leveldb分析——Arena内存管理

leveldb中实现了一个简单的内存管理工具Arena,其基本思想为：先预先向系统申请一块内存，此后需要申请内存时，直接到预先分配的内存中申请。

那么这样做的目的是什么呢？

(1)避免了频率地进行malloc/new和free/delete操作，同时对于内存管理变得简单，对于内存的释放工作交给Arena。

(2)避免造成大量的内存碎片。(还需去了解一下)

下面看具体的源码分析：

Arena定义：

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&);
};

Arena提供两种分配方式：所分配的内存严格对齐、不一定严格对齐的分配方式。每次预先分配的4K（为什么是4K？）保存到blocks_ vector中，最后统一释放。这种内存管理方式是具有一定的适用范围，如需不断分配小内存，最终一并全释放的场景。对于leveldb来说，memtable恰好就是这样的，每次向memtable中insert一条k/v时，就申请一块内存，当memtable被flush到磁盘且不再使用时，将整个memtable释放掉。

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);   //预先分配的不足
}

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);  ///对于大内存，直接单独给分配一块，原先预分配的内存还能使用
return result;
}

// We waste the remaining space in the current block.  预分配的内存剩下的已很少，所以直接重新分配一块，也就是说浪费了一点内存
alloc_ptr_ = AllocateNewBlock(kBlockSize);
alloc_bytes_remaining_ = kBlockSize;

char* result = alloc_ptr_;
alloc_ptr_ += bytes;
alloc_bytes_remaining_ -= bytes;
return result;
}

下面来看下严格对齐的分配方式

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);  ///计算出alloc_ptr_ % align
size_t slop = (current_mod == 0 ? 0 : align - current_mod);                ///对齐还需要向前移动的大小
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;
}