为什么是多线程慢？ [英] Why is multithreaded slower?

问题描述

所以我想写发现素数的程序。该项目的真正目的就是了解多线程。首先，我写了一个单线程程序，它在1分钟内发现到13633943。我的多线程版本只得到了10025627。

So I am trying to write a program that finds prime numbers. The real purpose of the project is just to learn about multithreading. First I wrote a single thread program and it finds up to 13,633,943 in 1 minute. My multithreaded version only got to 10,025,627.

下面是我的code为单线程程序

Here is my code for the single threaded program

#include <iostream>

using namespace std;

bool isprime(long num)
{
    long lim = num/2;
    if(num == 1)
    {
        return 0;
    }
    for(long i = 2; i <= lim; i++)
    {
        if (num % i == 0)
        {
            return 0;
        }
        else{ lim = num/i; }
    }
    return 1;
}

int main()
{
    long lim;
    cout << "How many numbers should I test: ";
    cin >> lim;
    for(long i = 1; i <= lim || lim == 0; i++)
    {
        if(isprime(i))
        {
            cout << i << endl;
        }
    }
}

下面是我的code为我的多线程程序。

Here is my code for my multithreaded program.

extern"C"
{
    #include <pthread.h>
    #include <unistd.h>
}
#include <iostream>

using namespace std;

bool isprime(long num);
void * iter1(void * arg);
void * iter2(void * arg);
void * iter3(void * arg);
void * iter4(void * arg);


int main()
{
    //long lim;
    //cout << "How many numbers should I test: ";
    //cin >> lim;
    pthread_t t1;
    char mem1[4096];//To avoid false sharing. Needed anywhere else?
    pthread_t t2;
    char mem2[4096];//These helped but did not solve problem.
    pthread_t t3;
    pthread_create(&t1, NULL, iter1, NULL);
    pthread_create(&t2, NULL, iter2, NULL);
    pthread_create(&t3, NULL, iter3, NULL);
    iter4(0);
}

bool isprime(long num)
{
    long lim = num/2;
    if(num == 1)
    {
        return 0;
    }
    for(long i = 2; i <= lim; i++)
    {
        if (num % i == 0)
        {
            return 0;
        }
        else{ lim = num/i; }
    }
    return 1;
}

void * iter1(void * arg)
{
    for(long i = 1;; i = i + 4)
    {
        if(isprime(i))
        {
            cout << i << endl;
        }
    }
return 0;
}

void * iter2(void * arg)
{
    for(long i = 2;; i = i + 4)
    {
        if(isprime(i))
        {
            cout << i << endl;
        }
    }
return 0;
}

void * iter3(void * arg)
{
    for(long i = 3;; i = i + 4)
    {
        if(isprime(i))
        {
            cout << i << endl;
        }
    }
return 0;
}

void * iter4(void * arg)
{
    for(long i = 4;; i = i + 4)
    {
        if(isprime(i))
        {
            cout << i << endl;
        }
    }
return 0;
}

东西尤其是混淆我是该系统监视器报告为单线程和100％使用了多线程25％的CPU使用率。不应该意味着它是做多4倍的计算？

Something that especially confuses me is that system monitor reports 25% CPU usage for the single thread and 100% usage for the multithread. Shouldn't that mean it is doing 4 times as many calculation?

推荐答案

我相当肯定 COUT 充当共享资源 - 即使它实际上打印正确每个号码并在正确的命令，它会减慢速度非常这么做。

I'm fairly sure cout acts a shared resource - and even if it actually prints each number correctly and in the right order, it slows things down VERY much to do so.

我也做类似的东西（这是更灵活，并使用一个原子操作挑下一个数字），它是几乎完全4X我的四核机器上快。但是，这只是如果我不显示任何信息。如果它打印到控制台，它是慢了很多 - 因为大量的时间用来洗牌像素，而不是实际计算。

I have done something similar (it is more flexible, and uses an atomic operation to "pick the next number"), and it's almost exactly 4x faster on my quad core machine. But that's only if I don't print anything. If it prints to the console, it's a lot slower - because a lot of the time is used shuffling pixels rather than actually calculating.

注释掉 COUT＆LT;＆LT; I＆LT;＆LT; ENDL; 行，它会更快的运行。

Comment out the cout << i << endl; line, and it will run much quicker.

编辑：用我的测试程序，具有打印：

using my test program, with printing:

Single thread: 15.04s. 
Four threads: 11.25s

如果没有打印：

Single threads: 12.63s.
Four threads: 3.69s.

3.69 * 4 = 14.76s，但时间我的Linux机器上的命令显示12.792s总运行时间，所以很明显存在的，当所有线程AREN时间一点点'T运行 - 或某些会计差错...

3.69 * 4 = 14.76s, but the time command on my Linux machine shows 12.792s total runtime, so there is obviously a little bit of time when all threads aren't running - or some accounting errors...

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