leveldb中有且只有一个进程单独做compact，当主线程触发compact，调用void DBImpl::MaybeScheduleCompaction()，如果compact正在运行或者DB正在退出，直接返回；检查version中是否存在需要compact，有则触发后台调度env_->schedele(…)

void DBImpl::MaybeScheduleCompaction() { mutex_.AssertHeld(); if (bg_compaction_scheduled_) { // Already scheduled } else if (shutting_down_.Acquire_Load()) { // DB is being deleted; no more background compactions } else if (!bg_error_.ok()) { // Already got an error; no more changes } else if (imm_ == NULL && manual_compaction_ == NULL && !versions_->NeedsCompaction()) { // No work to be done } else { bg_compaction_scheduled_ = true; env_->Schedule(&DBImpl::BGWork, this); } }

schedele把compact处理程序函数指针和db对象指针传入后台任务队列，BGWork 是compact处理函数，Schedule函数如下：

void PosixEnv::Schedule(void (*function)(void*), void* arg) { PthreadCall("lock", pthread_mutex_lock(&mu_)); // Start background thread if necessary if (!started_bgthread_) { started_bgthread_ = true; PthreadCall( "create thread", pthread_create(&bgthread_, NULL, &PosixEnv::BGThreadWrapper, this)); } // If the queue is currently empty, the background thread may currently be // waiting. if (queue_.empty()) { PthreadCall("signal", pthread_cond_signal(&bgsignal_)); } // Add to priority queue queue_.push_back(BGItem()); queue_.back().function = function; queue_.back().arg = arg; PthreadCall("unlock", pthread_mutex_unlock(&mu_)); }

将处理函数放入任务队列中，后台进程就可以不断地从queue_中取出任务函数，并执行。

实际compact处理进程是BackgroundCall和BackgroundCompaction。BackgroundCall完成一些判断，条件符合则调用BackgroundCompaction，compact完成后再次触发compact，重复上述过程。

void DBImpl::BackgroundCall() { MutexLock l(&mutex_); assert(bg_compaction_scheduled_); if (shutting_down_.Acquire_Load()) { // No more background work when shutting down. } else if (!bg_error_.ok()) { // No more background work after a background error. } else { BackgroundCompaction(); } bg_compaction_scheduled_ = false; // Previous compaction may have produced too many files in a level, // so reschedule another compaction if needed. MaybeScheduleCompaction(); bg_cv_.SignalAll(); }

实际compact流程：