访问类数据时互斥锁/解锁的替代方案
我试图像这样使 my_class 线程安全。
class my_class
{
const std::vector<double>&
get_data() const
{ //lock so that cannot get_data() while setting data
lock l(m_mutex);
return m_data;
}
void
run()
{
vector<double> tmp;
//some calculations on tmp.
{ //lock so that cannot get_data() while setting m_data
lock l(m_mutex);
m_data = tmp; //set the data
}
}
private:
std::vector<double> m_data;
mutex m_mutex;
my_class(); //non-copyable
}
run() 和 get_data() 可能由不同的 openmp 线程调用，因此我引入了一个锁。 （由于我使用 openmp，m_mutex 和 lock 是 omp_init_lock(); 等命令的 RAII 包装器）。
但是，创建和销毁 get_data () 上的锁的成本很高（当我分析代码时最昂贵的操作 - 我经常调用 get_data() ）。
是否可以重新组织 my_class 以删除 get_data() 中的锁？或者这个锁是并行代码不可避免的成本吗？
I am trying to make my_class thread-safe like so.
class my_class
{
const std::vector<double>&
get_data() const
{ //lock so that cannot get_data() while setting data
lock l(m_mutex);
return m_data;
}
void
run()
{
vector<double> tmp;
//some calculations on tmp.
{ //lock so that cannot get_data() while setting m_data
lock l(m_mutex);
m_data = tmp; //set the data
}
}
private:
std::vector<double> m_data;
mutex m_mutex;
my_class(); //non-copyable
}
run() and get_data() may be called by different openmp threads and so I introduce a lock.
(Since am using openmp, m_mutex and lock are RAII wrappers around omp_init_lock(); etc. commands).
However, the lock on get_data () is expensive to create and destroy (The most expensive operation when I profile my code - I call get_data() a lot).
Is is possible to reorganise my_class to remove the lock in get_data()? Or is this lock the unavoidable cost of parallelising the code?
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第一步是研究读写锁：这样多个读取器就不会互相阻塞。
下一步将是使用无锁或无等待操作。网上有很多资源比我能更好地描述它们。请注意：无锁方法处理原子（互锁）操作，这意味着数据大小需要很小。如果您走这条路，您将自动替换指向向量的指针，而不是整个向量。这意味着您的类将变得更加复杂，并且将处理一些指针和内存管理。
First step would be to look into read-write locks: this way multiple readers will not block each other.
The next step would be using lock-free or wait-free operations. There are plenty of resources online describing them better than I would be able to. Just one note: lock-free approaches deal with atomic (interlocked) operations, which means the data size needs to be small. If you go this route, you'll be atomically replacing a pointer to your vector, not the whole vector. This means your class will get a bit more complex and will deal with some pointers and memory management.
在 get_data/run 函数周围使用关键部分可能会更便宜，您不会产生额外的设置/拆卸开销（因为关键部分是静态初始化的），但这也会同步该类的其他实例。
It may be cheaper to use a critical section around get_data/run functions, you will not incur additional setup/teardown overhead (as the critical section is statically initialized), but this would also synchronize other instances of the class.