如何并行化此代码？

Question

如何并行化此代码？

for (t = 0; t < _PB_TMAX; t++) {
    for (j = 0; j < _PB_NY; j++)
        ey[0][j] = _fict_[t];
    for (i = 1; i < _PB_NX; i++)
        for (j = 0; j < _PB_NY; j++)
            ey[i][j] = ey[i][j] - 0.5*(hz[i][j]-hz[i-1][j]);
    for (i = 0; i < _PB_NX; i++)
        for (j = 1; j < _PB_NY; j++)
            ex[i][j] = ex[i][j] - 0.5*(hz[i][j]-hz[i][j-1]);
    for (i = 0; i < _PB_NX - 1; i++)
        for (j = 0; j < _PB_NY - 1; j++)
            hz[i][j] = hz[i][j] - 0.7*  (ex[i][j+1] - ex[i][j] +
                       ey[i+1][j] - ey[i][j]);
}

当我并行运行时，它需要比串行运行更多的时间。