关于多线程一直没有搞懂，最近面试也被问到C++11的thread和linux的pthread，由于我之前只用过pthread，连thread咋用都不知道，
后来看了thread的用法，编译的时候也要加-lpthread，难道底层是pthread？无从得知，网上也找不到。可能只是部分用到了pthread吧。
为了搞懂多线程，就想找个题目实践一下。
然后我就找到一篇博客，用Java写的多线程并行计算的代码，就是简单的求1到N的和，N是90000000。
那篇博客地址:https://blog.csdn.net/whandwho/article/details/80159377
于是我就想用C++的多线程来实现一下。
就找到了这篇博客https://blog.csdn.net/whandwho/article/details/80159377, 里面用C++的thread实现了并行加法，
稍作改动，便解决了上面的问题。
然后，考虑用pthread来实现, 毕竟我之前做项目有用过pthread, 对它还算熟悉。经过好一番折腾，总算是做出来了。
下面奉上代码和运行结果比较。
为了方便对比，一开始就写了单线程计算的，其实后来多线程版本都支持输入线程数, 但方便对比就按原来的吧。
另外，N设为900000000，也就是9亿，比那个Java的多个0。
运行结果1 : 单线程，thread和pthread
运行结果2 : thread和pthread，不同线程数
可以看到，开启8个线程比开3个线程更快。事实上，我试了很多，基本上8个线程是最快的，这道题属于密集计算型任务，线程数跟CPU核心数量一样比较好。
还有，thread和pthread的速度也差不多。
1.单线程
#include <iostream>
#include <chrono>
#include <ctime>
#define N 900000000
using namespace std;
int main(){
long long sum = 0;
auto start = std::chrono::system_clock::now();
std::time_t start_time = std::chrono::system_clock::to_time_t(start);
cout << "开始时间: " << std::ctime(&start_time);
for(long long i = 1; i <= N; ++i){
sum += i;
}
auto end = std::chrono::system_clock::now();
std::chrono::duration<double> elapsed_seconds = end-start;
std::time_t end_time = std::chrono::system_clock::to_time_t(end);
cout << "sum = " << sum << endl;
cout << "结束时间: " << std::ctime(&end_time);
cout << "用 时: " << elapsed_seconds.count() << "s" << endl;
return 0;
}
2.std::thread
#include <iostream>
#include <vector>
#include <thread>
#include <future>
#include <condition_variable>
#include <mutex>
#include <chrono>
#include <ctime>
#include <algorithm>
using namespace std;
const long long N = 900000000;
mutex m;
long long run(long long from, long long to)
{
long long ret = 0;
for (long long i = from+1; i <= to; i++)
{
ret += i;
}
cout <<"线程ID："<< this_thread::get_id() <<", ret="<<ret<< endl;
return ret;
}
int main()
{
int n;
long long sum = 0;
cout << "CPU核心个数 : " << thread::hardware_concurrency() << endl;
cout << "请输入启动线程个数: ";
cin >> n;
//计时
auto start = std::chrono::system_clock::now();
vector<future<long long>> result;
std::time_t start_time = std::chrono::system_clock::to_time_t(start);
cout << "开始时间: " << std::ctime(&start_time);
for(long long i = 0; i < n; ++i){
result.push_back(async(run, i*N/n, (i+1)*N/n));
}
for(long long i = 0; i < n; ++i){
sum += result[i].get();
}
auto end = std::chrono::system_clock::now();
std::chrono::duration<double> elapsed_seconds = end-start;
std::time_t end_time = std::chrono::system_clock::to_time_t(end);
cout << "sum = " << sum << endl;
cout << "结束时间: " << std::ctime(&end_time);
cout << "用 时: " << elapsed_seconds.count() << "s" << endl;
return 0;
}
3.pthread
#include <iostream>
#include <vector>
#include <chrono>
#include <ctime>
#include <algorithm>
#include <pthread.h>
#include <unistd.h>
using namespace std;
const long long N = 900000000;
typedef struct A{
long long from;
long long to;
}a_t, *pa_t;
pthread_mutex_t mutex;
void* run(void *arg)
{
pa_t at = (pa_t)arg;
long long from = at->from, to = at->to;
pthread_mutex_unlock(&mutex);
long long ret = 0;
for (long long i = from+1; i <= to; i++)
{
ret += i;
}
//cout << "from = " << from << ", to = " << to << endl;
//cout << "ret = " << ret << endl;
//cout <<"线程ID："<< pthread_self() << " sum="<<ret<< endl;
return (void*)ret;
}
int main()
{
int n;
cout << "CPU核心个数 : " << sysconf(_SC_NPROCESSORS_CONF) << endl;
cout << "请输入启动线程个数: ";
cin >> n;
pthread_t pthid[n] = {0};
pthread_mutex_init(&mutex, NULL);
long long from, to;
//计时
auto start = std::chrono::system_clock::now();
std::time_t start_time = std::chrono::system_clock::to_time_t(start);
cout << "开始时间: " << std::ctime(&start_time);
for(int i = 0; i < n; ++i){
pthread_mutex_lock(&mutex);
a_t arg;
arg.from = i*N/n;
arg.to = (i+1)*N/n;
pthread_create(&pthid[i], NULL, run, (void*)&arg);
}
long long sum = 0;
void* ret;
for(int i = 0; i < n; ++i){
pthread_join(pthid[i], &ret);
cout << "pthid = "<< pthid[i] << ", ret = " << (long long)ret << endl;
sum += (long long)ret;
}
auto end = std::chrono::system_clock::now();
std::chrono::duration<double> elapsed_seconds = end-start;
std::time_t end_time = std::chrono::system_clock::to_time_t(end);
cout << "sum = " << sum << endl;
cout << "结束时间: " << std::ctime(&end_time);
cout << "用 时: " << elapsed_seconds.count() << "s" << endl;
return 0;
}
