时间:2021-07-01 10:21:17 帮助过:13人阅读
- <span style="font-size: 14px">1 Status DBImpl::Write(const WriteOptions& options, WriteBatch* my_batch) {
- 2 // -----A begin-------
- 3 Writer w(&mutex_);
- 4 w.batch = my_batch;
- 5 w.sync = options.sync;
- 6 w.done = false;
- 7 // -----A end --------
- 8
- 9
- 10 // -----B begin-------
- 11 MutexLock l(&mutex_);
- 12 writers_.push_back(&w);
- 13 while (!w.done && &w != writers_.front()) {
- 14 w.cv.Wait();
- 15 }
- 16 if (w.done) {
- 17 return w.status;
- 18 }
- 19 // -----B end -------
- 20
- 21 // May temporarily unlock and wait.
- 22 Status status = MakeRoomForWrite(my_batch == NULL);
- 23 uint64_t last_sequence = versions_->LastSequence();
- 24 Writer* last_writer = &w;
- 25 if (status.ok() && my_batch != NULL) { // NULL batch is for compactions
- 26 WriteBatch* updates = BuildBatchGroup(&last_writer);
- 27 WriteBatchInternal::SetSequence(updates, last_sequence + 1);
- 28 last_sequence += WriteBatchInternal::Count(updates);
- 29
- 30 // Add to log and apply to memtable. We can release the lock
- 31 // during this phase since &w is currently responsible for logging
- 32 // and protects against concurrent loggers and concurrent writes
- 33 // into mem_.
- 34 {
- 35 // -----C begin-------
- 36 mutex_.Unlock();
- 37 // -----C end -------
- 38 status = log_->AddRecord(WriteBatchInternal::Contents(updates));
- 39 if (status.ok() && options.sync) {
- 40 status = logfile_->Sync();
- 41 }
- 42 if (status.ok()) {
- 43 status = WriteBatchInternal::InsertInto(updates, mem_);
- 44 }
- 45 // -----D begin-------
- 46 mutex_.Lock();
- 47 // -----D end -------
- 48 }
- 49 if (updates == tmp_batch_) tmp_batch_->Clear();
- 50
- 51 versions_->SetLastSequence(last_sequence);
- 52 }
- 53
- 54 // -----E begin-------
- 55 while (true) {
- 56 Writer* ready = writers_.front();
- 57 writers_.pop_front();
- 58 if (ready != &w) {
- 59 ready->status = status;
- 60 ready->done = true;
- 61 ready->cv.Signal();
- 62 }
- 63 if (ready == last_writer) break;
- 64 }
- 65 // -----E end -------
- 66
- 67
- 68 // -----F begin-------
- 69 // Notify new head of write queue
- 70 if (!writers_.empty()) {
- 71 writers_.front()->cv.Signal();
- 72 }
- 73 // -----F end-------
- 74
- 75 return status;
- 76 }</span>
如上,A段代码定义一个Writer w, w的成员包括了batch信息,同时初始化了一个条件变量成员(port::CondVar
)
假设同时有w1, w2, w3, w4, w5, w6 并发请求写入。
B段代码让竞争到mutex资源的w1获取了锁。添加到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c段代码时,mutex互斥锁释放,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,之所以这里在无锁状况下时安全的,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。 当w1完成log和memtable写入后,进入d段代码,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。
进入E段代码后,w1被pop出来,由于reader==w, 并且ready==last_writer,所以直接到F段代码,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,c段代码运行后,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).
C段和D段间的代码将w3, w5, w2, w4整个的batch写入log和memtable. 到E段时,分别对w5, w2, w4进行了一次cond signal.当判断到完w4 == lastwriter时,则退出E段代码。F段则对队首的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。
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