leveldb阅读-arena内存池

arena内存缓冲池采用一种比较简单的分配方案，只提供了Allocate() 和AllocateAligned()接口，系统申请的缓冲池随着arena的销毁而销毁。简单来说，arena不应该算作是缓冲池，应理解为一种内存管理机制，适合memtable的内存管理方案。

其中，arena.h头文件内容如下：

// Copyright (c) 2011 The LevelDB Authors. All rights reserved.// Use of this source code is governed by a BSD-style license that can be// found in the LICENSE file. See the AUTHORS file for names of contributors.#ifndef STORAGE_LEVELDB_UTIL_ARENA_H_#define STORAGE_LEVELDB_UTIL_ARENA_H_#include <vector>#include <assert.h>#include <stddef.h>#include <stdint.h>namespace leveldb {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  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_;  size_t alloc_bytes_remaining_;  // Array of new[] allocated memory blocks  std::vector<char*> blocks_;  // Bytes of memory in blocks allocated so far  size_t blocks_memory_;  // No copying allowed  Arena(const Arena&);  void operator=(const Arena&);};//直接申请空间，不处理对齐问题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);}}  // namespace leveldb#endif  // STORAGE_LEVELDB_UTIL_ARENA_H_

arena.cc文件内容如下：

// Copyright (c) 2011 The LevelDB Authors. All rights reserved.// Use of this source code is governed by a BSD-style license that can be// found in the LICENSE file. See the AUTHORS file for names of contributors.#include "util/arena.h"#include <assert.h>namespace leveldb {static const int kBlockSize = 4096;Arena::Arena() {  blocks_memory_ = 0;  alloc_ptr_ = NULL;  // First allocation will allocate a block  alloc_bytes_remaining_ = 0;}//释放管理的所有内存空间Arena::~Arena() {  for (size_t i = 0; i < blocks_.size(); i++) {    delete[] blocks_[i];  }}//判断分配的空间是否是大于1k，如果超过1k，那么申请独立的空间，否则就申请能够共享的空间char* Arena::AllocateFallback(size_t bytes) {  //如果大于1k，申请bytes大小的空间  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;  }  //系统中之前剩余的空间不要了，直接申请新的空间，可以参见allocate()函数实现  // 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;}char* Arena::AllocateAligned(size_t bytes) {  //至少8字节对齐  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);//取得对齐需要空出来的最小空间  size_t needed = bytes + slop;//需要的空间加上对齐所需要的空间  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);//确保对齐  return result;}char* Arena::AllocateNewBlock(size_t block_bytes) {  char* result = new char[block_bytes];  blocks_memory_ += block_bytes;  blocks_.push_back(result);  return result;}}  // namespace leveldb

arena.h虽然没有比较复杂的管理机制，但是与memtable的一些特性有比较密切的联系，memtable的所有操作都不会立刻清楚内存区域，因此这个内存管理机制还是很适合的，后面会逐一回过头来修改现在的理解。