使用异步 I/O 完成事件，线程池中的线程将只在收到数据时对数据进行处理，一旦处理完数据，该线程就会返回到线程池中。
若要进行异步 I/O 调用，必须将操作系统 I/O 句柄与线程池相关联，并且必须指定一个回调方法。当 I/O 操作完成后，线程池中的线程将调用该回调方法。
下面的 C# 代码示例说明了一个简单的异步 I/O 操作。
注意 该示例需要 100MB 以上的可用内存。
[C#]
using System;
using System.IO;
using System.Threading;
using System.Runtime.InteropServices;
public class BulkImageProcAsync{
public const String ImageBaseName = "tmpImage-";
public const int numImages = 200;
public const int numPixels = 512*512;
// ProcessImage has a simple O(N) loop, and you can vary the number
// of times you repeat that loop to make the application more CPU-bound
// or more I/O-bound.
public static int processImageRepeats = 20;
// Threads must decrement NumImagesToFinish, and protect
// their access to it through a mutex.
public static int NumImagesToFinish = numImages;
public static Object NumImagesMutex = new Object[0];
// WaitObject is signalled when all image processing is done.
public static Object WaitObject = new Object[0];
public class ImageStateObject{
public byte[] pixels;
public int imageNum;
public FileStream fs;
}
public static void MakeImageFiles(){
int sides = (int) Math.Sqrt(numPixels);
Console.Write("Making "+numImages+" "+sides+"x"+sides+" images... ");
byte[] pixels = new byte[numPixels];
for(int i=0; i<numPixels; i++)
pixels[i] = (byte) i;
for(int i=0; i<numImages; i++){
FileStream fs = new FileStream(
ImageBaseName+i+".tmp",
FileMode.Create,
FileAccess.Write,
FileShare.None,
8192,
false
);
fs.Write(pixels, 0, pixels.Length);
FlushFileBuffers(fs.Handle);
fs.Close();
}
Console.WriteLine("Done.");
}
public static void ReadInImageCallback(IAsyncResult asyncResult){
ImageStateObject state = (ImageStateObject) asyncResult.AsyncState;
Stream stream = state.fs;
int bytesRead = stream.EndRead(asyncResult);
if (bytesRead != numPixels)
throw new Exception("In ReadInImageCallback, got wrong number of bytes from the image! got: " + bytesRead);
ProcessImage(state.pixels, state.imageNum);
stream.Close();
// Now write out the image. No async here.
FileStream fs = new FileStream(
ImageBaseName + state.imageNum + ".done",
FileMode.Create,
FileAccess.Write,
FileShare.None,
4096,
false
);
fs.Write(state.pixels, 0, numPixels);
fs.Close();
// This application model uses too much memory.
// Releasing memory as soon as possible is a good idea, especially global
// state.
state.pixels = null;
// Record that an image is done now.
lock(NumImagesMutex){
NumImagesToFinish--;
if (NumImagesToFinish==0){
lock(WaitObject){
Monitor.Pulse(WaitObject);
}
}
}
}
public static void ProcessImage(byte[] pixels, int imageNum){
Console.WriteLine("ProcessImage "+imageNum);
// Do some CPU-intensive operation on the image.
for(int i=0; i<processImageRepeats; i++)
for(int j=0; j<numPixels; j++)
pixels[j] += 1;
Console.WriteLine("ProcessImage "+imageNum+" done.");
}
public static void ProcessImagesInBulk(){
Console.WriteLine("Processing images... ");
long t0 = Environment.TickCount;
NumImagesToFinish = numImages;
AsyncCallback readImageCallback = new AsyncCallback(ReadInImageCallback);
for(int i=0; i<numImages; i++){
ImageStateObject state = new ImageStateObject();
state.pixels = new byte[numPixels];
state.imageNum = i;
// Very large items are read only once, so the
// buffer on the file stream can be very small to save memory.
FileStream fs = new FileStream(
ImageBaseName+i+".tmp",
FileMode.Open,
FileAccess.Read,
FileShare.Read,
1,
true
);
state.fs = fs;
fs.BeginRead(state.pixels, 0, numPixels, readImageCallback, state);
}
// Determine whether all images are done being processed.
// If not, block until all are finished.
bool mustBlock = false;
lock (NumImagesMutex){
if (NumImagesToFinish > 0)
mustBlock = true;
}
if (mustBlock){
Console.WriteLine(
"All worker threads are queued... Blocking until they complete. numLeft: " + NumImagesToFinish
);
lock(WaitObject){
Monitor.Pulse(WaitObject);
}
}
long t1 = Environment.TickCount;
Console.WriteLine("Total time processing images: {0} ms", (t1-t0));
}
public static void Cleanup(){
for(int i=0; i<numImages; i++){
File.Delete(ImageBaseName+i+".tmp");
File.Delete(ImageBaseName+i+".done");
}
}
public static void TryToClearDiskCache(){
// Try to force all pending writes to disk, and clear the
// disk cache of any data.
byte[] bytes = new byte[100*(1<<20)];
for(int i=0; i<bytes.Length; i++)
bytes[i] = 0;
bytes = null;
GC.Collect();
Thread.Sleep(2000);
}
public static void Main(String[] args){
Console.WriteLine("Bulk image processing sample application, using asynchronous I/O");
Console.WriteLine("Simulates applying a simple transformation to "+numImages+" \"images\"");
Console.WriteLine("(ie, Async FileStream & Threadpool benchmark)");
Console.WriteLine("Warning - this test requires "+(numPixels * numImages * 2)+" bytes of tmp space");
if (args.Length==1){
processImageRepeats = Int32.Parse(args[0]);
Console.WriteLine("ProcessImage inner loop - "+processImageRepeats);
}
MakeImageFiles();
TryToClearDiskCache();
ProcessImagesInBulk();
Cleanup();
}
[DllImport("KERNEL32", SetLastError=true)]
private static extern void FlushFileBuffers(IntPtr handle);
}
