# Faster

Here we have a serial program`serial.c`:

{% code title="serial.c" %}

```c
#include <stdio.h>
#include <omp.h>
#include <time.h>
#include <sys/time.h>
void test() {
    int a = 0;
    int i;
    for(i=0; i<1000000000; i++) {
        a++;
    }
}

int main() {
    struct timeval start, end;
    gettimeofday(&start, NULL);
    int i;

    for(i=0; i<8; i++) {
        test();
    }
    gettimeofday(&end, NULL);
    printf("time = %f seconds\n", (double)((end.tv_sec * 1000000 + end.tv_usec)- (start.tv_sec * 1000000 + start.tv_usec))/1000000);
}
```

{% endcode %}

```c
# compile with gnu gcc
gcc -o serial serial.c -fopenmp
# compile with clang (on mac)
clang -Xpreprocessor -fopenmp serial.c -lomp -oserial
```

```
$ ./serial
time = 20.059342 seconds
```

*Notice that the output time may be different on your own computer, that's OK.*

Now we use 4 (the default setting of my machine) threads to see how fast we can get!

{% code title="parallel\_with\_openmp.c" %}

```c
#include <stdio.h>
#include <omp.h>
#include <time.h>
#include <sys/time.h>
void test() {
    int a = 0;
    int i;
    for(i=0; i<1000000000; i++) {
        a++;
    }
}

int main() {
    struct timeval start, end;
    gettimeofday(&start, NULL);
    int i;
    #pragma omp parallel for
    for(i=0; i<8; i++) {
        test();
    }
    gettimeofday(&end, NULL);
    printf("time = %f seconds\n", (double)((end.tv_sec * 1000000 + end.tv_usec)- (start.tv_sec * 1000000 + start.tv_usec))/1000000);
}
```

{% endcode %}

```c
# compile with gnu gcc
gcc -o parallel parallel_with_openmp.c -fopenmp
# compile with clang (on mac)
clang -Xpreprocessor -fopenmp parallel_with_openmp.c -lomp -oparallel
```

```
$ ./parallel
time = 5.337185 seconds
```

That's amazing! As I know when a biology scientist wants to calculate the structure of some proteins, it can even take him/her time-cost from months down to weeks!


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