【leveldb】arena内存结构

本文详细探讨了leveldb中的arena内存管理机制,包括数据结构、内存分配策略以及为什么arena内存只能增长而不能释放。在数据结构部分,讲解了arena如何存储和跟踪内存块。内存分配部分阐述了如何通过Allocate方法有效地分配内存,避免浪费。arena的内存释放策略是在arena对象销毁时统一释放,确保内存的增长性。

本文介绍arena内存管理的设计和实现。

arena是leveldb中一个非常简单的内存池,以下做详细介绍。

1 数据结构

arena数据结构如下,其中,blocks_保存着所有创建的block;blocks_memory_保存所有block申请的总空间;alloc_ptr_和alloc_bytes_remaining_分别是当前block的内存分配起始地址 和 可用空间大小。

  // 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_;

2 分配内存

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);
}
// Allocate memory with the normal align
leveldb实现解析 淘宝-核心系统研发-存储 那岩 neveray@gmail.com 2011-12-13 目录 一、 代码目录结构 ....................................................................... 1 1. doc/ ............................................................................ 1 2. include/leveldb/ ................................................................. 1 3. db/ ............................................................................. 1 4. table/........................................................................... 1 5. port/ ........................................................................... 1 6. util/ ........................................................................... 1 7. helper/memenv/ ................................................................... 1 二、 基本概念 .......................................................................... 1 1. Slice (include/leveldb/slice.h) .................................................. 1 2. Option (include/leveldb/option.h) .............................................. 2 3. Env (include/leveldb/env.h util/env_posix.h) .................................... 3 4. varint (util/coding.h) ........................................................... 3 5. ValueType (db/dbformat.h) ...................................................... 3 6. SequnceNnumber (db/dbformat.h) ................................................. 4 7. user key......................................................................... 4 8. ParsedInternalKey (db/dbformat.h) .............................................. 4 9. InternalKey (db/dbformat.h) ...................................................... 4 10. LookupKey (db/dbformat.h) ........................................................ 4 11. Comparator (include/leveldb/comparator.h util/comparator.cc) .................... 4 12. InternalKeyComparator (db/dbformat.h) ............................................ 5 13. WriteBatch (db/write_batch.cc) ................................................. 5 14. Memtable (db/memtable.cc db/skiplist.h) .......................................... 5 15. Sstable (table/table.cc) ......................................................... 5 16. FileMetaData (db/version_edit.h) ............................................... 5 17. block (table/block.cc) ........................................................... 6 18. BlockHandle(table/format.h) ...................................................... 6 19. FileNumber(db/dbformat.h) ...................................................... 6 20. filename (db/filename.cc) ........................................................ 6 21. level-n (db/version_set.h) ....................................................... 7 22. Compact (db/db_impl.cc db/version_set.cc) ........................................ 7 23. Compaction(db/version_set.cc) .................................................... 7 24. Version (db/version_set.cc) ...................................................... 8 25. VersionSet (db/version_set.cc) ................................................. 9 26. VersionEdit(db/version_edit.cc) ................................................. 10 27. VersionSet::Builder (db/version_set.cc) ......................................... 11 28. Manifest(descriptor)(db/version_set.cc)...................................... 12 29. TableBuilder/BlockBuilder(table/table_builder.cc table/block_builder.cc) ......... 12 30. Iterator (include/leveldb/iterator.h) ........................................... 12 三、 存储结构的格式定义与操作 .......................................................... 12 1. memtable (db/skiplist.h db/memtable) ............................................ 12 2. block of sstable (table/block_builder.cc table/block.cc) ......................... 14 3. sstable (table/table_bulder.cc/table.cc) ........................................ 16 4. block of log (db/log_format.h db/log_writer.cc db/log_reader.cc) .............. 18 5. log (db/log_format.h db/log_writer.cc db/log_reader.cc) ........................ 18 6. cache(util/cache.cc) .......................................................... 19 7. Snapshot (include/leveldb/snapshot.h) ......................................... 19 8. Iterator (include/leveldb/iterator.h) ........................................... 19 四、 主要流程 ......................................................................... 24 1. open ........................................................................... 24 2. put ............................................................................ 25 3. get ............................................................................ 25 4. delete.......................................................................... 26 5. snapshot........................................................................ 26 6. NewIterator ..................................................................... 26 7. compact......................................................................... 26 五、 总结 ............................................................................. 30 1. 设计/实现中的优化 ............................................................... 30 2. 可以做的优化 .................................................................... 31
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