leveldb 合并写详解
leveldb的write代码初看瞎搞一堆,细看则实为短小精悍。
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 }
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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