linux cpu usage - programs taking more time in machines with more processors

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http://stackoverflow.com – I am facing an issue. my programs in C for any reason has taking more time in some machines than in other, when i expect to be more fast because has more processors. I mean, I have 3 machines: - machine 1 with 2 processor, and ubuntu, and a process is taking me 20 minutes - machine 2 with 4 processors, and ubuntu, running the same programs is taking 34 minutes. - machine 3 with 8 processors, and ubuntu, and the same programs is taking 40 minutes. checking the %CPU with top in the three machines: machine 1: %CPU 100% machine 2: %CPU 20% machine 3: %CPU 15% My programs is using pthreads, mutex (HowTos)