多线程——线程同步

数据竞争问题

void test_2(){
    int sum = 0;
    #pragma omp parallel for
    for(int i=0;i<100;i++){
        sum += 1;
        #pragma omp critical
        cout<<"tid:"<<omp_get_thread_num()<<" sum:"<<sum<<endl;
    }
    cout<<"sum:"<<sum<<endl;
}

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线程互斥同步——critical

void test_2(){

    int sum = 0;
    #pragma omp parallel for
    for(int i=0;i<100;i++){
        // 线程互斥同步，保证某一时刻只有1个线程访问操作
        #pragma omp critical
        {
            sum += 1;
            cout<<"tid:"<<omp_get_thread_num()<<" sum:"<<sum<<endl;
        }
        
    }

    cout<<"sum:"<<sum<<endl;

}

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线程互斥同步——atmoic

void test_3(){
    int sum = 0;
    #pragma omp parallel for
    for(int i=0;i<100;i++){
        // 线程互斥同步, 原子操作
        #pragma omp atomic
        sum += 1;
        cout<<"tid:"<<omp_get_thread_num()<<" sum:"<<sum<<endl;
    }

    cout<<"sum:"<<sum<<endl;
}

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线程互斥同步——线程互斥锁函数

void test_4(){
    omp_lock_t lock;            
    omp_init_lock(&lock);   // 初始化创建互斥锁

    int sum = 0;
    #pragma omp parallel for
    for(int i=0;i<100;i++){
        omp_set_lock(&lock);    // 加锁
        sum += 1;
        cout<<"tid:"<<omp_get_thread_num()<<" sum:"<<sum<<endl;
        omp_unset_lock(&lock);  // 解锁
    }

    cout<<"sum:"<<sum<<endl;
    omp_destroy_lock(&lock);    // 销毁线程锁
}

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reduction

void test_5(){

    int a[] = {1, 3, 5, 7, 9};
    int b[] = {0, 2, 4, 6, 8};

    int sum = 0;

    #pragma omp parallel for
    for(int i=0;i<5;i++){
        sum += a[i] + b[i];
        printf("tid:%d %d+%d=%d sum=%d\n", omp_get_thread_num(), a[i], b[i], a[i]+b[i], sum);
    }

    printf("sum=%d\n", sum);
}

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void test_5(){

    int a[] = {1, 3, 5, 7, 9};
    int b[] = {0, 2, 4, 6, 8};

    int sum = 0;

    #pragma omp parallel for reduction(+:sum)
    for(int i=0;i<5;i++){
        sum += a[i] + b[i];
        printf("tid:%d %d+%d=%d sum=%d\n", omp_get_thread_num(), a[i], b[i], a[i]+b[i], sum);
    }

    printf("sum=%d\n", sum);
}

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