asymmetric linear system solver by using LU decomposition + partial pivoting.
this is a direct method, so not so suitable for parallelization. (just for my study)
parallelized the code by OpenMP, but not so good(there are some unparallelized loops, and many loops were parallelized in inner loops)
and this is for a study of FEM.

$ make
$ OMP_NUM_THREADS=1 ./a.out
 size:        1024
 LU decomposition maximum error:  1.598721155460225E-014
 time[s]:   2.92910000000000
 check the result: calc res = b - A*x
 residual:  2.310554965218229E-013

$ OMP_NUM_THREADS=16 KMP_AFFINITY=compact ./a.out
 size:        1024
 LU decomposition maximum error:  1.598721155460225E-014
 time[s]:  0.320800000000000
 check the result: calc res = b - A*x
 residual:  1.856472502915098E-013