LevelDB的源码阅读（二） Open操作

在Linux上leveldb的安装和使用中我们写了一个测试代码，内容如下：

#include "leveldb/db.h"
#include <cassert>
#include <iostream>

using namespace std;
using namespace leveldb;

int main() {
    leveldb::DB *db;
    leveldb::Options options;
    options.create_if_missing = true;
    leveldb::Status status = leveldb::DB::Open(options, "testdb", &db);
    assert(status.ok());

    status = db->Put(WriteOptions(), "test", "Hello World!");
    assert(status.ok());
    string res;
    status = db->Get(ReadOptions(), "test", &res);
    assert(status.ok());
    cout << res << endl;

    delete db;
    return 0;
}

其中db.h中定义了leveldb对外接口，定义了class DB,这个类只是一个接口类，leveldb::DB::Open操作来自leveldb源代码db文件夹下db_impl.cc文件，源码内容如下：

Status DB::Open(const Options& options, const std::string& dbname,
                DB** dbptr) {
  *dbptr = NULL;

  DBImpl* impl = new DBImpl(options, dbname);
  impl->mutex_.Lock();
  VersionEdit edit;
  // Recover handles create_if_missing, error_if_exists
  bool save_manifest = false;
  Status s = impl->Recover(&edit, &save_manifest);
  if (s.ok() && impl->mem_ == NULL) {
    // Create new log and a corresponding memtable.
    uint64_t new_log_number = impl->versions_->NewFileNumber();
    WritableFile* lfile;
    s = options.env->NewWritableFile(LogFileName(dbname, new_log_number),
                                     &lfile);
    if (s.ok()) {
      edit.SetLogNumber(new_log_number);
      impl->logfile_ = lfile;
      impl->logfile_number_ = new_log_number;
      impl->log_ = new log::Writer(lfile);
      impl->mem_ = new MemTable(impl->internal_comparator_);
      impl->mem_->Ref();
    }
  }
  if (s.ok() && save_manifest) {
    edit.SetPrevLogNumber(0);  // No older logs needed after recovery.
    edit.SetLogNumber(impl->logfile_number_);
    s = impl->versions_->LogAndApply(&edit, &impl->mutex_);
  }
  if (s.ok()) {
    impl->DeleteObsoleteFiles();
    impl->MaybeScheduleCompaction();
  }
  impl->mutex_.Unlock();
  if (s.ok()) {
    assert(impl->mem_ != NULL);
    *dbptr = impl;
  } else {
    delete impl;
  }
  return s;
}

DB::Open函数创建的是一个DBImpl类，具体的操作由DBImpl类来处理.DBImpl构造函数如下：

DBImpl::DBImpl(const Options& raw_options, const std::string& dbname)
    : env_(raw_options.env), // Env* const
      internal_comparator_(raw_options.comparator), // const InternalKeyComparator
      internal_filter_policy_(raw_options.filter_policy), // const InternalFilterPolicy
      options_(SanitizeOptions(dbname, &internal_comparator_, // const Options
                               &internal_filter_policy_, raw_options)),
      owns_info_log_(options_.info_log != raw_options.info_log), // bool
      owns_cache_(options_.block_cache != raw_options.block_cache), // bool
      dbname_(dbname), // const std::string
      db_lock_(NULL), // FileLock*
      shutting_down_(NULL), // port::AtomicPointer
      bg_cv_(&mutex_), // port::CondVar
      mem_(NULL), // MemTable*
      imm_(NULL), // MemTable*
      logfile_(NULL), // WritableFile*
      logfile_number_(0), // uint64_t
      log_(NULL), // log::Writer*
      seed_(0), // uint32_t
      tmp_batch_(new WriteBatch), // WriteBatch*
      bg_compaction_scheduled_(false), // bool
      manual_compaction_(NULL) { // ManualCompaction*
  has_imm_.Release_Store(NULL);

  // Reserve ten files or so for other uses and give the rest to TableCache.
  const int table_cache_size = options_.max_open_files - kNumNonTableCacheFiles;
  table_cache_ = new TableCache(dbname_, &options_, table_cache_size);

  versions_ = new VersionSet(dbname_, &options_, table_cache_,
                             &internal_comparator_);
}

以下是成员变量的含义：

• env_, 负责所有IO, 比如建立文件
• internal_comparator_, 用来比较不同key的大小
• internal_filter_policy_, 可自定义BloomFilter
• options_, 对调用者传入的options进行调整
• db_lock_, 文件锁
• shutting_down_, 基于memory barrier的原子指针
• bg_cv_, 多线程的条件
• mem_ = memtable
• imm = immemtable
• tmp_batch_, 所有Put都是以batch写入, 这里建立临时的batch
• manual_compaction_, 内部开发者调用时的参数, 可以不用理会
• has_imm_, 用于判断是否有等待或者正在写入硬盘的immemtable
• table_cache_, SSTable查询缓存
• versions_, 数据库MVCC

接下来说一下memory barrier的问题.

程序经由编译器处理后变成了一条条的机器指令，每条指令又对应了更基础的几个硬件阶段，各个指令在这些硬件阶段里排队处理，组成流水线.由于不同指令对应的硬件阶段不同，导致有些时候流水线填不满，影响性能.因此硬件会主动的对指令顺序做微调，提升流水线的效率，也就是乱序执行.乱序执行是有规则的，在单线程下几乎不需要软件介入，但多线程就不一样了.比如说我们的程序这么写：

thread1:
1: some_task = 123;
2: complete = true;

thread2:
  while (!complete) sleep(1);
  printf("got %d
", task_result);

程序本意是等任务完成后输出 got 123，但结果很可能是 got 0，原因就是 1 和 2 两条指令被调整了顺序，乱序不当.为解决这样的问题，cpu 增加了用于控制乱序执行的指令，称为内存栅栏（memory barrier）.在 1、2 之间插入栅栏就会强制 cpu 不做乱序，从而保证程序的正确性.

在leveldb中port文件夹下atomic_pointer.h里出现了memory barrier.

首先是memory barrier的定义问题，leveldb提供了不同环境下的多种定义方式：

// Define MemoryBarrier() if available
// Windows on x86
#if defined(OS_WIN) && defined(COMPILER_MSVC) && defined(ARCH_CPU_X86_FAMILY)
// windows.h already provides a MemoryBarrier(void) macro
// http://msdn.microsoft.com/en-us/library/ms684208(v=vs.85).aspx
#define LEVELDB_HAVE_MEMORY_BARRIER

// Mac OS
#elif defined(__APPLE__)
inline void MemoryBarrier() {
  OSMemoryBarrier();
}
#define LEVELDB_HAVE_MEMORY_BARRIER

// Gcc on x86
#elif defined(ARCH_CPU_X86_FAMILY) && defined(__GNUC__)
inline void MemoryBarrier() {
  // See http://gcc.gnu.org/ml/gcc/2003-04/msg01180.html for a discussion on
  // this idiom. Also see http://en.wikipedia.org/wiki/Memory_ordering.
  __asm__ __volatile__("" : : : "memory");
}
#define LEVELDB_HAVE_MEMORY_BARRIER

// Sun Studio
#elif defined(ARCH_CPU_X86_FAMILY) && defined(__SUNPRO_CC)
inline void MemoryBarrier() {
  // See http://gcc.gnu.org/ml/gcc/2003-04/msg01180.html for a discussion on
  // this idiom. Also see http://en.wikipedia.org/wiki/Memory_ordering.
  asm volatile("" : : : "memory");
}
#define LEVELDB_HAVE_MEMORY_BARRIER

// ARM Linux
#elif defined(ARCH_CPU_ARM_FAMILY) && defined(__linux__)
typedef void (*LinuxKernelMemoryBarrierFunc)(void);
// The Linux ARM kernel provides a highly optimized device-specific memory
// barrier function at a fixed memory address that is mapped in every
// user-level process.
//
// This beats using CPU-specific instructions which are, on single-core
// devices, un-necessary and very costly (e.g. ARMv7-A "dmb" takes more
// than 180ns on a Cortex-A8 like the one on a Nexus One). Benchmarking
// shows that the extra function call cost is completely negligible on
// multi-core devices.
//
inline void MemoryBarrier() {
  (*(LinuxKernelMemoryBarrierFunc)0xffff0fa0)();
}
#define LEVELDB_HAVE_MEMORY_BARRIER

// ARM64
#elif defined(ARCH_CPU_ARM64_FAMILY)
inline void MemoryBarrier() {
  asm volatile("dmb sy" : : : "memory");
}
#define LEVELDB_HAVE_MEMORY_BARRIER

// PPC
#elif defined(ARCH_CPU_PPC_FAMILY) && defined(__GNUC__)
inline void MemoryBarrier() {
  // TODO for some powerpc expert: is there a cheaper suitable variant?
  // Perhaps by having separate barriers for acquire and release ops.
  asm volatile("sync" : : : "memory");
}
#define LEVELDB_HAVE_MEMORY_BARRIER

// MIPS
#elif defined(ARCH_CPU_MIPS_FAMILY) && defined(__GNUC__)
inline void MemoryBarrier() {
  __asm__ __volatile__("sync" : : : "memory");
}
#define LEVELDB_HAVE_MEMORY_BARRIER

#endif

 例如，对于X86内存模型而言，__asm__ __volatile__(" : : : "memory")的意思就是告诉编译器，memory在执行这端inline 汇编代码之后，已经失效了.也就是告诉编译器，不要在这个地方优化有关访问内存的指令.

在atomic_pointer.h中 memory barrier的使用方式如下：

// AtomicPointer built using platform-specific MemoryBarrier()
#if defined(LEVELDB_HAVE_MEMORY_BARRIER)
class AtomicPointer {
 private:
  void* rep_;
 public:
  AtomicPointer() { }
  explicit AtomicPointer(void* p) : rep_(p) {}
  inline void* NoBarrier_Load() const { return rep_; }
  inline void NoBarrier_Store(void* v) { rep_ = v; }
  inline void* Acquire_Load() const {
    void* result = rep_;
    MemoryBarrier();
    return result;
  }
  inline void Release_Store(void* v) {
    MemoryBarrier();
    rep_ = v;
  }
};

DBImpl中的has_imm_就是上面描述的atomic pointer.

再次回到db_impl.cc文件，读完:DB::Open的源码：

Status DB::Open(const Options& options, const std::string& dbname,
                DB** dbptr) { 
  *dbptr = NULL; // 设置结果默认值, 指针传值
  DBImpl* impl = new DBImpl(options, dbname);
  impl->mutex_.Lock(); // 数据恢复时上锁, 禁止所有可能的后台任务
  VersionEdit edit;
  // Recover handles create_if_missing, error_if_exists
  bool save_manifest = false;
  Status s = impl->Recover(&edit, &save_manifest); // //调用DBImpl的恢复数据接口，读取元数据，恢复日志数据
  if (s.ok() && impl->mem_ == NULL) {
    // Create new log and a corresponding memtable. 复位
    uint64_t new_log_number = impl->versions_->NewFileNumber();
    WritableFile* lfile;
    //创建新的写操作日志文件
    s = options.env->NewWritableFile(LogFileName(dbname, new_log_number),
                                     &lfile);
    if (s.ok()) {
      edit.SetLogNumber(new_log_number);
      impl->logfile_ = lfile;
      impl->logfile_number_ = new_log_number;
      impl->log_ = new log::Writer(lfile);
      impl->mem_ = new MemTable(impl->internal_comparator_);
      impl->mem_->Ref();
    }
  }
  if (s.ok() && save_manifest) {
    edit.SetPrevLogNumber(0);  // No older logs needed after recovery.
    edit.SetLogNumber(impl->logfile_number_);
    s = impl->versions_->LogAndApply(&edit, &impl->mutex_); //添加VersionEdit,初始化时会将现在的VersionSet的状态写入新的manifest文件，并更新Current文件
  }
  if (s.ok()) {
    impl->DeleteObsoleteFiles(); // 清理无用文件
    impl->MaybeScheduleCompaction(); // 有写入就有可能要compact
  }
  impl->mutex_.Unlock(); // 初始化完毕
  if (s.ok()) {
    assert(impl->mem_ != NULL);
    *dbptr = impl;
  } else {
    delete impl;
  }
  return s;
}

参考文献：

1.https://zhuanlan.zhihu.com/jimderestaurant?topic=LevelDB

2.https://www.zhihu.com/question/24301047

3.https://www.zhihu.com/question/49039919

4.http://blog.csdn.net/joeyon1985/article/details/47154249