leveldb - 并发写入处理

在并发写入的时候，leveldb巧妙地利用一个时间窗口做batch写入，这部分代码值得一读：

Status DBImpl::Write(const WriteOptions& options, WriteBatch* my_batch) {
  // A begin 
  Writer w(&mutex_);
  w.batch = my_batch;
  w.sync = options.sync;
  w.done = false;
  // A end

  // B begin 
  MutexLock l(&mutex_);
  writers_.push_back(&w);
  while (!w.done && &w != writers_.front()) {
    w.cv.Wait();
  }
  if (w.done) {
    return w.status;
  }
  // B end

  // May temporarily unlock and wait.
  Status status = MakeRoomForWrite(my_batch == NULL);
  uint64_t last_sequence = versions_->LastSequence();
  Writer* last_writer = &w;
  if (status.ok() && my_batch != NULL) {  // NULL batch is for compactions
    WriteBatch* updates = BuildBatchGroup(&last_writer);
    WriteBatchInternal::SetSequence(updates, last_sequence + 1);
    last_sequence += WriteBatchInternal::Count(updates);

    // Add to log and apply to memtable.  We can release the lock
    // during this phase since &w is currently responsible for logging
    // and protects against concurrent loggers and concurrent writes
    // into mem_.
    {
      mutex_.Unlock();
      status = log_->AddRecord(WriteBatchInternal::Contents(updates));
      bool sync_error = false;
      if (status.ok() && options.sync) {
        status = logfile_->Sync();
        if (!status.ok()) {
          sync_error = true;
        }
      }
      if (status.ok()) {
        status = WriteBatchInternal::InsertInto(updates, mem_);
      }
      mutex_.Lock();
      if (sync_error) {
        // The state of the log file is indeterminate: the log record we
        // just added may or may not show up when the DB is re-opened.
        // So we force the DB into a mode where all future writes fail.
        RecordBackgroundError(status);
      }
    }
    if (updates == tmp_batch_) tmp_batch_->Clear();

    versions_->SetLastSequence(last_sequence);
  }

  while (true) {
    Writer* ready = writers_.front();
    writers_.pop_front();
    if (ready != &w) {
      ready->status = status;
      ready->done = true;
      ready->cv.Signal();
    }
    if (ready == last_writer) break;
  }

  // Notify new head of write queue
  if (!writers_.empty()) {
    writers_.front()->cv.Signal();
  }

  return status;
}

     假设同时有w1, w2, w3, w4, w5, w6 并发请求写入。

　　B部分代码让竞争到mutex资源的w1获取了锁。w1将它要写的数据添加到了writers_队列里去，此时队列只有一个w1, 从而其顺利的进行buildbatchgroup。当运行到34行时mutex_互斥锁释放，之所以这儿可以释放mutex_，是因为其它的写操作都不满足队首条件，进而不会进入log和memtable写入阶段。这时(w2, w3, w4, w5, w6)会竞争锁，由于B段代码中不满足队首条件，均等待并释放锁了。从而队列可能会如(w3, w5, w2, w4).

　　继而w1进行log写入和memtable写入。 当w1完成log和memtable写入后，进入46行代码，则mutex_又锁住，这时B段代码中队列因为获取不到锁则队列不会修改。

　　随后59行开始，w1被pop出来，由于ready==w, 并且ready==last_writer,所以直接到71行代码，唤醒了此时处于队首的w3.

      w3唤醒时，发现自己是队首，可以顺利的进行进入buildbatchgroup，在该函数中，遍历了目前所有的队列元素，形成一个update的batch，即将w3, w5, w2, w4合并为一个batch. 并将last_writer置为此时处于队尾的最后一个元素w4，34行代码运行后，因为释放了锁资源，队列可能随着dbimpl::write的调用而更改，如队列状况可能为(w3, w5, w2, w4, w6, w9, w8).

 　　35-45行的代码将w3, w5, w2, w4整个的batch写入log和memtable. 到65行，分别对w5, w2, w4进行了一次cond signal.当判断到完w4 == lastwriter时，则退出循环。72行则对队首的w6唤醒，从而按上述步骤依次进行下去。

　　这样就形成了多个并发write 合并为一个batch写入log和memtable的机制。