VectorMemory< VectorType > Class Template Reference

#include <deal.II/lac/vector_memory.h>

Inheritance diagram for VectorMemory< VectorType >:

Classes
class	Pointer

Public Member Functions
virtual	~VectorMemory () override=default
virtual VectorType *	alloc ()=0
virtual void	free (const VectorType *const)=0

Static Public Member Functions
static ::ExceptionBase &	ExcNotAllocatedHere ()

Subscriptor functionality
Classes derived from Subscriptor provide a facility to subscribe to this object. This is mostly used by the SmartPointer class.
void	subscribe (std::atomic< bool > *const validity, const std::string &identifier="") const
void	unsubscribe (std::atomic< bool > *const validity, const std::string &identifier="") const
unsigned int	n_subscriptions () const
template<typename StreamType >
void	list_subscribers (StreamType &stream) const
void	list_subscribers () const
template<class Archive >
void	serialize (Archive &ar, const unsigned int version)
static ::ExceptionBase &	ExcInUse (int arg1, std::string arg2, std::string arg3)
static ::ExceptionBase &	ExcNoSubscriber (std::string arg1, std::string arg2)
using	map_value_type = decltype(counter_map)::value_type
using	map_iterator = decltype(counter_map)::iterator
std::atomic< unsigned int >	counter
std::map< std::string, unsigned int >	counter_map
std::vector< std::atomic< bool > * >	validity_pointers
const std::type_info *	object_info
static std::mutex	mutex
void	check_no_subscribers () const noexcept

Detailed Description

template<typename VectorType = ::Vector<double>>
class VectorMemory< VectorType >

Memory management base class for vectors. This is an abstract base class used, among other places, by all iterative methods to allocate space for auxiliary vectors.

The purpose of this class is as follows: in iterative solvers and other places, one needs to allocate temporary storage for vectors, for example for auxiliary vectors. One could allocate and release them anew every time, but this may be expensive in some situations if it has to happen very frequently. A common case for this is when an iterative method is used to invert a matrix in each iteration of an outer solver, such as when inverting a matrix block for a Schur complement solver. (step-20 does this, for example, but instead just keeps a vector around permanently for temporary storage.)

In such situations, allocating and deallocating vectors anew in each call to the inner solver is expensive and leads to memory fragmentation. The present class allows to avoid this by offering an interface that other classes can use to allocate and deallocate vectors. Different derived classes then implement different strategies to provide temporary storage vectors to using classes.

For example, the PrimitiveVectorMemory class simply allocates and deallocates vectors via the operating system facilities (i.e., using new and delete) each time it is asked for a vector. It is an appropriate implementation to use for iterative solvers that are called only once, or very infrequently.

On the other hand, the GrowingVectorMemory class never returns memory space to the operating system memory management subsystem during its lifetime; it only marks them as unused and allows them to be reused next time a vector is requested.

Practical use

Classes derived from this base class return pointers to new vectors via the VectorMemory::alloc() function, and re-claim the vector when it is returned via VectorMemory::free(). These two functions therefore play a similar role as new and delete. This includes the usual drawbacks: It is simple to forget to call VectorMemory::free() at the end of a function that uses this facility, or to forget it in an if branch of the function where one has an early return from the function. In both cases, this results in a memory leak: a correct piece of code has to call VectorMemory::free() for all allocated vectors at all possible exit points. This includes places where a function is left because an exception is thrown further down in the call stack and not explicitly handled here.

In other words, vectors allocated via VectorMemory::alloc() have the same issue as raw pointers allocated via new: It is easy to write code that has memory leaks. In the case of raw pointers, the common solution is to use the std::unique_ptr class instead (see http://en.cppreference.com/w/cpp/memory/unique_ptr). In the case of the current class, the VectorMemory::Pointer class is the solution: it is a class that for all practical purposes looks like a pointer, but upon destruction also returns the vector back to the VectorMemory object from which it got it. Since destruction of the VectorMemory::Pointer class happens whenever it goes out of scope (whether because the function explicitly returns, or because control flow leaves it due to an exception), a memory leak cannot happen: the vector the VectroMemory::Pointer object points to is always returned.

Definition at line 104 of file vector_memory.h.

Constructor & Destructor Documentation

~VectorMemory()

template<typename VectorType = ::Vector<double>>

virtual VectorMemory< VectorType >::~VectorMemory ( )

overridevirtualdefault

Virtual destructor. This destructor is declared virtual to allow destroying objects of derived type through pointers to this base class.

Member Function Documentation

alloc()

template<typename VectorType = ::Vector<double>>

virtual VectorType * VectorMemory< VectorType >::alloc ( )

pure virtual

Return a pointer to a new vector. The number of elements or their subdivision into blocks (if applicable) is unspecified and users of this function should reset vectors to their proper size. The same holds for the contents of vectors: they are unspecified. In other words, the place that calls this function will need to resize or reinitialize it appropriately.

Warning

Just like using new and delete explicitly in code invites bugs where memory is leaked (either because the corresponding delete is forgotten altogether, or because of exception safety issues), using the alloc() and free() functions explicitly invites writing code that accidentally leaks memory. You should consider using the VectorMemory::Pointer class instead, which provides the same kind of service that std::unique provides for arbitrary memory allocated on the heap.

Implemented in PrimitiveVectorMemory< VectorType >, GrowingVectorMemory< VectorType >, GrowingVectorMemory< Vector< double > >, GrowingVectorMemory< BlockVector< number > >, and GrowingVectorMemory< VectorType >.

free()

template<typename VectorType = ::Vector<double>>

virtual void VectorMemory< VectorType >::free ( const VectorType * const  )

pure virtual

Return a vector and indicate that it is not going to be used any further by the place that called alloc() to get a pointer to it.

Warning

Just like using new and delete explicitly in code invites bugs where memory is leaked (either because the corresponding delete is forgotten altogether, or because of exception safety issues), using the alloc() and free() functions explicitly invites writing code that accidentally leaks memory. You should consider using the VectorMemory::Pointer class instead, which provides the same kind of service that std::unique provides for arbitrary memory allocated on the heap.

Implemented in PrimitiveVectorMemory< VectorType >, GrowingVectorMemory< VectorType >, and GrowingVectorMemory< VectorType >.

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The documentation for this class was generated from the following file:

• include/deal.II/lac/vector_memory.h