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rocksdb/util/arena.h

91 lines
3.0 KiB

// Copyright (c) 2013, Facebook, Inc. All rights reserved.
// This source code is licensed under the BSD-style license found in the
// LICENSE file in the root directory of this source tree. An additional grant
// of patent rights can be found in the PATENTS file in the same directory.
//
// 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.
// Arena is an implementation of Arena class. For a request of small size,
// it allocates a block with pre-defined block size. For a request of big
// size, it uses malloc to directly get the requested size.
#pragma once
#include <cstddef>
#include <vector>
#include <assert.h>
#include <stdint.h>
#include "util/arena.h"
namespace rocksdb {
class Arena {
public:
// No copying allowed
Arena(const Arena&) = delete;
void operator=(const Arena&) = delete;
static const size_t kMinBlockSize;
static const size_t kMaxBlockSize;
explicit Arena(size_t block_size = kMinBlockSize);
~Arena();
char* Allocate(size_t bytes);
char* AllocateAligned(size_t bytes);
// Returns an estimate of the total memory usage of data allocated
// by the arena (exclude the space allocated but not yet used for future
// allocations).
size_t ApproximateMemoryUsage() const {
return blocks_memory_ + blocks_.capacity() * sizeof(char*) -
alloc_bytes_remaining_;
}
size_t MemoryAllocatedBytes() const { return blocks_memory_; }
private:
// Number of bytes allocated in one block
const size_t kBlockSize;
// Array of new[] allocated memory blocks
typedef std::vector<char*> Blocks;
Blocks blocks_;
// Stats for current active block.
// For each block, we allocate aligned memory chucks from one end and
// allocate unaligned memory chucks from the other end. Otherwise the
// memory waste for alignment will be higher if we allocate both types of
// memory from one direction.
char* unaligned_alloc_ptr_ = nullptr;
char* aligned_alloc_ptr_ = nullptr;
// How many bytes left in currently active block?
size_t alloc_bytes_remaining_ = 0;
char* AllocateFallback(size_t bytes, bool aligned);
char* AllocateNewBlock(size_t block_bytes);
// Bytes of memory in blocks allocated so far
size_t blocks_memory_ = 0;
};
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_) {
unaligned_alloc_ptr_ -= bytes;
alloc_bytes_remaining_ -= bytes;
return unaligned_alloc_ptr_;
}
return AllocateFallback(bytes, false /* unaligned */);
}
// check and adjust the block_size so that the return value is
// 1. in the range of [kMinBlockSize, kMaxBlockSize].
// 2. the multiple of align unit.
extern size_t OptimizeBlockSize(size_t block_size);
} // namespace rocksdb