如何在Linux中比单线程慢解决OpenMP？

Question

我使用OpenMP并行程序，但它比单线程慢。我使用的代码是#pragma omp parallel for。我不知道该怎么解决。我希望你能帮助我。谢谢！

我在子函数（xorencrypt和xordecode）中使用了OpenMP代码。我使用c99 -o NEWRSULT openMPtest.c -fopenmp进行编译。该程序与密码学加密和解码有关

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <omp.h>
#include <time.h>

int char_to_index(char c)
{
    /*if(c>='A' && c<='Z'){
        c=c+32;
    }*/
    char x = c;
    char result = x - 'a';
    return (int)result;
}

char index_to_char(int c)
{
    int result = c + 97;
    return (char)result;
}

char * xorencrypt(char * message, char * key) {
    size_t messagelen = strlen(message);
    size_t keylen = strlen(key);

    char * encrypted = malloc(messagelen+1);

    int i;
    #pragma omp parallel for
    for (i = 0; i < messagelen; i++) {
        int buf1 = char_to_index(message[i]);
        int buf2 = char_to_index(key[i % keylen]);
        char r = index_to_char(buf1 ^ buf2);
        encrypted[i] = r;
    }

    encrypted[messagelen] = '\0';

    return encrypted;
}

char * xordecode(char * cript, char * key) {
    size_t criptlen = strlen(cript);
    size_t keylen = strlen(key);

    char * message = malloc(criptlen+1);

    int i;
    #pragma omp parallel for
    for (i = 0; i < criptlen; i++) {
        int buf1 = char_to_index(cript[i]);
        int buf2 = char_to_index(key[i % keylen]);
        char r = index_to_char(buf1 ^ buf2);
        message[i] = r;
    }

    message[criptlen] = '\0';

    return message;
}

int main(int argc, const char * argv[]) {

    clock_t start, end;
    start = clock();

    char * message = "Is this Rayban?";
    printf("Input    : %s\n",message);
    char * key = "key";

    char * encrypted = xorencrypt(message, key);
    printf("Encrypted: %s\n", encrypted);

    char * message1 = xordecode(encrypted, key);
    printf("Decoded  : %s\n", message1);

    end = clock();
    double diff = end - start; // ms
        printf(" %f  ms" , diff);
        printf(" %f  sec", diff / CLOCKS_PER_SEC );

    return 0;
}