Question

以下是我正在玩的玩具示例，以了解如何使用TBB。 Parallel :: operator（）应该并行运行，但它有一个关键区域，一次只能由一个处理器访问，因此它打印的消息不会被扰乱。我的问题是它无法编译，编译器消息对我没有多大帮助。我做错了什么？

另外，这被认为是在parallel_for中实现互斥的正确方法吗？

#include <iostream>
#include <vector>
#include <cmath>
#include <tbb/tbb.h>

typedef tbb::queuing_mutex Mutex;

struct Parallel
{
    Mutex mutex;
    std::vector<int> * values;

    Parallel(std::vector<int> * values_) : values(values_) {}

    void operator()( tbb::blocked_range< unsigned int > & range ) const {
        for(unsigned int i = range.begin(); i < range.end(); ++i) {
            {
                Mutex::scoped_lock lock(mutex);
                if ( (*values)[i] > 40)
                {
                    std::cout << "NO SCRAMBLING ALLOWED!\n";
                    std::cout.flush();
                }
                lock.release();
            }
        }
    }
};

int main() {
    const int someValue = 20000;

    std::vector<int> data(someValue);
    for(int i = 0; i < someValue; ++i) {
        data[i] = std::rand();
    }

    tbb::parallel_for( tbb::blocked_range<unsigned int>(0, data.size()),
                       Parallel(&data) );
}

Bellow是错误信息：

/path-to-src/main.cpp: In member function 'void Parallel::operator()(tbb::blocked_range<unsigned int>&) const':
/path-to-src/main.cpp:20:46: error: no matching function for call to 'tbb::queuing_mutex::scoped_lock::scoped_lock(const Mutex&)'
/path-to-src/main.cpp:20:46: note: candidates are:
In file included from /usr/include/tbb/tbb.h:65:0,
                 from /path-to-src/main.cpp:4:
/usr/include/tbb/queuing_mutex.h:80:9: note: tbb::queuing_mutex::scoped_lock::scoped_lock(tbb::queuing_mutex&)
/usr/include/tbb/queuing_mutex.h:80:9: note:   no known conversion for argument 1 from 'const Mutex {aka const tbb::queuing_mutex}' to 'tbb::queuing_mutex&'
/usr/include/tbb/queuing_mutex.h:77:9: note: tbb::queuing_mutex::scoped_lock::scoped_lock()
/usr/include/tbb/queuing_mutex.h:77:9: note:   candidate expects 0 arguments, 1 provided
/usr/include/tbb/queuing_mutex.h:66:11: note: tbb::queuing_mutex::scoped_lock::scoped_lock(const tbb::queuing_mutex::scoped_lock&)
/usr/include/tbb/queuing_mutex.h:66:11: note:   no known conversion for argument 1 from 'const Mutex {aka const tbb::queuing_mutex}' to 'const tbb::queuing_mutex::scoped_lock&'

Answer 1

tbb :: parallel_for被设计为不编写示例。它可以防止代码中的错误。 tbb :: parallel_for的范围形式将仿函数按值复制到多个任务对象中。因此，每个任务都有一个单独的互斥锁副本，因此互斥锁不会提供预期的同步。

修复代码的方法是在struct Parallel之外声明互斥，并通过指针传递它，类似于“values”的指针。

Answer 2

一切都是正确的，但mutex必须是mutable才能将其用于锁定。此外，没有必要释放锁。

typedef tbb::queuing_mutex Mutex;

struct Parallel
{
    mutable Mutex mutex;              // mutable so that we can use it in const member
    std::vector<int> * values;

    Parallel(std::vector<int> * values_) : values(values_) {}

    // note: this is a const member
    void operator()( tbb::blocked_range< unsigned int > & range ) const
    {
        for(unsigned int i = range.begin(); i < range.end(); ++i)
        {
            Mutex::scoped_lock lock(mutex);       // requires non-const argument
            if ( (*values)[i] > 40)
            {
                std::cout << "NO SCRAMBLING ALLOWED!\n";
                std::cout.flush();
            }
            // no need to release the lock: the destructor will do that for you
        }
    }
};

或者，您当然可以使成员函数非常量。

在tbb :: parallel_for中使用tbb :: queuing mutex的简单示例程序无法编译

2 个答案: