LevelDB源码分析5-Arena.md

Arena为leveldb的内存管理单元，避免频繁的new/delete,减少内存申请和释放带来的开销。

整体类

class Arena { public:  Arena();  ~Arena();  // Return a pointer to a newly allocated memory block of "bytes" bytes.  char* Allocate(size_t bytes);  // Allocate memory with the normal alignment guarantees provided by malloc  //yanke:保证字节对齐  char* AllocateAligned(size_t bytes);  // Returns an estimate of the total memory usage of data allocated  // by the arena (including space allocated but not yet used for user  // allocations).  size_t MemoryUsage() const {    return blocks_memory_ + blocks_.capacity() * sizeof(char*);  } private:  char* AllocateFallback(size_t bytes);  char* AllocateNewBlock(size_t block_bytes);  // Allocation state  char* alloc_ptr_;     //指向当前内存块剩余空间的首地址  size_t alloc_bytes_remaining_;//剩余的内存数  // Array of new[] allocated memory blocks  std::vector<char*> blocks_; //blocks_  // Bytes of memory in blocks allocated so far  size_t blocks_memory_;        //分配的总内存  // No copying allowed  //yanke:只声明不实现，防止编译器隐式拷贝  Arena(const Arena&);  void operator=(const Arena&);};

具体分析

成员变量

其中的blocks_是一个char*的vector,这个vector中每个元素都是指向一块内存(大小为4096，4K)

其他的成员变量

char* alloc_ptr_;//指向当前内存块剩余空间的首地址size_t alloc_bytes_remaining_;//剩余的内存数std::vector<char*> blocks_; //blocks_size_t blocks_memory_;      //分配的总内存

构造函数和析构函数

//构造函数：分配内存为0Arena::Arena() {  blocks_memory_ = 0;  alloc_ptr_ = NULL;  // First allocation will allocate a block  alloc_bytes_remaining_ = 0;}//析构函数：释放blocks_中每一个元素指向的内存Arena::~Arena() {  for (size_t i = 0; i < blocks_.size(); i++) {    delete[] blocks_[i];  }}

内存分配

Arena维护一个char*的vector,每次需要内存的时候就向操作系统申请4K的空间，
这样就不用每次申请一个小的空间时都malloc。

//内存分配函数：//1、如果分配的内存小于等于alloc_bytes_remaining_内存，那么直接分配就行了//2、否则调用AllocateFallback函数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);}char* Arena::AllocateFallback(size_t bytes) {    //如果比kBlockSize / 4还要多，直接给这个内存单独分配一个内存。    //防止内存碎片  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.  //处理剩余空间的方法：直接放弃掉，开辟一个新的4K的空间  alloc_ptr_ = AllocateNewBlock(kBlockSize);    //向操作系统申请4K空间  alloc_bytes_remaining_ = kBlockSize;          //然后这就是当前的块剩余的空间4K  char* result = alloc_ptr_;                    //这是新的空间  alloc_ptr_ += bytes;        alloc_bytes_remaining_ -= bytes;              //新的剩余空间为kBlockSize-bytes  return result;                                //返回}//分配新的内存块函数：需求是block_byteschar* Arena::AllocateNewBlock(size_t block_bytes) {  char* result = new char[block_bytes]; //分配内存，block_bytes  blocks_memory_ += block_bytes;//总的内存增加  blocks_.push_back(result);//添加到内存指针数组中  return result;}

内存使用量

包含放弃的小于1K的空间

  size_t MemoryUsage() const {    return blocks_memory_ + blocks_.capacity() * sizeof(char*);  }

内存对齐分配

char* Arena::AllocateAligned(size_t bytes) {    //根据void*的大小对齐  const int align = sizeof(void*);    // We'll align to pointer size  //align应该为2的幂，如果不为2的幂，说明出错了，检测到了如此地步。  assert((align & (align-1)) == 0);   // Pointer size should be a power of 2  //又用到&和%计算的指是一样的，但是&快  size_t current_mod = reinterpret_cast<uintptr_t>(alloc_ptr_) & (align-1);  //计算需要添加的字节数  size_t slop = (current_mod == 0 ? 0 : align - current_mod);  //因此内存对齐后需要的字节为bytes+slop  size_t needed = bytes + slop;  char* result;  //如果需求小于剩余的bytes，那么直接分配  if (needed <= alloc_bytes_remaining_) {    result = alloc_ptr_ + slop;    alloc_ptr_ += needed;    alloc_bytes_remaining_ -= needed;  } else {    // 否则调用AllocateFallback函数    result = AllocateFallback(bytes);  }  assert((reinterpret_cast<uintptr_t>(result) & (align-1)) == 0);  return result;}