what is the command line argument for running process in multiple cores (linux)

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http://stackoverflow.com – I am trying to run my program in more than one core. I have 12 cores in my desktop and want the computer to use 1,2,3,4,......,11,12 one by one and want to test how the program performs in different number of cores. I tried using -t4 or -t 4 after the executable like. ./a.out -t4 but I cannot make sure whether its using exactly 4 or not. Can anyone please help me out understand this or point me in right direction. (HowTos)