From: Wolfgang Bangerth Date: Thu, 29 Jul 2021 01:41:45 +0000 (-0600) Subject: Introduce deleter objects for AlignedVector. X-Git-Tag: v9.4.0-rc1~1055^2~6 X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=2717483de36819ea288079f5a28283968576d301;p=dealii.git Introduce deleter objects for AlignedVector. --- diff --git a/include/deal.II/base/aligned_vector.h b/include/deal.II/base/aligned_vector.h index b0e248ba2e..06ef40941c 100644 --- a/include/deal.II/base/aligned_vector.h +++ b/include/deal.II/base/aligned_vector.h @@ -465,9 +465,200 @@ public: private: /** - * Pointer to actual data array. + * A class that is used as the "deleter" for a `std::unique_ptr` object that + * AlignedVector uses to store the memory used for the elements. + * + * There are two ways the AlignedVector class can handle memory: + * - Allocation via `new[]` in reserve() where we call `posix_memalign()` + * to obtain a chunk of memory and then do placement-`new` to initialize + * memory. If this is what we have done, then we need to call the + * destructors of the currently active elements by hand, and then + * call `std::free()` to return memory. In order to call the destructors + * of currently used elements, the deleter object needs to have access + * to the owning `AlignedVector` object to know which of the allocated + * elements are currently actually used. + * - We have called `replicate_across_communicator()`, in which case the + * elements have been moved into a memory "window" managed by MPI. + * In that case, one process (the root process of an MPI communicator + * that ties together all processes on one node) needs to call the + * destructor of all elements in this shared memory space, and then + * all processes on that communicator need to first destroy the + * MPI window object, and then the MPI communicator for the shared + * memory node's processes. In this approach, we need to store the + * following data: A pointer to the owning AlignedVector object to know + * which elements need to be destroyed, copies of the MPI window + * and communicator objects, and a couple of ancillary pieces of data. + * + * A common idiom towards using `std::unique_ptr` with complex de-allocation + * semantics is to use `std::unique_ptr` + * and then use a lambda function to initialize the `std::function` + * deleter object. This approach is used in numerous other places in + * deal.II, but this has two downsides: + * - `std::function` is relatively memory-hungry. It takes 24 bytes by + * itself, but then also needs to allocate memory dynamically, for + * example to store the capture object of a lambda function. This ends + * up to be quite a lot of memory given that we frequently use small + * Vector objects (which build on AlignedVector). + * - More importantly, this breaks move operations. In a move constructor + * or move assignment of AlignedVector, we want to steal the memory pointed + * to, but we then need to install a new deleter object because the deleter + * needs to know about the owning object to determine which elements to + * call the destructor for. The problem is that we can't use the old + * deleter (it is a lambda function that still points to the previous + * owner that we are moving *from*) but we also don't know what deleter + * to install otherwise -- we don't know the innards of the lambda function + * that was previously installed; in fact, we don't even know whether it + * was for regular or MPI shared-memory data management. + * + * The way out of this is to write a custom deleter. It stores a pointer to + * the owning AlignedVector object. It then also stores a `std::unique_ptr` + * to an object of a class derived from a base class that implements the + * concrete action necessary to facilitate the "deleter" action. Based on + * the arguments given to the constructor of the Deleter class, the + * constructor then either allocates a "regular" or an MPI-based action + * object, and the action is facilitated by an overloaded `virtual` function. + * + * In the end, this solves both of our problems: + * - The deleter object only requires 16 bytes (two pointers) plus whatever + * dynamic memory is necessary to store the "action" objects. + * - In move operations, the only thing that needs to be done is to tell + * the deleter about the change of owning AlignedVector object, which we + * can achieve via the reset_owning_object() function. + */ + class Deleter + { + public: + /** + * Constructor. When this constructor is called, it installs an + * action that corresponds to "regular" memory allocation that + * needs to be handled by using `std::free()`. + */ + Deleter(AlignedVector *owning_object); + + /** + * Constructor. When this constructor is called, it installs an + * action that corresponds to MPI-based shared memory allocation that + * needs to be handled by letting MPI de-allocate the shared memory + * window, plus destroying the MPI communicator, and doing other + * clean-up work. + */ + Deleter(AlignedVector *owning_object, + const bool is_shmem_root, + const size_type array_size, + T * aligned_shmem_pointer, + MPI_Comm shmem_group_communicator, + MPI_Win shmem_window); + + + /** + * The operator called by `std::unique_ptr` to destroy the data it + * is storing. This function dispatches to the different actions that + * this class implements. + */ + void + operator()(T *ptr); + + /** + * Reset the pointer to the owning AlignedVector object. This function + * is used in move operations when the pointer to the data is transferred + * from one AlignedVector object -- i.e., the pointer itself remains + * unchanged, but the deleter object needs to be updated to know who + * the new owner now is. + */ + void + reset_owning_object(const AlignedVector *new_aligned_vector_ptr); + + private: + /** + * Base class for the action necessary to de-allocate memory. + */ + class DeleterActionBase + { + public: + /** + * Destructor, made `virtual` to allow for derived classes. + */ + virtual ~DeleterActionBase() = default; + + /** + * The function that implements the action of de-allocating memory. + * It receives as arguments a pointer to the owning AlignedVector object + * as well as a pointer to the memory being de-allocated. + */ + virtual void + delete_array(const AlignedVector *owning_aligned_vector, T *ptr) = 0; + }; + + /** + * A class that implements the deleter action for "regularly" allocated + * memory. + */ + class DefaultDeleterAction : public DeleterActionBase + { + public: + /** + * The function that implements the action of de-allocating memory. + * It receives as arguments a pointer to the owning AlignedVector object + * as well as a pointer to the memory being de-allocated. + */ + virtual void + delete_array(const AlignedVector *aligned_vector, T *ptr); + }; + + /** + * A class that implements the deleter action for MPI shared-memory + * allocated data. + */ + class MPISharedMemDeleterAction : public DeleterActionBase + { + public: + /** + * Constructor. Store the various pieces of information necessary to + * identify the MPI window in which the data resides. + */ + MPISharedMemDeleterAction(const bool is_shmem_root, + const size_type array_size, + T * aligned_shmem_pointer, + MPI_Comm shmem_group_communicator, + MPI_Win shmem_window); + + /** + * The function that implements the action of de-allocating memory. + * It receives as arguments a pointer to the owning AlignedVector object + * as well as a pointer to the memory being de-allocated. + */ + virtual void + delete_array(const AlignedVector *aligned_vector, T *ptr); + + private: + /** + * Variables necessary to identify the MPI shared-memory window plus + * all ancillary information to destroy this window. + */ + const bool is_shmem_root; + const size_type array_size; + T * aligned_shmem_pointer; + MPI_Comm shmem_group_communicator; + MPI_Win shmem_window; + }; + + /** + * A pointer to the object that facilitates the actual action of + * destroying the memory. + */ + std::unique_ptr deleter_action_object; + + /** + * A (non-owned) pointer to the surrounding AlignedVector object that owns + * the memory this deleter object is responsible for deleting. + */ + const AlignedVector *owning_aligned_vector; + }; + + /** + * Pointer to actual data array, using the custom deleter class above. */ - std::unique_ptr> elements; + std::unique_ptr elements; /** * Pointer to one past the last valid value. @@ -836,9 +1027,111 @@ namespace internal #ifndef DOXYGEN + +template +inline AlignedVector::Deleter::Deleter(AlignedVector *owning_object) + : deleter_action_object(std::make_unique()) + , owning_aligned_vector(owning_object) +{} + + + +template +inline AlignedVector::Deleter::Deleter(AlignedVector *owning_object, + const bool is_shmem_root, + const size_type array_size, + T * aligned_shmem_pointer, + MPI_Comm shmem_group_communicator, + MPI_Win shmem_window) + : deleter_action_object( + std::make_unique(is_shmem_root, + array_size, + aligned_shmem_pointer, + shmem_group_communicator, + shmem_window)) + , owning_aligned_vector(owning_object) +{} + + + +template +inline void +AlignedVector::Deleter::operator()(T *ptr) +{ + Assert(deleter_action_object != nullptr, ExcInternalError()); + deleter_action_object->delete_array(owning_aligned_vector, ptr); +} + + + +template +inline void +AlignedVector::Deleter::reset_owning_object( + const AlignedVector *new_aligned_vector_ptr) +{ + owning_aligned_vector = new_aligned_vector_ptr; +} + + + +template +inline void +AlignedVector::Deleter::DefaultDeleterAction::delete_array( + const AlignedVector *aligned_vector, + T * ptr) +{ + if (ptr != nullptr) + { + Assert(aligned_vector->used_elements_end != nullptr, ExcInternalError()); + + if (std::is_trivial::value == false) + for (T *p = aligned_vector->used_elements_end - 1; p >= ptr; --p) + p->~T(); + } + + std::free(ptr); +} + + + +template +inline AlignedVector::Deleter::MPISharedMemDeleterAction:: + MPISharedMemDeleterAction(const bool is_shmem_root, + const size_type array_size, + T * aligned_shmem_pointer, + MPI_Comm shmem_group_communicator, + MPI_Win shmem_window) + : is_shmem_root(is_shmem_root) + , array_size(array_size) + , aligned_shmem_pointer(aligned_shmem_pointer) + , shmem_group_communicator(shmem_group_communicator) + , shmem_window(shmem_window) +{} + + + +template +inline void +AlignedVector::Deleter::MPISharedMemDeleterAction::delete_array( + const AlignedVector *aligned_vector, + T * ptr) +{ + if (is_shmem_root) + for (unsigned int i = 0; i < array_size; ++i) + aligned_shmem_pointer[i].~T(); + + int ierr; + ierr = MPI_Win_free(&shmem_window); + AssertThrowMPI(ierr); + + ierr = MPI_Comm_free(&shmem_group_communicator); + AssertThrowMPI(ierr); +} + + template inline AlignedVector::AlignedVector() - : elements(nullptr, [](T *) { Assert(false, ExcInternalError()); }) + : elements(nullptr, Deleter(this)) , used_elements_end(nullptr) , allocated_elements_end(nullptr) {} @@ -847,7 +1140,7 @@ inline AlignedVector::AlignedVector() template inline AlignedVector::AlignedVector(const size_type size, const T &init) - : elements(nullptr, [](T *) { Assert(false, ExcInternalError()); }) + : elements(nullptr, Deleter(this)) , used_elements_end(nullptr) , allocated_elements_end(nullptr) { @@ -859,7 +1152,7 @@ inline AlignedVector::AlignedVector(const size_type size, const T &init) template inline AlignedVector::AlignedVector(const AlignedVector &vec) - : elements(nullptr, [](T *) { Assert(false, ExcInternalError()); }) + : elements(nullptr, Deleter(this)) , used_elements_end(nullptr) , allocated_elements_end(nullptr) { @@ -904,23 +1197,13 @@ AlignedVector::operator=(AlignedVector &&vec) noexcept clear(); // Move the actual data in the 'elements' object. One problem is that this - // also moves the deleter object, but the deleter object is a lambda function - // that references 'this' (i.e., the 'this' pointer of the *moved-from* - // object). So what we actually do is steal the pointer via - // std::unique_ptr::release() and then install our own deleter object that - // mirrors the one used in reserve() below. - elements = decltype(elements)(vec.elements.release(), [this](T *ptr) { - if (ptr != nullptr) - { - Assert(this->used_elements_end != nullptr, ExcInternalError()); - - if (std::is_trivial::value == false) - for (T *p = this->used_elements_end - 1; p >= ptr; --p) - p->~T(); - } - - std::free(ptr); - }); + // also moves the deleter object, but the deleter object + // references 'this' (i.e., the 'this' pointer of the *moved-from* + // object). The way this is implemented is that we have to move the + // deleter as well, and then reset the pointer inside the deleter + // that references the outer object. + elements = std::move(vec.elements); + elements.get_deleter().reset_owning_object(this); // Then also steal the other pointers and clear them in the original object: used_elements_end = vec.used_elements_end; @@ -1063,18 +1346,7 @@ AlignedVector::reserve(const size_type new_allocated_size) // reverse order, and then release the memory. Note that we catch the // 'this' pointer because the number of elements currently alive might // change over time. - auto deleter = [this](T *ptr) { - if (ptr != nullptr) - { - Assert(this->used_elements_end != nullptr, ExcInternalError()); - - if (std::is_trivial::value == false) - for (T *p = this->used_elements_end - 1; p >= ptr; --p) - p->~T(); - } - - std::free(ptr); - }; + Deleter deleter(this); // copy whatever elements we need to retain if (new_allocated_size > 0) @@ -1088,9 +1360,10 @@ AlignedVector::reserve(const size_type new_allocated_size) // Note that at the time of releasing the old memory, 'used_elements_end' // still points to its previous value, and this is important for the // deleter object of the previously allocated array (see how it loops over - // the to-be-destroyed elements a few lines above). - elements = decltype(elements)(new_data_ptr, deleter); - used_elements_end = elements.get() + old_size; + // the to-be-destroyed elements a the Deleter::DefaultDeleterAction + // class). + elements = decltype(elements)(new_data_ptr, std::move(deleter)); + used_elements_end = elements.get() + old_size; allocated_elements_end = elements.get() + new_size; } else if (new_allocated_size == 0) @@ -1559,23 +1832,12 @@ AlignedVector::replicate_across_communicator(const MPI_Comm & communicator, // that is encapsulated in the following call where the deleter makes copies // of the arguments in the lambda capture. elements = decltype(elements)(aligned_shmem_pointer, - [is_shmem_root, - array_size, - aligned_shmem_pointer, - shmem_group_communicator, - shmem_window](T *) mutable { - if (is_shmem_root) - for (size_type i = 0; i < array_size; ++i) - aligned_shmem_pointer[i].~T(); - - int ierr; - ierr = MPI_Win_free(&shmem_window); - AssertThrowMPI(ierr); - - ierr = - MPI_Comm_free(&shmem_group_communicator); - AssertThrowMPI(ierr); - }); + Deleter(this, + is_shmem_root, + array_size, + aligned_shmem_pointer, + shmem_group_communicator, + shmem_window)); // We then also have to set the other two pointers that define the state of // the current object. Note that the new buffer size is exactly as large as