Why is sys+user > real in "time command"?

I have a program that uses pthread library to do the matrix multiplication of 500x500 matrix. Each thread calculates 50 rows of the matrix.

When I time its execution:

shadyabhi@shadyabhi-desktop:~$ time ./a.out

real    0m0.383s
user    0m0.810s
sys     0m0.000s
shadyabhi@shadyabhi-desktop:~$

Why is sys+user is 开发者_C百科greater than real time?

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It is greater, because it adds the time from all the cores together.