TensorFlow内存管理bfc算法

1. 基本介绍

• tensorflow设备内存管理模块实现了一个best-fit with coalescing算法（后文简称bfc算法）。
• bfc算法是Doung Lea’s malloc(dlmalloc)的一个非常简单的版本。
• 它具有内存分配、释放、碎片管理等基本功能。

2. bfc基本算法思想

1. 数据结构

• 整个内存空间由一个按基址升序排列的Chunk双向链表来表示，它们的直接前趋和后继必须在地址连续的内存空间。Chunk结构体里含有实际大小、请求大小、是否被占用、基址、直接前趋、直接后继、Bin索引等信息。

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2. 申请

• 用户申请一个内存块（malloc)。根据chunk双链表找到一个合适的内存块，如果该内存块的大小是用户申请的大小的二倍以上，那么就将该内存块切分成两块，这就是split操作。
  
  • 返回其中一块给用户，并将该内存块标识为占用
  • Spilt操作会新增一个chunk，所以需要修改chunk双链表以维持前驱和后继关系
• 如果用户申请512的空间，正好有一块1024的chunk2是空闲的，由于1024/512 =2，所以chunk2 被split为2块：chunk2_1和chunk2_2。返回chunk2_1给用户并将其标志位占用状态。

3. 释放

• 用户释放一个内存块（free）。先将该块标记为空闲。然后根据chunk数据结构中的信息找到其前驱和后继内存块。如果前驱和后继块中有空闲的块，那么将刚释放的块和空闲的块合并成一个更大的chunk（这就是merge操作，合并当前块和其前后的空闲块）。再修改双链表结构以维持前驱后继关系。这就做到了内存碎片的回收。
• 如果用户要free chunk3，由于chunk3的前驱chunk2也是空闲的，所以将chunk2和chunk3合并得到一个新的chunk2’，大小为chunk2和chunk3之和。

3. bins

1. bins数据结构

• bfc算法采取的是被动分块的策略。最开始整个内存是一个chunk，随着用户申请空间的次数增加，最开始的大chunk会被不断的split开来，从而产生越来越多的小chunk。当chunk数量很大时，为了寻找一个合适的内存块而遍历双链表无疑是一笔巨大的开销。为了实现对空闲块的高效管理，bfc算法设计了bin这个抽象数据结构。

• 每个bin都有一个size属性，一个bin是一个拥有chunk size >= binsize的空闲chunk的集合。集合中的chunk按照chunk size的升序组织成单链表。bfc算法维护了一个bin的集合：bins。它由多个bin以及从属于每个bin的chunks组成。内存中所有的空闲chunk都由bins管理。

• 

• 图中每一列表示一个bin，列首方格中的数字表示bin的size。bin size的大小都是256的2^n的倍。每个bin下面挂载了一系列的空闲chunk，每个chunk的chunk size都大于等于所属的bin的bin size，按照chunk size的升序挂载成单链表。

2. bins操作

• bfc算法针对bins这个集合设计了三个操作：search、insert、delete。
  
  • search
    给定一个chunk size，从bins中找到大于等于该chunksize的最小的那个空闲chunk。Search操作具体流程如下。如果bin以数组的形式组织，那么可以从index = chunk size /256 >>2的那个bin开始查找。最好的情况是开始查找的那个bin的chunk链表非空，那么直接返回链表头即可。这种情况时间复杂度是常数级的。最坏的情况是遍历bins数组中所有的bin。对于一般大小的内存来说，bins数组元素非常少，比如4G空间只需要23个bin就足够了（256 * 2 ^ 23 > 4G），因此也很快能返回结果。总体来说search操作是非常高效的。对于固定大小内存来说，查找时间是常数量级的。
  • insert
    将一个空闲的chunk插入到一个bin所挂载的chunk链表中，同时需要维持chunk链表的升序关系。具体流程是直接将chunk插入到index = chunk size /256 >>2的那个bin中即可。
  • delete
    将一个空闲的chunk从bins中移除。

4. 总结

• 将内存分块管理，按块进行空间分配和释放。
• 通过split操作将大内存块分解成用户需要的小内存块。
• 通过merge操作合并小的内存块，做到内存碎片回收
• 通过bin这个抽象数据结构实现对空闲块高效管理。

5. 代码分析

1. 代码地址

• https://github.com/tensorflow/tensorflow/tree/master/tensorflow/core/common_runtime

2. 数据结构

• Chunk

static const int kInvalidChunkHandle = -1;...struct Chunk {    size_t size = 0;  // Full size of buffer.    // We sometimes give chunks that are larger than needed to reduce    // fragmentation.  requested_size keeps track of what the client    // actually wanted so we can understand whether our splitting    // strategy is efficient.    size_t requested_size = 0;    // allocation_id is set to -1 when the chunk is not in use. It is assigned a    // value greater than zero before the chunk is returned from    // AllocateRaw, and this value is unique among values assigned by    // the parent allocator.    int64 allocation_id = -1;    void* ptr = nullptr;  // pointer to granted subbuffer.    // If not kInvalidChunkHandle, the memory referred to by 'prev' is directly    // preceding the memory used by this chunk.  E.g., It should start    // at 'ptr - prev->size'    ChunkHandle prev = kInvalidChunkHandle;    // If not kInvalidChunkHandle, the memory referred to by 'next' is directly    // following the memory used by this chunk.  E.g., It should be at    // 'ptr + size'    ChunkHandle next = kInvalidChunkHandle;    // What bin are we in?    BinNum bin_num = kInvalidBinNum;    bool in_use() const { return allocation_id != -1; }};

• Bin

// A Bin is a collection of similar-sized free chunks.struct Bin {    // All chunks in this bin have >= bin_size memory.    size_t bin_size = 0;    struct ChunkComparator {        explicit ChunkComparator(BFCAllocator* allocator)            : allocator_(allocator) {}        // Sort first by size and then use pointer address as a tie breaker.        bool operator()(const ChunkHandle ha,                        const ChunkHandle hb) const NO_THREAD_SAFETY_ANALYSIS {            const Chunk* a = allocator_->ChunkFromHandle(ha);            const Chunk* b = allocator_->ChunkFromHandle(hb);            if (a->size != b->size) {                return a->size < b->size;            }            return a->ptr < b->ptr;        }        private:            BFCAllocator* allocator_;  // The parent allocator    };    typedef std::set<ChunkHandle, ChunkComparator> FreeChunkSet;    // List of free chunks within the bin, sorted by chunk size.    // Chunk * not owned.    FreeChunkSet free_chunks;    Bin(BFCAllocator* allocator, size_t bs)        : bin_size(bs), free_chunks(ChunkComparator(allocator)) {}};

• AllocationRegion
  
  • AllocationRegion给一个连续的内存区域做指针到ChunkHandle的映射。
• RegionManager
  
  • RegionManager聚集了一个或多个AllocationRegion，并提供一个从指针到基础ChunkHandle的间接层，这个间接层可在多个不连续的内存区域进行分配。

3. 分配大小

• 将每次分配的内存大小调整为kMinAllocationSize的N倍，这样所有内存地址都是很好地按字节对齐了。

// kMinAllocationSize = 256static const size_t kMinAllocationBits = 8;static const size_t kMinAllocationSize = 1 << kMinAllocationBits;...size_t BFCAllocator::RoundedBytes(size_t bytes) {    size_t rounded_bytes =        (kMinAllocationSize *        ((bytes + kMinAllocationSize - 1) / kMinAllocationSize));    DCHECK_EQ(size_t{0}, rounded_bytes % kMinAllocationSize);    return rounded_bytes;}

4. 初始化bin

typedef int BinNum;static const int kInvalidBinNum = -1;static const int kNumBins = 21;...// 二进制2^8往左移0，1，2位// (static_cast<size_t>(256) << 0) = 256// (static_cast<size_t>(256) << 1) = 512// (static_cast<size_t>(256) << 2) = 1024size_t BinNumToSize(BinNum index) {    return static_cast<size_t>(256) << index;}...char bins_space_[sizeof(Bin) * kNumBins];// Map from bin size to BinBin* BinFromIndex(BinNum index) {    return reinterpret_cast<Bin*>(&(bins_space_[index * sizeof(Bin)]));}...// We create bins to fit all possible ranges that cover the// memory_limit_ starting from allocations up to 256 bytes to// allocations up to (and including) the memory limit.for (BinNum b = 0; b < kNumBins; b++) {    size_t bin_size = BinNumToSize(b);    VLOG(1) << "Creating bin of max chunk size "            << strings::HumanReadableNumBytes(bin_size);    new (BinFromIndex(b)) Bin(this, bin_size);    CHECK_EQ(BinForSize(bin_size), BinFromIndex(b));    CHECK_EQ(BinForSize(bin_size + 255), BinFromIndex(b));    CHECK_EQ(BinForSize(bin_size * 2 - 1), BinFromIndex(b));    if (b + 1 < kNumBins) {        CHECK_NE(BinForSize(bin_size * 2), BinFromIndex(b));    }}

5. 查找bin

// 求属于第几个binBinNum BinNumForSize(size_t bytes) {    uint64 v = std::max<size_t>(bytes, 256) >> kMinAllocationBits;    int b = std::min(kNumBins - 1, Log2FloorNonZero(v));    return b;}// 最高位非零的二进制位数，eg: 0001 0101B 为5inline int Log2FloorNonZero(uint64 n) {#if defined(__GNUC__)    return 63 ^ __builtin_clzll(n);#elif defined(PLATFORM_WINDOWS)    unsigned long index;    _BitScanReverse64(&index, n);    return index;#else    int r = 0;    while (n > 0) {        r++;        n >>= 1;    }    return r;#endif}

6. 查找Chunk

// 先加锁mutex_lock l(lock_);void* ptr = FindChunkPtr(bin_num, rounded_bytes, num_bytes);if (ptr != nullptr) {    return ptr;}// FindChunkPtr函数内部void* BFCAllocator::FindChunkPtr(BinNum bin_num, size_t rounded_bytes,                                 size_t num_bytes) {    // First identify the first bin that could satisfy rounded_bytes.    for (; bin_num < kNumBins; bin_num++) {        // Start searching from the first bin for the smallest chunk that fits        // rounded_bytes.        Bin* b = BinFromIndex(bin_num);        for (auto citer = b->free_chunks.begin(); citer != b->free_chunks.end();                ++citer) {            // 从之前得到的Bin索引开始，查找合适的空闲Chunk：            const BFCAllocator::ChunkHandle h = (*citer);            BFCAllocator::Chunk* chunk = ChunkFromHandle(h);            DCHECK(!chunk->in_use());            if (chunk->size >= rounded_bytes) {                // We found an existing chunk that fits us that wasn't in use, so remove                // it from the free bin structure prior to using.                RemoveFreeChunkIterFromBin(&b->free_chunks, citer);                // If we can break the size of the chunk into two reasonably                // large pieces, do so.                //                // TODO(vrv): What should be the criteria when deciding when                // to split?                 // 具体实现后面会分析                if (chunk->size >= rounded_bytes * 2) {                    SplitChunk(h, rounded_bytes);                    chunk = ChunkFromHandle(h);  // Update chunk pointer in case it moved                }                // The requested size of the returned chunk is what the user                // has allocated.                chunk->requested_size = num_bytes;                // Assign a unique id and increment the id counter, marking the                // chunk as being in use.                chunk->allocation_id = next_allocation_id_++;                // Update stats.                ++stats_.num_allocs;                stats_.bytes_in_use += chunk->size;                stats_.max_bytes_in_use =                    std::max(stats_.max_bytes_in_use, stats_.bytes_in_use);                stats_.max_alloc_size =                    std::max<std::size_t>(stats_.max_alloc_size, chunk->size);                VLOG(4) << "Returning: " << chunk->ptr;                if (VLOG_IS_ON(4)) {                    LOG(INFO) << "A: " << RenderOccupancy();                }                return chunk->ptr;            }        }    }    return nullptr;}

7. 拆分Chunk

• 如果Chunk的大小大于等于申请内存大小的2倍，那么将该Chunk拆分成2个：第一个Chunk的大小等于申请内存大小，第二个Chunk作为它的直接后继。

if (chunk->size >= rounded_bytes * 2) {    SplitChunk(h, rounded_bytes);    chunk = ChunkFromHandle(h);  // Update chunk pointer in case it moved}void BFCAllocator::SplitChunk(BFCAllocator::ChunkHandle h, size_t num_bytes) {    // Allocate the new chunk before we do any ChunkFromHandle    ChunkHandle h_new_chunk = AllocateChunk();    Chunk* c = ChunkFromHandle(h);    CHECK(!c->in_use() && (c->bin_num == kInvalidBinNum));    // Create a new chunk starting num_bytes after c    BFCAllocator::Chunk* new_chunk = ChunkFromHandle(h_new_chunk);    new_chunk->ptr = static_cast<void*>(static_cast<char*>(c->ptr) + num_bytes);    region_manager_.set_handle(new_chunk->ptr, h_new_chunk);    // Set the new sizes of the chunks.    new_chunk->size = c->size - num_bytes;    c->size = num_bytes;    // The new chunk is not in use.    new_chunk->allocation_id = -1;    // Maintain the pointers.    // c <-> c_neighbor becomes    // c <-> new_chunk <-> c_neighbor    BFCAllocator::ChunkHandle h_neighbor = c->next;    new_chunk->prev = h;    new_chunk->next = h_neighbor;    c->next = h_new_chunk;    if (h_neighbor != kInvalidChunkHandle) {        Chunk* c_neighbor = ChunkFromHandle(h_neighbor);        c_neighbor->prev = h_new_chunk;    }    // Add the newly free chunk to the free bin.    InsertFreeChunkIntoBin(h_new_chunk);}

8. 回收chunk

• 加锁，获得ChunkHandle

mutex_lock l(lock_);BFCAllocator::ChunkHandle h = region_manager_.get_handle(ptr);FreeAndMaybeCoalesce(h);

• FreeAndMaybeCoalesce

void BFCAllocator::FreeAndMaybeCoalesce(BFCAllocator::ChunkHandle h) {    Chunk* c = ChunkFromHandle(h);    CHECK(c->in_use() && (c->bin_num == kInvalidBinNum));    // Mark the chunk as no longer in use    c->allocation_id = -1;    // Updates the stats.    stats_.bytes_in_use -= c->size;    // This chunk is no longer in-use, consider coalescing the chunk    // with adjacent chunks.    ChunkHandle chunk_to_reassign = h;    // If the next chunk is free, coalesce the two    if (c->next != kInvalidChunkHandle) {        Chunk* cnext = ChunkFromHandle(c->next);        if (!cnext->in_use()) {        //      VLOG(8) << "Chunk at " << cnext->ptr << " merging with c " <<        //      c->ptr;        chunk_to_reassign = h;        // Deletes c->next        RemoveFreeChunkFromBin(c->next);        Merge(h, ChunkFromHandle(h)->next);        }    }    // If the previous chunk is free, coalesce the two    c = ChunkFromHandle(h);    if (c->prev != kInvalidChunkHandle) {        Chunk* cprev = ChunkFromHandle(c->prev);        if (!cprev->in_use()) {        //      VLOG(8) << "Chunk at " << c->ptr << " merging into c->prev "        //       << cprev->ptr;        chunk_to_reassign = c->prev;        // Deletes c        RemoveFreeChunkFromBin(c->prev);        Merge(ChunkFromHandle(h)->prev, h);        c = ChunkFromHandle(h);        }    }    InsertFreeChunkIntoBin(chunk_to_reassign);}

• Merge

// Merges h1 and h2 when Chunk(h1)->next is h2 and Chunk(h2)->prev is c1.// We merge Chunk(h2) into Chunk(h1).void BFCAllocator::Merge(BFCAllocator::ChunkHandle h1,                         BFCAllocator::ChunkHandle h2) {    Chunk* c1 = ChunkFromHandle(h1);    Chunk* c2 = ChunkFromHandle(h2);    // We can only merge chunks that are not in use.    CHECK(!c1->in_use() && !c2->in_use());    // c1's prev doesn't change, still points to the same ptr, and is    // still not in use.    // Fix up neighbor pointers    //    // c1 <-> c2 <-> c3 should become    // c1 <-> c3    BFCAllocator::ChunkHandle h3 = c2->next;    c1->next = h3;    CHECK(c2->prev == h1);    if (h3 != kInvalidChunkHandle) {        BFCAllocator::Chunk* c3 = ChunkFromHandle(h3);        c3->prev = h1;    }    // Set the new size    c1->size += c2->size;    DeleteChunk(h2);}

6. 参考

• https://github.com/tensorflow/tensorflow
• http://www.cnblogs.com/yao62995/p/5773166.html
• https://deepreve.github.io/articles/20161007.html