leveldb 之 内存管理

leveldb 内存管理部分主要是arena.h 和arena.cc 部分完成

整体的内存管理策略是按照block管理。大于1/4的都按照block分配。作为一个单独的block维护。

当小于1/4block的尽量复用剩余内存。 

不太理解为什么不是那种直接分配大内存的方式(这样估计还是会产生很多内存空洞)。

arena.h文件如下：

// 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 <cstddef>
#include <vector>
#include <assert.h>
#include <stdint.h>

namespace leveldb {

/*
 * 内存管理的类
 */
class Arena {
 public:
  Arena();
  ~Arena();

  // Return a pointer to a newly allocated memory block of "bytes" bytes.
  /*
   * 分配内存
   */
  char* Allocate(size_t bytes);

  // Allocate memory with the normal alignment guarantees provided by malloc
  /*
   * 分配内存，内存对齐
   */
  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).
  /*
   * 返回分配的内存数
   */
  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&);
};

/*
 * bytes 必须大于0
 * 当bytes 小于alloc_bytes_remaining_ 时，使用alloc_bytes_remaining_
 * 否则调用AllocateFallback 分配内存。
 */
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);
}

}  // namespace leveldb

#endif  // STORAGE_LEVELDB_UTIL_ARENA_H_

　　

arena.cc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 {

/*
 * block 大小 4K
 */
static const int kBlockSize = 4096;

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

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

/*
 * 大于1/4 块大小的时候,分配单独块
 * 否则，分配一个快，使用邋alloc_ptr指向带分贝位置。
 * 使用alloc_bytes_remaining_ 保存剩余内存大小
 * 
 */
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;
}

/*
 * 考虑内存对齐的内存分配。
 *  align必须为2的指数次方
 */
char* Arena::AllocateAligned(size_t bytes) {
  const int align = sizeof(void*);    // We'll align to pointer size
  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;
}

/*
 * 分配新的内存块
 */
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