By default I/O requests are assigned Normal priority. Windows Vista defines five priority classes – Very Low, Low, Normal, High and Critical. Disk I/O requests in Windows Vista are assigned priorities a higher priority request is given preferential treatment, over a request that has a lower priority, during the execution of the request. Windows Vista also implements I/O scheduling as prioritized I/O. Applications that cancel the operation on user feedback may prefer to enable user feedback during the time the issuing thread is suspended for usability. In Windows Vista the application may issue a cancellation request. During a synchronous I/O request, the application is blocked until the request is serviced or fails. Windows Vista also introduces synchronous I/O cancellation. If this thread is the one that handles the data after the I/O request completes, then a thread-switch, which causes a performance hit, may be avoided. With this, a single thread can issue all the I/O requests, and then switch to a different worker thread. With the new asynchronous I/O APIs, a thread, different from the one that issued the I/O request, can be notified when the operation completes. Vista modifies the behavior of asynchronous I/O operations.
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