boost::managed_mapped_file 无法分配所有成长的 space

Question

我正在尝试增加一个内存映射文件，我成功地增加了它，但是我无法分配我请求的所有额外 space - 我只是得到一个 std::bad_alloc .

这是一个示例，显示了 g++ 对 Linux 的影响（我在 MSVC 上的 'real' 代码中也看到了相同的效果）：

#include <memory>
#include <sstream>

#include <boost/interprocess/managed_mapped_file.hpp>
#include <boost/interprocess/allocators/allocator.hpp>
#include <boost/interprocess/containers/vector.hpp>
#include <boost/filesystem/operations.hpp>
#include <boost/uuid/uuid_io.hpp>
#include <boost/uuid/random_generator.hpp>

namespace
{
using MMapManager = boost::interprocess::basic_managed_mapped_file<
    char,
    boost::interprocess::rbtree_best_fit<boost::interprocess::null_mutex_family,
                                         boost::interprocess::offset_ptr<void>,
                                         16u>,
    boost::interprocess::iset_index>;

using MMapAllocatorType = boost::interprocess::allocator<
    std::size_t,
    MMapManager::segment_manager>;

using MMapContainerType = boost::interprocess::vector<
    std::size_t,
    MMapAllocatorType>;

// I've measured this at 256 bytes for my example configuration, but it's not
// documented anywhere, so let's overcompensate
constexpr auto ManagedFileOverhead = 1024u;

boost::filesystem::path getTemporaryFilePath()
{
    auto ss = std::stringstream{};
    ss << "MMap_test_" << boost::uuids::random_generator{}();

    return boost::filesystem::temp_directory_path() / ss.str();
}
}

int main()
{
    // Create memory mapped file, initially for 100 items
    auto capacity = 100u;
    const auto size = (capacity * sizeof(std::size_t)) + ManagedFileOverhead;
    const auto path = getTemporaryFilePath();

    auto file = std::make_unique<MMapManager>(
        boost::interprocess::create_only,
        path.string().c_str(),
        size);
    auto data = file->construct<MMapContainerType>("data_")(file->get_segment_manager());

    // Fill with stuff
    data->reserve(capacity);
    for (auto i = 0u; i < capacity; ++i) {
        data->push_back(i);
    }

    // Let's grow to hold 162 items (100 * golden ratio)
    capacity = 162u;
    const auto newFileSize = (capacity * sizeof(std::size_t)) + ManagedFileOverhead;
    const auto oldFileSize = boost::filesystem::file_size(path);
    const auto extraBytes = newFileSize - oldFileSize;

    // Unmap from the process, and grow
    file.reset();
    MMapManager::grow(path.string().c_str(), extraBytes);

    // Reopen it to re-map it into this process
    file = std::make_unique<MMapManager>(
        boost::interprocess::open_only,
        path.string().c_str());
    data = file->find<MMapContainerType>("data_").first;

    // Allocate it all
    data->reserve(capacity); // Bang, you're dead

    // Close down
    file.reset();
    boost::system::error_code ec;
    boost::filesystem::remove(path, ec);

    return EXIT_SUCCESS;
}

将储备（增长后）设置为 155 个项目有效，再多一个就会触发 std::bad_alloc。

为什么这不起作用？增长是否会导致映射文件中的额外管理开销导致我比预期早 space 运行？

Answer 1

您只是对分配器假设过多。

映射文件的增长将就地进行。增长向量不会。因此，虽然您只需要 extraBytes after 增加保留大小，但在保留期间您需要足够的 space 来容纳 both 旧分配和新分配。

使用以下方法证明：

MMapManager::grow(path.string().c_str(), oldFileSize + extraBytes);

或者先清除旧容器：

{
    auto file = std::make_unique<MMapManager>(boost::interprocess::open_only, path.string().c_str());
    file->destroy<MMapContainerType>("data_");
    auto data = file->construct<MMapContainerType>("data_")(file->get_segment_manager());
}

MMapManager::grow(path.string().c_str(), extraBytes);

boost::managed_mapped_file 无法分配所有成长的 space

boost::managed_mapped_file cannot allocate all of grown space

c++

boost

memory-mapped-files