leveldb 通用功能实现之arena
首先亮出leveldb arena 源码:
/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. #ifndef STORAGE_LEVELDB_UTIL_ARENA_H_ #define STORAGE_LEVELDB_UTIL_ARENA_H_ #include <cstddef> #include <vector> #include <assert.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&); }; 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_
/util/arena.cc
// 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) { // 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; } 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; } 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