LevelDB源码分析5-Arena.md

Arena为leveldb的内存管理单元，避免频繁的new/delete,减少内存申请和释放带来的开销。

整体类

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
//yanke:保证字节对齐
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).
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_;     //指向当前内存块剩余空间的首地址
size_t alloc_bytes_remaining_;//剩余的内存数

// Array of new[] allocated memory blocks
std::vector<char*> blocks_; //blocks_

// Bytes of memory in blocks allocated so far
size_t blocks_memory_;        //分配的总内存

// No copying allowed
//yanke:只声明不实现，防止编译器隐式拷贝
Arena(const Arena&);
void operator=(const Arena&);
};

具体分析

成员变量

其中的blocks_是一个char*的vector,这个vector中每个元素都是指向一块内存(大小为4096，4K)



其他的成员变量

char* alloc_ptr_;//指向当前内存块剩余空间的首地址
size_t alloc_bytes_remaining_;//剩余的内存数
std::vector<char*> blocks_; //blocks_
size_t blocks_memory_;      //分配的总内存

构造函数和析构函数

//构造函数：分配内存为0
Arena::Arena() {
blocks_memory_ = 0;
alloc_ptr_ = NULL;  // First allocation will allocate a block
alloc_bytes_remaining_ = 0;
}

//析构函数：释放blocks_中每一个元素指向的内存
Arena::~Arena() {
for (size_t i = 0; i < blocks_.size(); i++) {
delete[] blocks_[i];
}
}

内存分配

Arena维护一个

char*

的vector,每次需要内存的时候就向操作系统申请4K的空间，

这样就不用每次申请一个小的空间时都malloc。

//内存分配函数：
//1、如果分配的内存小于等于alloc_bytes_remaining_内存，那么直接分配就行了
//2、否则调用AllocateFallback函数
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);
}

char* Arena::AllocateFallback(size_t bytes) {
//如果比kBlockSize / 4还要多，直接给这个内存单独分配一个内存。
//防止内存碎片
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.
//处理剩余空间的方法：直接放弃掉，开辟一个新的4K的空间
alloc_ptr_ = AllocateNewBlock(kBlockSize);    //向操作系统申请4K空间
alloc_bytes_remaining_ = kBlockSize;          //然后这就是当前的块剩余的空间4K

char* result = alloc_ptr_;                    //这是新的空间
alloc_ptr_ += bytes;
alloc_bytes_remaining_ -= bytes;              //新的剩余空间为kBlockSize-bytes
return result;                                //返回
}

//分配新的内存块函数：需求是block_bytes
char* Arena::AllocateNewBlock(size_t block_bytes) {
char* result = new char[block_bytes]; //分配内存，block_bytes
blocks_memory_ += block_bytes;//总的内存增加
blocks_.push_back(result);//添加到内存指针数组中
return result;
}

内存使用量

包含放弃的小于1K的空间

size_t MemoryUsage() const {
return blocks_memory_ + blocks_.capacity() * sizeof(char*);
}

内存对齐分配

char* Arena::AllocateAligned(size_t bytes) {
//根据void*的大小对齐
const int align = sizeof(void*);    // We'll align to pointer size
//align应该为2的幂，如果不为2的幂，说明出错了，检测到了如此地步。
assert((align & (align-1)) == 0);   // Pointer size should be a power of 2
//又用到&和%计算的指是一样的，但是&快
size_t current_mod = reinterpret_cast<uintptr_t>(alloc_ptr_) & (align-1);
//计算需要添加的字节数
size_t slop = (current_mod == 0 ? 0 : align - current_mod);
//因此内存对齐后需要的字节为bytes+slop
size_t needed = bytes + slop;
char* result;
//如果需求小于剩余的bytes，那么直接分配
if (needed <= alloc_bytes_remaining_) {
result = alloc_ptr_ + slop;
alloc_ptr_ += needed;
alloc_bytes_remaining_ -= needed;
} else {
// 否则调用AllocateFallback函数
result = AllocateFallback(bytes);
}
assert((reinterpret_cast<uintptr_t>(result) & (align-1)) == 0);
return result;
}