leveldb arena

arena

a simple memery pool

结构

class Arena {
  // Allocation state
  char* alloc_ptr_;  // 指向当前可分配的空间
  size_t alloc_bytes_remaining_; // 当前块剩余可分配的空间
  // Array of new[] allocated memory blocks
  std::vector<char*> blocks_; // 所有历史创建的块
  // ...
}

构造函数

Arena::Arena() : memory_usage_(0) {
  alloc_ptr_ = NULL;  // First allocation will allocate a block
  alloc_bytes_remaining_ = 0;
}

析构函数

Arena::~Arena() {
  for (size_t i = 0; i < blocks_.size(); i++) {
    delete[] blocks_[i];
  }
}

获取内存

inline char* Arena::Allocate(size_t bytes) {
  // The semantics of what to return are a bit messy if we allow
  // 0-byte allocations, so we disallow them here (we don't need
  // them for our internal use).
  assert(bytes > 0);
  if (bytes <= alloc_bytes_remaining_) {
    char* result = alloc_ptr_;
    alloc_ptr_ += bytes;
    alloc_bytes_remaining_ -= bytes;
    return result;
  }
  return AllocateFallback(bytes);
}

获取一块对齐的内存

char* Arena::AllocateAligned(size_t bytes) {
  const int align = (sizeof(void*) > 8) ? sizeof(void*) : 8;
  assert((align & (align-1)) == 0);   // Pointer size should be a power of 2
  size_t current_mod = reinterpret_cast<uintptr_t>(alloc_ptr_) & (align-1);
  size_t slop = (current_mod == 0 ? 0 : align - current_mod);
  size_t needed = bytes + slop;
  char* result;
  if (needed <= alloc_bytes_remaining_) {
    result = alloc_ptr_ + slop;
    alloc_ptr_ += needed;
    alloc_bytes_remaining_ -= needed;
  } else {
    // AllocateFallback always returned aligned memory
    result = AllocateFallback(bytes);
  }
  assert((reinterpret_cast<uintptr_t>(result) & (align-1)) == 0);
  return result;
}

获取内存失败, 重新申请一块

char* Arena::AllocateFallback(size_t bytes) {
  if (bytes > kBlockSize / 4) {
    // Object is more than a quarter of our block size.  Allocate it separately
    // to avoid wasting too much space in leftover bytes.
    char* result = AllocateNewBlock(bytes);
    return result;
  }

  // We waste the remaining space in the current block.
  alloc_ptr_ = AllocateNewBlock(kBlockSize);
  alloc_bytes_remaining_ = kBlockSize;

  char* result = alloc_ptr_;
  alloc_ptr_ += bytes;
  alloc_bytes_remaining_ -= bytes;
  return result;
}

获取一块新的内存

char* Arena::AllocateNewBlock(size_t block_bytes) {
  char* result = new char[block_bytes];
  blocks_.push_back(result);
  memory_usage_.NoBarrier_Store(
      reinterpret_cast<void*>(MemoryUsage() + block_bytes + sizeof(char*)));
  return result;
}

总结

arena是一个简单的内存池,简单在于没有考虑任何复用和回收的问题,并且对于空间的利用率不是很高,优点在于大量的占用空间小的值的分配速度