qr code generator vb.net 2010 Part V in .NET framework

Encoder QR Code in .NET framework Part V

Part V
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Threading
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FileStream-Specific Issues
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When you create a FileStream object, you get to specify whether you want to communicate using synchronous or asynchronous operations via the FileOptions.Asynchronous flag (which is equivalent to calling the Win32 CreateFile function and passing into it the FILE_FLAG_OVERLAPPED flag) . If you do not specify this flag, Windows performs all operations against the file synchronously . Of course, you can still call FileStream s BeginRead method, and to your application, it looks as if the operation is being performed asynchronously, but internally, the FileStream class uses another thread to emulate asynchronous behavior . This additional thread is wasteful and hurts performance . On the other hand, you can create a FileStream object by specifying the FileOptions. Asynchronous flag . Then you can call FileStream s Read method to perform a synchronous operation . Internally, the FileStream class emulates this behavior by starting an asynchronous operation and then immediately puts the calling thread to sleep until the operation is complete . This is also inefficient, but it is not as inefficient as calling BeginRead by using a FileStream constructed without the FileOptions.Asynchronous flag . So, to summarize: When working with a FileStream, you must decide up front whether you intend to perform synchronous or asynchronous I/O against the file and indicate your choice by specifying the FileOptions.Asynchronous flag (or not) . If you specify this flag, always call BeginRead . If you do not specify this flag, always call Read . This will give you the best performance . If you intend to make some synchronous and some asynchronous operations against the FileStream, it is more efficient to construct it using the FileOptions.Asynchronous flag . Alternatively, you can create two FileStream objects over the same file; open one FileStream for asynchronous I/O and open the other FileStream for synchronous I/O . You should also be aware that the NTFS file system device driver performs some operations synchronously no matter how you open the file . For more information about this, see http://support.microsoft.com/default.aspx scid=kb%3Ben-us%3B156932.
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In 25, Thread Basics, I showed how setting thread priorities affects how threads are scheduled . However, threads also perform I/O requests to read and write data from various hardware devices . If a low-priority thread gets CPU time, it could easily queue hundreds or thousands of I/O requests in a very short time . Because I/O requests typically require time to process, it is possible that a low-priority thread could significantly affect the responsiveness of the system by suspending high-priority threads, which prevents them from getting their work done . Because of this, you can see a machine become less responsive when executing long-running low-priority services such as disk defragmenters, virus scanners, content indexers, and so on .9
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The Windows SuperFetch feature takes advantage of low-priority I/O requests .
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27 I/O-Bound Asynchronous Operations
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Starting with Windows Vista, it is now possible for a thread to specify a priority when making I/O requests . For more details about I/O priorities, refer to the white paper at http://www.microsoft.com/whdc/driver/priorityio.mspx. Unfortunately, the FCL does not include this functionality yet; hopefully, it will be added in a future version . However, you can still take advantage of this feature by P/Invoking out to native Win32 functions . Here is the P/Invoke code:
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internal static class ThreadIO { public static BackgroundProcessingDisposer BeginBackgroundProcessing( Boolean process = false) { ChangeBackgroundProcessing(process, true); return new BackgroundProcessingDisposer(process); } public static void EndBackgroundProcessing(Boolean process = false) { ChangeBackgroundProcessing(process, false); } private static void ChangeBackgroundProcessing(Boolean process, Boolean start) { Boolean ok = process SetPriorityClass(GetCurrentWin32ProcessHandle(), start ProcessBackgroundMode.Start : ProcessBackgroundMode.End) : SetThreadPriority(GetCurrentWin32ThreadHandle(), start ThreadBackgroundgMode.Start : ThreadBackgroundgMode.End); if (!ok) throw new Win32Exception(); } // This struct lets C#'s using statement end the background processing mode public struct BackgroundProcessingDisposer : IDisposable { private readonly Boolean m_process; public BackgroundProcessingDisposer(Boolean process) { m_process = process; } public void Dispose() { EndBackgroundProcessing(m_process); } }
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// See Win32 s THREAD_MODE_BACKGROUND_BEGIN and THREAD_MODE_BACKGROUND_END private enum ThreadBackgroundgMode { Start = 0x10000, End = 0x20000 } // See Win32 s PROCESS_MODE_BACKGROUND_BEGIN and PROCESS_MODE_BACKGROUND_END private enum ProcessBackgroundMode { Start = 0x100000, End = 0x200000 } [DllImport("Kernel32", EntryPoint = "GetCurrentProcess", ExactSpelling = true)] private static extern SafeWaitHandle GetCurrentWin32ProcessHandle(); [DllImport("Kernel32", ExactSpelling = true, SetLastError = true)] [return: MarshalAs(UnmanagedType.Bool)] private static extern Boolean SetPriorityClass( SafeWaitHandle hprocess, ProcessBackgroundMode mode); [DllImport("Kernel32", EntryPoint = "GetCurrentThread", ExactSpelling = true)] private static extern SafeWaitHandle GetCurrentWin32ThreadHandle();
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