并行性

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并行性：cpu处理指令是并行的，前一条指令处理时，紧接着前一条的指令也会开始处理，如果后一条指令和前一条指令无关，那么程序性能会得到提升，我们常常会把一个操作分解成几条指令并行来提升性能。

实例代码：

long long getSystemTime() {        struct timeb t;        ftime(&t);        return 1000 * t.time + t.millitm;}int main(){int                     * b_test_data = (int *)malloc(80000000*sizeof(int));        int                             b_loop;        long long               b_begin, b_end, b_ttl1 = 1, b_ttl2 = 1;        for(b_loop = 0; b_loop < 80000000; b_loop++)        {                b_test_data[b_loop] = 1;        }        b_begin = getSystemTime();        for(b_loop = 0; b_loop < 80000000 - 1; b_loop += 2)        {                b_ttl1 = b_ttl1*b_test_data[b_loop]*b_test_data[b_loop + 1];                     }                for(; b_loop < 80000000; b_loop++)        {                b_ttl1 = b_ttl1*b_test_data[b_loop];        }        b_end = getSystemTime();        printf("time = %lu, b_ttl = %lu\n", b_end - b_begin, b_ttl1);        return 0;}

运行结果：

并行：

int main(){int                     * b_test_data = (int *)malloc(80000000*sizeof(int));        int                             b_loop;        long long               b_begin, b_end, b_ttl1 = 1, b_ttl2 = 1;        for(b_loop = 0; b_loop < 80000000; b_loop++)        {                b_test_data[b_loop] = 1;        }        b_begin = getSystemTime();        for(b_loop = 0; b_loop < 80000000 - 1; b_loop += 2)        {                b_ttl1 = b_ttl1*b_test_data[b_loop];                b_ttl2 = b_ttl2*b_test_data[b_loop + 1];        }        b_ttl1 *= b_ttl2;        for(; b_loop < 80000000; b_loop++)        {                b_ttl1 = b_ttl1*b_test_data[b_loop];        }        b_end = getSystemTime();        printf("time = %lu, b_ttl = %lu\n", b_end - b_begin, b_ttl1);        return 0;}

运行结果：

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