C 中央调度中心给了我一个较慢的执行时间

所以，我大约99%肯定我实施了错误的东西，但事情是这样的

我一直在玩GrandCentralDispatch，并做了一个计算MD5哈希的实验。我正在使用i5运行macbook air，因此有4个内核可用。这让我相信，使用Grand Central Dispatch计算散列的速度将快约4倍。但是，出于某种原因，它似乎要慢一些

代码如下

使用GCD

查看Activity Monitor，我可以看到脚本运行时，所有4个内核的最大值都已用完

现在，评论一下调度

#include <stdio.h>
#include <time.h>
#import <CommonCrypto/CommonDigest.h>
#import <dispatch/dispatch.h>

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

    int i,j,k,l,a;
    int num_chars = 4, total;
    clock_t start, end;
    double elap;

    printf("Calculating hashes for %d chars\n", num_chars);

    total = num_chars ^ 64;

    printf("Calculating %d hashes\n", total);

    dispatch_queue_t queue = dispatch_get_global_queue(0,0);
    dispatch_queue_t main = dispatch_get_main_queue();
    dispatch_group_t group = dispatch_group_create();

    start = clock();

   printf("Starting calculation queue\n");
   for(i=0;i<62;i++) {
        for(j=0;j<62;j++) {
            for(k=0;k<62;k++) {
                for(l=0;l<62;l++) {

                    //dispatch_group_async(group, queue, ^{

                        char *letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
                        char buffer[10];
                        char out[100];
                        unsigned char hash[16];

                        sprintf(buffer, "%c%c%c%c", letters[i], letters[j], letters[k], letters[l]);
                        CC_MD5(buffer, strlen(buffer), hash); 

                    //});




   }
        }
    }
}

printf("Finished calculation queue\n");
//dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

end = clock();

elap = ((double) end - start) / CLOCKS_PER_SEC;

printf("Time taken  %2f\n", elap);
return 0;
}

---

查看Activity Monitor，当脚本运行时，它只显示1个核心处于活动状态。

调度中可能完成的工作太少，因此调度所涉及的开销不值得。我会尝试增加每次发送的工作量。我不知道这是否有用，但请尝试：

将分派向上移动几个循环，或者将

k

或

j

循环包装在分派的块中，以使其执行更多的工作

删除调用

sprintf

和

strlen

。事实上，该区块可简化为：

static const char *letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
unsigned char hash[16];
char buffer[4] = { letters[i], letters[j], letters[k], letters[l] };

CC_MD5(buffer, sizeof buffer, hash);

#include <stdio.h>
#include <time.h>
#import <CommonCrypto/CommonDigest.h>
#import <dispatch/dispatch.h>

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

    int i,j,k,l,a;
    int num_chars = 4, total;
    clock_t start, end;
    double elap;

    printf("Calculating hashes for %d chars\n", num_chars);

    total = num_chars ^ 64;

    printf("Calculating %d hashes\n", total);

    dispatch_queue_t queue = dispatch_get_global_queue(0,0);
    dispatch_queue_t main = dispatch_get_main_queue();
    dispatch_group_t group = dispatch_group_create();

    start = clock();

   printf("Starting calculation queue\n");
   for(i=0;i<62;i++) {
        for(j=0;j<62;j++) {
            for(k=0;k<62;k++) {
                for(l=0;l<62;l++) {

                    //dispatch_group_async(group, queue, ^{

                        char *letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
                        char buffer[10];
                        char out[100];
                        unsigned char hash[16];

                        sprintf(buffer, "%c%c%c%c", letters[i], letters[j], letters[k], letters[l]);
                        CC_MD5(buffer, strlen(buffer), hash); 

                    //});




   }
        }
    }
}

printf("Finished calculation queue\n");
//dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

end = clock();

elap = ((double) end - start) / CLOCKS_PER_SEC;

printf("Time taken  %2f\n", elap);
return 0;
}

gcc -o b.out main.c
./b.out
Calculating hashes for 4 chars
Calculating 68 hashes
Starting calculation queue
Finished calculation queue
Time taken  7.511273

static const char *letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
unsigned char hash[16];
char buffer[4] = { letters[i], letters[j], letters[k], letters[l] };

CC_MD5(buffer, sizeof buffer, hash);