使用I/O完成端口模型监测磁盘文件状态

作者：Frank

众所周知，I/O完成端口(IOCP)是目前性能最好的一种I/O模型。其大体的思路如下：在程序处理的过程中，阻塞类型的操作有很多，如(Socket Send/Recv), 磁盘读写，外部硬件接口（如打印机，扫描仪）等；以往的模型在处理阻塞事件时，为了提高程序在阻塞同时的并发性，经常使用多线程，这样就有很多可调度的线程并行在系统中，OS内核会花费大量的时间在线程的Context切换中，线程本身工作处理的时间会很少（可称工作饱和度很低），极端的情况下线程切换的时间甚至可能会大于本身线程的运行周期。

IOCP模型是windows提供的一种阻塞事件异步处理的模型，应用程序可以根据CPU的核数预先开N个线程，存储在线程池中。然后将所有的I/O请求都投递到一个完成端口上，与此同时N个工作线程逐一地从完成端口中读取完成的阻塞事件并加以处理，这样就极大程度的提高了线程的工作饱和度。

目前在性能上号称数倍优于Apache的服务器模型nginx，其实使用的也是这种机制，只是windows内部支持完成端口，而linux的支持为epool，所有的请求简化为阻塞操作和非阻塞操作，所有需要阻塞请求的部分全部由epool触发相应事件，非阻塞（处理耗时短）部分用主线程一直执行，直到遇到阻塞部分就停止，交由阻塞部分监听异步完成事件，这样就构成了nginx中的事件驱动模型。

平时讨论的比较多的是网络服务模型，即在server端使用IOCP管理socket链接，其实IOCP是可以应用于所有支持异步执行的阻塞操作中，下面介绍一个用来监测文件状态的实例：

应用场景之一：很多网盘应用提供同步功能，由于大家时常进行本地文件和网盘文件的upload/download操作，这样很不方便，设定同步功能可以自动将其关联，实现文件的实时更新，用户也体验大大提高。

代码使用VS2008实现，提供一个cFileUpdateChecker类，可以指定监测路径，每当其路径发生变化都会获取到通知事件，如copy, paste, create, remove, renaming, property modification等等，使用Windows API : ReadDirectoryChangesW的异步方式进行监测，所有请求有IOCP方式处理。

被观察文件的结构定义：

typedef struct directory_info {        HANDLE    hDir;        CHAR          szDirName[260];        CHAR          szBuffer[4096];        DWORD      dwBufLength;        OVERLAPPED Overlapped;  // 此处需要定义overlapp结构 }DIRECTORY_INFO, *LPDIRECTORY_INFO; #define DIRECTORY_PATH      L"C://TestFolder"

 

实现观察的具体类定义：

 

class cFileUpdateChecker { public: cFileUpdateChecker():hThread(NULL), dwThread(0), hComp(NULL), pdir(NULL)     {}        ~cFileUpdateChecker()        {               if (hThread)         CloseHandle(hThread);                   if (hComp)          CloseHandle(hComp);                    if (pdir)               {                      PostQueuedCompletionStatus(hComp, 0, NULL, NULL);                      CloseHandle(pdir->hDir);                      HeapFree(GetProcessHeap(), 0, pdir);               }        }          // 定义IOCP的工作线程        static DWORD WINAPI  ThreadFunc(void* p)        {               cFileUpdateChecker* pHost  = (cFileUpdateChecker*)p;                 LPDIRECTORY_INFO lpdir = NULL;               DWORD nbytes;               LPOVERLAPPED lpOverlapped = NULL;               PFILE_NOTIFY_INFORMATION fni = NULL;               DWORD wait = 1000;                             while (true)               { BOOL bRet = GetQueuedCompletionStatus(pHost->hComp, &nbytes, (PULONG_PTR)&lpdir, &lpOverlapped, wait);                      if ( FALSE == bRet && NULL == lpOverlapped && NULL != pHost->hComp )                      {                             wait = INFINITE;                             continue;                      }                      else                      {                             if (lpdir)                             {                                    fni = (PFILE_NOTIFY_INFORMATION)lpdir->szBuffer;                                      switch(fni->Action)                                    {                                    case FILE_ACTION_ADDED: //The file was added to the directory.                                           break;                                    case FILE_ACTION_REMOVED: //The file was removed from the directory.                                           break;                                    case FILE_ACTION_MODIFIED: //The file was modified. This can be a change in the time stamp or attributes.                                           break;                                    case FILE_ACTION_RENAMED_OLD_NAME: //The file was renamed and this is the old name.                                           break;                                    case FILE_ACTION_RENAMED_NEW_NAME: //The file was renamed and this is the new name.                                           break;                                    default:                                           break;                                    }                             ZeroMemory(&(pHost->pdir->Overlapped), sizeof(OVERLAPPED));                                    ZeroMemory(pHost->pdir->szBuffer, 4096);                                    ZeroMemory(pHost->pdir->szDirName, 260);                                      ReadDirectoryChangesW(pHost->pdir->hDir,                                           pHost->pdir->szBuffer,                                           sizeof(pHost->pdir->szBuffer),                                           TRUE,                                           FILE_NOTIFY_CHANGE_FILE_NAME |                                           FILE_NOTIFY_CHANGE_DIR_NAME |                                           FILE_NOTIFY_CHANGE_ATTRIBUTES |                                           FILE_NOTIFY_CHANGE_SIZE |                                           FILE_NOTIFY_CHANGE_LAST_ACCESS |                                           FILE_NOTIFY_CHANGE_CREATION |                                           FILE_NOTIFY_CHANGE_SECURITY |                                           FILE_NOTIFY_CHANGE_LAST_WRITE,                                           &(pHost->pdir->dwBufLength),                                           (OVERLAPPED*)(&(pHost->pdir->Overlapped)),                                           NULL                                           );                             }                      }               }                 return 0;        }                 bool StartChecker()        { hComp = CreateIoCompletionPort(INVALID_HANDLE_VALUE, NULL, 0, 0);   pdir = (LPDIRECTORY_INFO)HeapAlloc(GetProcessHeap(), HEAP_ZERO_MEMORY, sizeof(DIRECTORY_INFO));               if (NULL == pdir)               {                      return false;               }                 pdir->hDir = CreateFile(DIRECTORY_PATH,                                                         FILE_LIST_DIRECTORY, FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE,                                           NULL,                                           OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OVERLAPPED,                                           NULL);               if (pdir->hDir == INVALID_HANDLE_VALUE)               {                      return false;               }                 // 启动工作线程               hThread = CreateThread(NULL, 0, ThreadFunc, this,0, &dwThread);               if (NULL == hThread)               {                      return false;               }        // 此处投递IO请求的时候必须清零，开始因为没有清零总是收不到完成事件 ZeroMemory(&(pdir->Overlapped), sizeof(OVERLAPPED));               ZeroMemory(pdir->szBuffer, 4096);               ZeroMemory(pdir->szDirName, MAX_PATH);                 // 投递I/O请求到完成端口 HANDLE hRet = CreateIoCompletionPort(pdir->hDir, hComp, (ULONG_PTR)pdir, 0);               if (NULL == hComp)               {                      return false;               }                 BOOL bRet = ReadDirectoryChangesW(pdir->hDir,                      pdir->szBuffer,                      4096,                      TRUE,                      FILE_NOTIFY_CHANGE_FILE_NAME |                      FILE_NOTIFY_CHANGE_DIR_NAME |                      FILE_NOTIFY_CHANGE_ATTRIBUTES |                      FILE_NOTIFY_CHANGE_SIZE |                      FILE_NOTIFY_CHANGE_LAST_ACCESS |                      FILE_NOTIFY_CHANGE_CREATION |                      FILE_NOTIFY_CHANGE_SECURITY |                      FILE_NOTIFY_CHANGE_LAST_WRITE,                      &pdir->dwBufLength,                      &(pdir->Overlapped),                      NULL                      );                 if (FALSE == bRet)               {                      return false;               }               return true;        }                 private:        HANDLE hThread;        DWORD dwThread;        HANDLE  hComp;        LPDIRECTORY_INFO pdir; };

 

参考资料：

1. MSDN 网站 - http://msdn.microsoft.com/en-us/library/aa365198%28VS.85%29.aspx