live555 運行調度機制

live555 運行調度機制

在運行live555時，整體的簡單結構是這樣的：

TaskScheduler* scheduler = BasicTaskScheduler::createNew();
創建server
創建mediasession
創建submediasession
添加submediasession到mediasession
添加mediasession到server
env->taskScheduler().doEventLoop(); 

運行調度相關的類

TaskScheduler <-----BasicTaskScheduler0 <------BasicTaskScheduler
DelayQueueEntry<----DelayQueue
DelayQueueEntry<----AlarmHandler

超時隊列

超時隊列的組織

maxSchedulerGranularity默認爲10ms，這裏主要關注scheduleDelayedTask()函數它向DelayQueue fDelayQueue中添加Alarm_handler。

BasicTaskScheduler* BasicTaskScheduler::createNew(unsigned maxSchedulerGranularity) {
	return new BasicTaskScheduler(maxSchedulerGranularity);
}

BasicTaskScheduler::BasicTaskScheduler(unsigned maxSchedulerGranularity)
  : fMaxSchedulerGranularity(maxSchedulerGranularity), fMaxNumSockets(0)
#if defined(__WIN32__) || defined(_WIN32)
  , fDummySocketNum(-1)
#endif
{
  FD_ZERO(&fReadSet);
  FD_ZERO(&fWriteSet);
  FD_ZERO(&fExceptionSet);

  if (maxSchedulerGranularity > 0) schedulerTickTask(); // ensures that we handle events frequently
}
void BasicTaskScheduler::schedulerTickTask() {
  scheduleDelayedTask(fMaxSchedulerGranularity, schedulerTickTask, this);
}

超時隊列的組織利用的時linux 2.4內核定時器隊列的管理方式。隊列中保存的超時時間是該節點的超時時間與前一個節點的超時間的差值，該節點的實際的超時時間是從該節點起到head節點的超時值的和。查詢到期時，只需要判斷第一個節點是否到期，到期就把該節點移除；插入時，將節點的超時值與從head開始的節點超時值的和比較找到合適的位置插入。

如上圖，1號節點的超時時間爲2，2號爲2+6，3號爲2+6+10，4號爲2+6+10+20

TaskToken BasicTaskScheduler0::scheduleDelayedTask(int64_t microseconds,
						 TaskFunc* proc,
						 void* clientData) {
  if (microseconds < 0) microseconds = 0;
  DelayInterval timeToDelay((long)(microseconds/1000000), (long)(microseconds%1000000));
  AlarmHandler* alarmHandler = new AlarmHandler(proc, clientData, timeToDelay);
  fDelayQueue.addEntry(alarmHandler);

  return (void*)(alarmHandler->token());
}

void DelayQueue::addEntry(DelayQueueEntry* newEntry) {
  synchronize(); //同步動作用來防止沒有及時執行timeout節點

  DelayQueueEntry* cur = head();
  while (newEntry->fDeltaTimeRemaining >= cur->fDeltaTimeRemaining) {
    newEntry->fDeltaTimeRemaining -= cur->fDeltaTimeRemaining;
    cur = cur->fNext;
  }

  cur->fDeltaTimeRemaining -= newEntry->fDeltaTimeRemaining;

  // Add "newEntry" to the queue, just before "cur":
  newEntry->fNext = cur;
  newEntry->fPrev = cur->fPrev;
  cur->fPrev = newEntry->fPrev->fNext = newEntry;
}

在創建TaskScheduler是就向延遲隊列中加入一個alarmhandler，它的超時動作是添加另一個與自己一模一樣的handler。

運行

env->taskScheduler().doEventLoop() 函數

void BasicTaskScheduler0::doEventLoop(char volatile* watchVariable) {
  // Repeatedly loop, handling readble sockets and timed events:
  while (1) {
    if (watchVariable != NULL && *watchVariable != 0) break;
    SingleStep();
  }
}

在SingleStep() 中在select()後做三件事情：

• 處理加入的HandlerDescriptor，每個HandlerDescriptor跟socket fd相關
• 處理事件觸發
• 處理延遲隊列

void BasicTaskScheduler::SingleStep(unsigned maxDelayTime) {
  fd_set readSet = fReadSet; // make a copy for this select() call
  fd_set writeSet = fWriteSet; // ditto
  fd_set exceptionSet = fExceptionSet; // ditto

  DelayInterval const& timeToDelay = fDelayQueue.timeToNextAlarm();
  struct timeval tv_timeToDelay;
  tv_timeToDelay.tv_sec = timeToDelay.seconds();
  tv_timeToDelay.tv_usec = timeToDelay.useconds();
  // Very large "tv_sec" values cause select() to fail.
  // Don't make it any larger than 1 million seconds (11.5 days)
  const long MAX_TV_SEC = MILLION;
  if (tv_timeToDelay.tv_sec > MAX_TV_SEC) {
    tv_timeToDelay.tv_sec = MAX_TV_SEC;
  }
  // Also check our "maxDelayTime" parameter (if it's > 0):
  if (maxDelayTime > 0 &&
      (tv_timeToDelay.tv_sec > (long)maxDelayTime/MILLION ||
       (tv_timeToDelay.tv_sec == (long)maxDelayTime/MILLION &&
	tv_timeToDelay.tv_usec > (long)maxDelayTime%MILLION))) {
    tv_timeToDelay.tv_sec = maxDelayTime/MILLION;
    tv_timeToDelay.tv_usec = maxDelayTime%MILLION;
  }

//select 超時時間設置爲延遲隊列的最近超時時間
  int selectResult = select(fMaxNumSockets, &readSet, &writeSet, &exceptionSet, &tv_timeToDelay);
  if (selectResult < 0) { //出錯
#if defined(__WIN32__) || defined(_WIN32)
    int err = WSAGetLastError();
    // For some unknown reason, select() in Windoze sometimes fails with WSAEINVAL if
    // it was called with no entries set in "readSet".  If this happens, ignore it:
    if (err == WSAEINVAL && readSet.fd_count == 0) {
      err = EINTR;
      // To stop this from happening again, create a dummy socket:
      if (fDummySocketNum >= 0) closeSocket(fDummySocketNum);
      fDummySocketNum = socket(AF_INET, SOCK_DGRAM, 0);
      FD_SET((unsigned)fDummySocketNum, &fReadSet);
    }
    if (err != EINTR) {
#else
    if (errno != EINTR && errno != EAGAIN) {
#endif
	// Unexpected error - treat this as fatal:
#if !defined(_WIN32_WCE)
	perror("BasicTaskScheduler::SingleStep(): select() fails");
	// Because this failure is often "Bad file descriptor" - which is caused by an invalid socket number (i.e., a socket number
	// that had already been closed) being used in "select()" - we print out the sockets that were being used in "select()",
	// to assist in debugging:
	fprintf(stderr, "socket numbers used in the select() call:");
	for (int i = 0; i < 10000; ++i) {
	  if (FD_ISSET(i, &fReadSet) || FD_ISSET(i, &fWriteSet) || FD_ISSET(i, &fExceptionSet)) {
	    fprintf(stderr, " %d(", i);
	    if (FD_ISSET(i, &fReadSet)) fprintf(stderr, "r");
	    if (FD_ISSET(i, &fWriteSet)) fprintf(stderr, "w");
	    if (FD_ISSET(i, &fExceptionSet)) fprintf(stderr, "e");
	    fprintf(stderr, ")");
	  }
	}
	fprintf(stderr, "\n");
#endif
	internalError();
      }
  }

  // Call the handler function for one readable socket:
  HandlerIterator iter(*fHandlers);
  HandlerDescriptor* handler;
  // To ensure forward progress through the handlers, begin past the last
  // socket number that we handled:
  if (fLastHandledSocketNum >= 0) {
    while ((handler = iter.next()) != NULL) {
    //找到剛處理的handler，如果有則退出
      if (handler->socketNum == fLastHandledSocketNum) break;
    }
    if (handler == NULL) {
      fLastHandledSocketNum = -1;
      iter.reset(); // start from the beginning instead
    }
  }
  //從已經處理過的handler之後進行查找進行處理
  while ((handler = iter.next()) != NULL) {
    int sock = handler->socketNum; // alias
    int resultConditionSet = 0;
    if (FD_ISSET(sock, &readSet) && FD_ISSET(sock, &fReadSet)/*sanity check*/) resultConditionSet |= SOCKET_READABLE;
    if (FD_ISSET(sock, &writeSet) && FD_ISSET(sock, &fWriteSet)/*sanity check*/) resultConditionSet |= SOCKET_WRITABLE;
    if (FD_ISSET(sock, &exceptionSet) && FD_ISSET(sock, &fExceptionSet)/*sanity check*/) resultConditionSet |= SOCKET_EXCEPTION;
    if ((resultConditionSet&handler->conditionSet) != 0 && handler->handlerProc != NULL) {
      fLastHandledSocketNum = sock;
          // Note: we set "fLastHandledSocketNum" before calling the handler,
          // in case the handler calls "doEventLoop()" reentrantly.
      (*handler->handlerProc)(handler->clientData, resultConditionSet);
      break;
    }
  }
  //如果沒有處理過handler，則從頭進行處理
  if (handler == NULL && fLastHandledSocketNum >= 0) {
    // We didn't call a handler, but we didn't get to check all of them,
    // so try again from the beginning:
    iter.reset();
    while ((handler = iter.next()) != NULL) {
      int sock = handler->socketNum; // alias
      int resultConditionSet = 0;
      if (FD_ISSET(sock, &readSet) && FD_ISSET(sock, &fReadSet)/*sanity check*/) resultConditionSet |= SOCKET_READABLE;
      if (FD_ISSET(sock, &writeSet) && FD_ISSET(sock, &fWriteSet)/*sanity check*/) resultConditionSet |= SOCKET_WRITABLE;
      if (FD_ISSET(sock, &exceptionSet) && FD_ISSET(sock, &fExceptionSet)/*sanity check*/) resultConditionSet |= SOCKET_EXCEPTION;
      if ((resultConditionSet&handler->conditionSet) != 0 && handler->handlerProc != NULL) {
	fLastHandledSocketNum = sock;
	    // Note: we set "fLastHandledSocketNum" before calling the handler,
            // in case the handler calls "doEventLoop()" reentrantly.
	(*handler->handlerProc)(handler->clientData, resultConditionSet);
	break;
      }
    }
    if (handler == NULL) fLastHandledSocketNum = -1;//because we didn't call a handler
  }

  // Also handle any newly-triggered event (Note that we do this *after* calling a socket handler, 處理新觸發的事件
  // in case the triggered event handler modifies The set of readable sockets.)
  if (fTriggersAwaitingHandling != 0) {
    if (fTriggersAwaitingHandling == fLastUsedTriggerMask) {
      // Common-case optimization for a single event trigger:
      fTriggersAwaitingHandling &=~ fLastUsedTriggerMask;
      if (fTriggeredEventHandlers[fLastUsedTriggerNum] != NULL) {
	(*fTriggeredEventHandlers[fLastUsedTriggerNum])(fTriggeredEventClientDatas[fLastUsedTriggerNum]);
      }
    } else {
      // Look for an event trigger that needs handling (making sure that we make forward progress through all possible triggers):
      unsigned i = fLastUsedTriggerNum;
      EventTriggerId mask = fLastUsedTriggerMask;

      do {
	i = (i+1)%MAX_NUM_EVENT_TRIGGERS;
	mask >>= 1;
	if (mask == 0) mask = 0x80000000;

	if ((fTriggersAwaitingHandling&mask) != 0) {
	  fTriggersAwaitingHandling &=~ mask;
	  if (fTriggeredEventHandlers[i] != NULL) {
	    (*fTriggeredEventHandlers[i])(fTriggeredEventClientDatas[i]);
	  }

	  fLastUsedTriggerMask = mask;
	  fLastUsedTriggerNum = i;
	  break;
	}
      } while (i != fLastUsedTriggerNum);
    }
  }

  // Also handle any delayed event that may have come due.
  //最後檢測延遲隊列
  fDelayQueue.handleAlarm();
}

在singlestep()的最後檢測延遲隊列，在創建TaskScheduler是加入的alarmHandler保證延遲隊列中一直會有一個10ms超時事件的handler存在，使得select()的超時事件最大是10ms，也就是說至少10ms檢測並執行一次singlestep的動作。

HandlerSet

在 BasicTaskScheduler0 與handler處理相關的元素：

 // To implement background reads:
  HandlerSet* fHandlers; 管理HandlerDescriptor鏈表
  int fLastHandledSocketNum;

參照上文調度類關係圖，handlerSet中管理handlerdescriptor雙向鏈表，HandlerDescriptor元素有socket號，和此handler需要檢測的fd變動條件：讀，寫，異常。handlerProc是回調函數。

事件觸發

在 BasicTaskScheduler0 中與事件觸發相關的元素

// To implement event triggers:
  EventTriggerId volatile fTriggersAwaitingHandling; // implemented as a 32-bit bitmap
  EventTriggerId fLastUsedTriggerMask; // implemented as a 32-bit bitmap
  TaskFunc* fTriggeredEventHandlers[MAX_NUM_EVENT_TRIGGERS]; 事件回調函數
  void* fTriggeredEventClientDatas[MAX_NUM_EVENT_TRIGGERS]; 響應回調函數的數據
  unsigned fLastUsedTriggerNum; // in the range [0,MAX_NUM_EVENT_TRIGGERS)

32個事件，用mask來管理到來的事件。