arena.h

// 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 <atomic>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <vector>

namespace leveldb {

// Arena 预先分配一大块内存，然后从中连续的进行分配。
// Arena 上分配出的内存块不能单独释放，只能随 Arena 整个释放
//
// 初始化 Arena:
// +-------------------------------+
// |          一大块内存             |
// +-------------------------------+
//
// 分配一个 4KB 的基本块:
// +-------------------------------+
// | 4KB |           free          |
// +-------------------------------+
//
// 再分配一个 16KB 的大块:
// +-------------------------------+
// | 4KB |   16KB  |     free      |
// +-------------------------------+
// 
// 最后把整个 Arena 一起释放掉
// Arena 可以显著的加快内存分配速度并减少内存碎片
class Arena {
 public:
  Arena();

  Arena(const Arena&) = delete;
  Arena& operator=(const Arena&) = delete;

  ~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.
  size_t MemoryUsage() const {
    return memory_usage_.load(std::memory_order_relaxed);
  }

 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_;

  // Total memory usage of the arena.
  //
  // TODO(costan): This member is accessed via atomics, but the others are
  //               accessed without any locking. Is this OK?
  std::atomic<size_t> memory_usage_;
};

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_