i=0; i<j; ++i) { boost::mutex::scoped_lock lock(M); sqrt(static_cast<double>(i)); } } int main() { thread A(bind(&functionA, 1000000000)), AA(bind(&functionA, 1000000000)), B(bind(&functionB, 100000000)), BB(bind(&functionA, 100000000)); A.join(); B.join(); AA.join(); BB.join(); } executed on a two processor machine uses the resourced differently depending if the line boost::mutex::scoped_lock lock(M); is comennted or not. If we do not lock the mutex then both processors are used on 100%. However, in case the line is uncommented (we lock the mutex) only one processor is used. Can anyone explain me why? Thanks in advance Pshemek -------------------------------------------------------- If you are not an intended recipient of this e-mail, please notify the sender, delete it and do not read, act upon, print, disclose, copy, retain or redistribute it. Click here for important additional terms relating to this e-mail. http://www.ml.com/email_terms/ --------------------------------------------------------