sysconf(_SC_NPROCESSORS_ONLN) is expensive in RHEL6

Issue

• Qt application that has a lot of threads and uses QReadWriteLocks to protect data that the threads are accessing. When the server is heavily loaded, the application spends about 60% of it's time in system time as opposed to user time. Further investigation showed that the QMutex class is calling sysconf(_SC_NPROCESSORS_ONLN) for each thread that tries to get a lock where that lock is already locked.