levelDB源码分析（内存池）
来源：互联网 发布：华硕网络视频电话 编辑：程序博客网 时间：2024/04/29 08:03
// 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). // 返回当前占用的内存，包括block以及std::vector占用 size_t MemoryUsage() const { return blocks_memory_ + blocks_.capacity() * sizeof(char*); } private: char* AllocateFallback(size_t bytes); // 如果旧的block空间不够，则申请一个新的block char* AllocateNewBlock(size_t block_bytes); // 申请一个新的block // Allocation state char* alloc_ptr_; //当前空闲block地址 size_t alloc_bytes_remaining_; //当前空闲的block剩余内存大小 // Array of new[] allocated memory blocks std::vector<char*> blocks_; //已经申请的block的地址集合 // Bytes of memory in blocks allocated so far size_t blocks_memory_; //已经申请的所有block的内存大小 // No copying allowed Arena(const Arena&); void operator=(const Arena&);};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_) {//检查当前的block是否可以分配，如果可以，直接分配 char* result = alloc_ptr_; alloc_ptr_ += bytes; alloc_bytes_remaining_ -= bytes; return result; } return AllocateFallback(bytes);}} // namespace leveldb#endif // STORAGE_LEVELDB_UTIL_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.#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;}Arena::~Arena() { for (size_t i = 0; i < blocks_.size(); i++) { delete[] blocks_[i]; }}char* Arena::AllocateFallback(size_t bytes) { if (bytes > kBlockSize / 4) { //Èç¹û´óÓÚkBlockSizeµÄ1/4£¨1K£©£¬ÔòÖ±½Ó·ÖÅäÄÚ´æ // 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); //°´kBlockSize£¨4k£©·ÖÅäÄڴ棬Ȼºóµ÷ÕûÖ¸Õë alloc_bytes_remaining_ = kBlockSize; char* result = alloc_ptr_; alloc_ptr_ += bytes; alloc_bytes_remaining_ -= bytes; return result;}char* Arena::AllocateAligned(size_t bytes) { 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); // ¼ì²éµ±Ç°µÄµØÖ·ÊÇ·ñ¶ÔÆä size_t slop = (current_mod == 0 ? 0 : align - current_mod); //µ±Ç°µØַû¶ÔÆ䣬ÐèÒªµ÷Õû£¬slopΪÐèÒªµ÷ÕûµÄ×Ö½ÚÊý size_t needed = bytes + slop; //¶ÔÆäºóÕæÕýÐèÒªÉêÇëµÄÄÚ´æ´óС char* result; //内存充足时直接分配 if (needed <= alloc_bytes_remaining_) { result = alloc_ptr_ + slop; //ÄÚ´æ¶ÔÆäºóµÄµØÖ· alloc_ptr_ += needed; alloc_bytes_remaining_ -= needed; } else { //内存不足时重新分配 // AllocateFallback always returned aligned memory result = AllocateFallback(bytes); } assert((reinterpret_cast<uintptr_t>(result) & (align-1)) == 0); return result;}//分配固定大小的内存放到vector里面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
0 0
