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。