參考連結：

1、https://blog.csdn.net/tigerisland45/article/details/54150950

2、https://blog.csdn.net/gengshenghong/article/details/7003110

1、首先配置.pro 檔案，以下是我的控制檯（console）.pro。注意紅色部分。

QT += core
QT -= gui

CONFIG += c++11

TARGET = testEigenProcess
CONFIG += console
CONFIG -= app_bundle

TEMPLATE = app

SOURCES += main.cpp

# The following define makes your compiler emit warnings if you use
# any feature of Qt which as been marked deprecated (the exact warnings
# depend on your compiler). Please consult the documentation of the
# deprecated API in order to know how to port your code away from it.
DEFINES += QT_DEPRECATED_WARNINGS

# You can also make your code fail to compile if you use deprecated APIs.
# In order to do so, uncomment the following line.
# You can also select to disable deprecated APIs only up to a certain version of Qt.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0

INCLUDEPATH += \
/home/david/MySoft/myeigen

QMAKE_CXXFLAGS += -fopenmp
LIBS += -fopenmp

2、程式碼示例如下：

//#define the EIGEN_USE_MKL_ALL

#include <QCoreApplication>
#include <Eigen/Dense>
#include <iostream>
#include <time.h>
#include <omp.h>


using namespace Eigen;
using namespace  std;

void print_xiao(char i)
{
    cout << "omp_get_num_threads : " << omp_get_num_threads() << endl;
    cout << "omp_get_max_threads: " << omp_get_max_threads() << endl;
    cout << "omp_get_thread_num: " << omp_get_thread_num() << endl;
    cout << i << endl;
}

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    initParallel();
    // use openMP
    cout << "before set openMP threads: " << omp_get_num_threads() << endl;
    int set_thread = 10;
    omp_set_num_threads(set_thread);
    cout << "after set openMP threads: " << omp_get_num_threads() << endl;
    #pragma omp parallel for
    for (char i = 'a'; i <= 'z'; i++)
        print_xiao(i);



    // test mult-threading
    int mat_size = 1000;
    MatrixXd matXd, save_mat;
    clock_t time_start = 0, time_end = 0;
    matXd.resize(mat_size, mat_size);
    matXd.setIdentity();
    //#pragma omp parallel for
    for(int i = 0; i < 10;i++)
    {
        time_start = clock();
//        save_mat = matXd.inverse();
        save_mat = matXd * matXd;
    //    cout << save_mat << endl;
        time_end = clock();
        cout <<i << " -> Elapsed time is "
                << (double)(time_end - time_start)/CLOCKS_PER_SEC<< " seconds." << endl;
    }




    return a.exec();
}

很多有趣的現象，開啟多執行緒後你會發現Eigen矩陣相乘變慢啦、。。。。。。。。哎