Reference documentation for deal.II version Git 2e98021fd4 2020-10-20 17:25:20 -0400
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Classes | Public Member Functions | Static Public Member Functions | List of all members
VectorMemory< VectorType > Class Template Referenceabstract

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

Inheritance diagram for VectorMemory< VectorType >:
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Classes

class  Pointer
 

Public Member Functions

virtual ~VectorMemory () override=default
 
virtual VectorTypealloc ()=0
 
virtual void free (const VectorType *const)=0
 
template<class Archive >
void serialize (Archive &ar, const unsigned int version)
 
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
 

Static Public Member Functions

static ::ExceptionBaseExcNotAllocatedHere ()
 
static ::ExceptionBaseExcInUse (int arg1, std::string arg2, std::string arg3)
 
static ::ExceptionBaseExcNoSubscriber (std::string arg1, std::string arg2)
 

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 GrowingVectorMemory< VectorType >, GrowingVectorMemory< BlockVector< number > >, and PrimitiveVectorMemory< 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 GrowingVectorMemory< VectorType >, GrowingVectorMemory< BlockVector< number > >, and PrimitiveVectorMemory< VectorType >.

◆ subscribe()

void Subscriptor::subscribe ( std::atomic< bool > *const  validity,
const std::string &  identifier = "" 
) const
inherited

Subscribes a user of the object by storing the pointer validity. The subscriber may be identified by text supplied as identifier.

Definition at line 136 of file subscriptor.cc.

◆ unsubscribe()

void Subscriptor::unsubscribe ( std::atomic< bool > *const  validity,
const std::string &  identifier = "" 
) const
inherited

Unsubscribes a user from the object.

Note
The identifier and the validity pointer must be the same as the one supplied to subscribe().

Definition at line 156 of file subscriptor.cc.

◆ n_subscriptions()

unsigned int Subscriptor::n_subscriptions ( ) const
inlineinherited

Return the present number of subscriptions to this object. This allows to use this class for reference counted lifetime determination where the last one to unsubscribe also deletes the object.

Definition at line 300 of file subscriptor.h.

◆ list_subscribers() [1/2]

template<typename StreamType >
void Subscriptor::list_subscribers ( StreamType &  stream) const
inlineinherited

List the subscribers to the input stream.

Definition at line 317 of file subscriptor.h.

◆ list_subscribers() [2/2]

void Subscriptor::list_subscribers ( ) const
inherited

List the subscribers to deallog.

Definition at line 204 of file subscriptor.cc.

◆ serialize()

template<class Archive >
void Subscriptor::serialize ( Archive &  ar,
const unsigned int  version 
)
inlineinherited

Read or write the data of this object to or from a stream for the purpose of serialization.

This function does not actually serialize any of the member variables of this class. The reason is that what this class stores is only who subscribes to this object, but who does so at the time of storing the contents of this object does not necessarily have anything to do with who subscribes to the object when it is restored. Consequently, we do not want to overwrite the subscribers at the time of restoring, and then there is no reason to write the subscribers out in the first place.

Definition at line 309 of file subscriptor.h.


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