pthread在Windows中比serial快,但在Linux中比serial慢 我试图在Windows和Linux上运行蒙特卡洛算法,用相同的线程数(4个线程和4个CPU)运行相同的C++并行代码。虽然并行代码在Windows上比串行实现快,但在Linux上要慢得多
节目如下:pthread在Windows中比serial快,但在Linux中比serial慢 我试图在Windows和Linux上运行蒙特卡洛算法,用相同的线程数(4个线程和4个CPU)运行相同的C++并行代码。虽然并行代码在Windows上比串行实现快,但在Linux上要慢得多,c++,linux,parallel-processing,pthreads,C++,Linux,Parallel Processing,Pthreads,节目如下: #include <iostream> #include <cstdlib> #include <ctime> #include <cmath> #include <pthread.h> #include <chrono> using namespace std; using ns = chrono::nanoseconds; using get_time = chrono::steady_clock; st
#include <iostream>
#include <cstdlib>
#include <ctime>
#include <cmath>
#include <pthread.h>
#include <chrono>
using namespace std;
using ns = chrono::nanoseconds;
using get_time = chrono::steady_clock;
static int thread_count = 4;
pthread_mutex_t myMutex;
struct args{
int id;
int random_count;
double *pi;
};
double compute_pi(long n)
{
double pi = 0;
double x, y;
for(long i=0; i<n; i++){
x = -1 + 2 * double(rand())/RAND_MAX;
y = -1 + 2 * double(rand())/RAND_MAX;
if (sqrt(x*x + y*y) <= 1.0) pi++;
}
return 4*pi/n;
}
void* threadFunc(void *argin){
args *inputs = (args*) argin;
double my_sum = 0;
double x, y;
for(int i=0; i<inputs->random_count; i++){
x = -1 + 2 * double(rand())/RAND_MAX;
y = -1 + 2 * double(rand())/RAND_MAX;
if (sqrt(x*x + y*y) <= 1.0) my_sum++;
}
pthread_mutex_lock(&myMutex);
*(inputs->pi) += my_sum;
pthread_mutex_unlock(&myMutex);
return nullptr;
}
double compute_pi_parallel(long n)
{
double pi = 0;
int count_per_thread = n/thread_count;
pthread_t *threads = new pthread_t[thread_count];
args *funcInputs = new args[thread_count];
pthread_mutex_init(&myMutex, nullptr);
for(int i=0; i<thread_count; i++){
funcInputs[i].id = i;
funcInputs[i].random_count = i<n%thread_count ? count_per_thread+1 :
count_per_thread;
funcInputs[i].pi = π
int rc = pthread_create(&threads[i], nullptr, threadFunc, (void *)
&funcInputs[i]);
if(rc) cerr << "error in thread creation!\n";
}
for(int i=0; i<thread_count; i++){
int rc = pthread_join(threads[i], nullptr);
if(rc) cerr << "Error in thread join!\n";
}
pthread_mutex_destroy(&myMutex);
delete [] funcInputs;
delete [] threads;
return 4*pi/n;
}
int main(int argc, char* argv[])
{
srand(time(nullptr));
long n = 100000000;
auto start = get_time::now();
if (argc > 1){
n = atol(argv[1]);
if (argc == 3){
thread_count = atoi(argv[2]);
cout << "pi(parallel) = " << compute_pi_parallel(n) << endl;
auto stop = get_time::now();
auto diff = stop - start;
cout<<"Elapsed time is : "<< chrono::duration_cast<ns>
(diff).count()/1e9<<" s "<<endl;
return 0;
}
}
cout << "pi = " << compute_pi(n) << endl;
auto stop_s = get_time::now();
auto diff_s = stop_s - start;
cout << "pi(parallel) = " << compute_pi_parallel(n) << endl;
auto stop_p = get_time::now();
auto diff = stop_p - stop_s;
cout<<"Elapsed time for serial is : "<< chrono::duration_cast<ns>
(diff_s).count()/1e9<<" s "<<endl;
cout<<"Number of threads: "<< thread_count<< endl;
cout<<"Elapsed time for parallel is : "<< chrono::duration_cast<ns>
(diff).count()/1e9<<" s "<<endl;
return 0;
}
在Linux上:$g++-std=c++11-g-Wall-omc mc.cpp-lpthread
输出:
pi = 3.14138
pi(parallel) = 3.14166
Elapsed time for serial is : 3.10837 s
Number of threads: 4
Elapsed time for parallel is : 19.8226 s
我用$lscpu检查Linux上的CPU数量,并用$top监视CPU使用情况。Linux似乎使用了所有可用的内核,但仍然比串行代码慢。我在Windows虚拟机上运行Ubuntu 16.04 LTS程序
我想知道我在Linux上是否做错了什么。您正在使用兰德。如果rand是线程安全的,则它是实现定义的。它可以简单地调用互斥锁。使用一个没有全局状态的现代C++随机数生成器。< /p>你在两个平台上都启用了优化吗?如果不是,那么从那里开始。为什么不用C++线程API?特别是,可能的复制,即代码> RAND()的GLUBC实现< /COD>确实使用互斥来同步线程之间的访问,所以在Linux上,您的解决方案只是序列化在<代码> RAND()/<代码>。
pi = 3.14138
pi(parallel) = 3.14166
Elapsed time for serial is : 3.10837 s
Number of threads: 4
Elapsed time for parallel is : 19.8226 s